ALBERT

Beschreibung: The objective of this study is to incorporate a time-dependent SCS CN method (SMA_CN) in Soil and Water Assessment Tool (SWAT) and compare its performance with the existing CN method in SWAT by simulating the hydrology of two agricultural watersheds in Indiana, United States. Results show that fusion of the SMA_CN method causes decrease in runoff volume and increase in profile soil moisture content, associated with larger groundwater contribution to the streamflow. In addition, the higher amount of moisture in the soil profile slightly elevates the actual evapotranspiration. The SMA-based SWAT configuration consistently produces improved goodness of fit scores and less uncertain outputs with respect to streamflow during both calibration and validation. The SMA_CN method exhibits better match with the observed data for all flow regimes, thereby addressing issues related to peak and low flow predictions by SWAT in many past studies. Comparison of the calibrated model outputs with field-scale soil moisture observations reveal that the SMA overhauling enables SWAT to represent soil moisture condition more accurately, with better response to the incident rainfall dynamics. While the results from the modification of the SCS method in SWAT are promising, more studies including watersheds with various physical and climatic settings are needed to validate the proposed approach. This article is protected by copyright. All rights reserved.

Beschreibung: We adapted Newton's Law of Cooling to model downstream water temperature change in response to stream-adjacent forest harvest on small and medium streams (average 327 ha in size) throughout the Oregon Coast Range, USA. The model requires measured stream gradient, width, depth and upstream control reach temperatures as inputs and contains two free parameters which were determined by fitting the model to measured stream temperature data. This model reproduces the measured downstream temperature responses to within 0.4 C ° for 15 of the 16 streams studied and provides insight into the physical sources of site-to-site variation among those responses. We also use the model to examine how the pre-to-post harvest change in daily maximum stream temperature depends on distance from the harvest reach. The model suggests that the pre-to-post harvest temperature change approximately 300  m downstream of the harvest will range from roughly 82% to less than 1% of that temperature change which occurred within the harvest reach, depending primarily on the downstream width, depth, and gradient. Using study-averaged values for these channel characteristics the model suggests that for a stream representative of those in the study, the temperature change approximately 300  m downstream of the harvest will be 56% of the temperature change which occurred within the harvest reach. This adapted Newton's Law of Cooling procedure represents a highly practical means for predicting stream temperature behavior downstream of timber harvests relative to conventional heat budget approaches, and is informative of the dominant processes affecting stream temperature. This article is protected by copyright. All rights reserved.

Beschreibung: Digital elevation models (DEMs) that are used in hydrological applications must be processed to remove sinks, mainly topographic depressions. Flow enforcement techniques include filling methods, which raise elevations within depressions, breaching, which carves channels through blockages, and hybrid methods. Despite previous research demonstrating the large impact to DEMs and subsequent analyses of depression filling, it is common practice apply this technique to flow enforcement. This is partly due to the greater efficiency of depression filling tools compared to breaching counterparts, which often limits breaching to applications of small- to moderate-sized DEMs. A new hybrid flow enforcement algorithm is presented in this study. The method can be run in complete breaching, selective breaching (either breached or filled), or constrained breaching (partial breaching) modes, allowing for greater flexibility in how practitioners enforce continuous flow paths. Algorithm performance was tested with DEMs of varying topography, spatial extents, and resolution. The sites included three moderate sized DEMs (52,000,000 to 190,000,000 cells) and three massive DEMs of the Iberian Peninsula, and the Amazon and Nile River basins, the largest containing nearly one billion cells. In complete breaching mode, the new algorithm required 87% of the time needed by a filling method to process the test DEMs, while the selective breaching and constrained breaching modes, operating with maximum breach depth constraints, increased run times by 8% and 27% respectively. Therefore, the new algorithm offers comparable performance to filling and the ability to process massive topographic data sets, while giving practitioners greater flexibility and lowering DEM impact. This article is protected by copyright. All rights reserved.

Beschreibung: Mountain snowpacks provide most of the annual discharge of western U.S. rivers, but the future of water resources in the western U.S. is tenuous, as climatic changes have resulted in earlier spring melts that have exacerbated summer droughts. Compounding changes to the physical environment are biotic disturbances including the mountain pine beetle (MPB), which has decimated millions of acres of western North American forests. At the watershed scale, MPB disturbance increases the peak hydrograph, and at the stand scale the ‘gray’ phase of MPB canopy disturbance decreases canopy snow interception, increases snow albedo, increases net shortwave radiation and decreases net longwave radiation versus the ‘red’ phase. Fewer studies have been conducted on the red phase of MPB disturbance, and in the mixed coniferous stands that may follow MPB-damaged forests. We measured the energy balance of four snowpacks representing different stages of MPB damage, management, and recovery: a lodgepole pine stand, a MPB-infested stand in the red phase, a mixed coniferous stand (representing one successional trajectory), and a clearcut (representing reactive management) in the Tenderfoot Creek Experimental Forest in Montana, USA. Net longwave radiation was lower in the MPB-infested stand despite higher basal area and plant area index of the other forests, suggesting that the dessicated needles serve as a less effective thermal buffer against longwave radiative losses. Eddy covariance observations of sensible and latent heat flux indicate that they are of similar but opposite magnitude, on the order of 20 MJ m −2 during the melt period. Further analyses reveal that net turbulent energy fluxes were near zero due to the temperature and atmospheric vapor pressure encountered during the melt period. Future research should place snow science in the context of forest succession and management, and address important uncertainties regarding the timing and magnitude of needlefall events. This article is protected by copyright. All rights reserved.

Beschreibung: As a result of climate change/variation and its aggravation by human activities over the past several decades, the hydrological conditions in the middle Yellow River in China have dramatically changed, which has led to a sharp decrease of streamflow and the drying up of certain tributaries. This paper simulated and analysed the impact of sediment-trapping dams (STDs, a type of large-sized check dam used to prevent sediment from entering the Yellow River main stem) on hydrological processes, and the study area was located in the 3,246 km 2 Huangfuchuan (HFC) River basin. Changes in the hydrological processes were analysed, and periods of natural and disturbed states were defined. Subsequently, the number and distribution of the STDs were determined based on data collected from statistical reports and identified from remote sensing images, and the topological relationships between the STDs and high-resolution river reaches were established. A hydrological model, the Digital Yellow River Integrated Model, was used to simulate the STD impact on the hydrological processes, and the maximum STD impact was evaluated through a comparison between the simulation results with and without the STDs, which revealed that the interception effect of the STDs contributed to the decrease of the streamflow by approximately 39%. This paper also analysed the relationship between the spatial distribution of the STDs and rainfall in the HFC River basin and revealed that future soil and water conservation measures should focus on areas with a higher average annual rainfall and higher number of rainstorm hours. This article is protected by copyright. All rights reserved.

Beschreibung: Distributed, continuous hydrologic models promote better understanding of hydrology and enable integrated hydrologic analyses by providing a more detailed picture of water transport processes across the varying landscape. However, such models are not widely used in routine modeling practices, due in part to the extensive data input requirements, computational demands, and complexity of routing algorithms. We developed a two-dimensional continuous hydrologic model, HYSTAR, using a time-area method within a grid-based spatial data model with the goal of providing an alternative way to simulate spatiotemporally varied watershed-scale hydrologic processes. The model calculates the direct runoff hydrograph by coupling a time-area routing scheme with a dynamic rainfall excess sub-model implemented here using a modified curve number method with an hourly time step, explicitly considering downstream ‘reinfiltration’ of routed surface runoff. Soil moisture content is determined at each time interval based on a water balance equation, and overland and channel runoff is routed on time-area maps, representing spatial variation in hydraulic characteristics for each time interval in a storm event. Simulating runoff hydrographs does not depend on unit hydrograph theory or on solution of the Saint Venant equation, yet retains the simplicity of a unit hydrograph approach and the capability of explicitly simulating two-dimensional flow routing. The model provided acceptable performance in predicting daily and monthly runoff for a 6-year period for a watershed in Virginia (USA) using readily available geographic information about the watershed landscape. Spatial and temporal variability in simulated effective runoff depth and time area maps dynamically show the areas of the watershed contributing to the direct runoff hydrograph at the outlet over time, consistent with the variable source area overland flow generation mechanism. The model offers a way to simulate watershed processes and runoff hydrographs using the time-area method, providing a simple, efficient, and sound framework that explicitly represents mechanisms of spatially and temporally varied hydrologic processes. This article is protected by copyright. All rights reserved.

Beschreibung: Given the importance of groundwater temperature to the biogeochemical health of aquatic ecosystems, a floodplain study was implemented to improve understanding of rural land use impacts on shallow groundwater (SGW) temperature. Study sites included a historic agricultural field (Ag) and bottomland hardwood forest (BHF), each with nine piezometers in an 80 × 80 m grid. Piezometers were equipped with pressure transducers to monitor SGW temperature and level at 30 minute intervals during the 2011, 2012, 2013, and 2014 water years. The study is one of the first to utilize long-term, continuous, automated, in situ monitoring to investigate rural land use impacts on shallow groundwater temperatures. Average SGW temperature during the study period was 11.1 and 11.2 °C at the Ag and BHF sites, respectively. However, temperature range at the Ag site was 72% greater than at the BHF site. Results indicate a greater responsiveness to seasonal climate fluctuations in Ag site SGW temperature related to absence of forest canopy. Patterns of intra-site groundwater temperature differences at both study sites illustrate the influence of stream-aquifer thermal conduction and occasional baseflow reversals. Considering similar surface soil temperature amplitudes and low average groundwater flow values at both sites, results suggest that contrasting rates of plant water use, groundwater recharge, and subsurface hydraulic conductivity are likely mechanistic causes for the observed SGW temperature differences. Results highlight the long-term impact of forest removal on subsurface hydrology and groundwater temperature regime. This article is protected by copyright. All rights reserved.

Beschreibung: Growing demand on groundwater resources and the semi-arid climate in the North China Plain (NCP) highlight the need for improved understanding of connections between regional climate change and groundwater recharge. Hydrologic time series of precipitation and groundwater levels were analyzed in three representative geographical zones throughout the NCP for the period of 1960-2008 using trend analysis and spectral analysis methods. A significant change point around 1975 is followed by a long term decline trend in precipitation time series, which coincides with the Pacific Decadal Oscillation (PDO) positive phase. However, the magnitudes of groundwater levels variability due to heavily pumping overwhelm the low-frequency signal of groundwater levels. Nonlinear trends that related to long-term climatic variability and anthropogenic activities are removed by using the Singular Spectrum Analysis (SSA) method. Spectral analyses of the detrended residuals demonstrate significant short-term oscillations at the frequencies of 2–7 years, which have strong correlations with the El Niño-Southern Oscillation (ENSO) modes. This study contributes to improved understanding of dynamic relationship between groundwater and climate variability modes in the NCP, and demonstrates the importance of reliable detrending methods for groundwater levels that are affected greatly by pumping. This article is protected by copyright. All rights reserved.

Beschreibung: On September 3, 1998, a glacial lake outburst flood (GLOF) that originated from Tam Pokhari occurred in the Hinku valley of the eastern Nepal Himalaya. This study analyzes the lake's geomorphic and hydrologic conditions prior to the outburst, and evaluates the conditions that could contribute to a future flood through photogrammetric techniques. We processed high-resolution Corona KH-4A (2.7 m) and ALOS PRISM (2.5 m) stereo-images taken before and after the GLOF event, and produced detailed topographic maps (2-m contour interval) and DEMs (5 m × 5 m). We (re-) constructed lake water surfaces before (4410 ± 5 m) and after (4356 ± 5 m) the outburst, and reliably estimated the lake water surface lowering (54 ± 5 m) and the water volume released (19.5 ± 2.2 × 10 6  m 3 ) from the lake, showing good agreement with the results obtained from ground-based measurements. The most relevant conditions that may have influenced the catastrophic drainage of Tam Pokhari in 1998 include the presence of: i) a narrow (75 ± 6 m), steep (up to 50°) and high (120 ± 5 m) moraine dam; ii) high lake level (8 ± 5 m of freeboard); and iii) a steep overhanging glacier (〉40°). The lake outburst substantially altered the immediate area, creating a low and wide (〉500 m) outwash plain below the lake, a wide lake outlet channel (~50 m) and a gentle channel slope (~3–5°). Our new data suggest that the likelihood of a future lake outburst is low. Our results demonstrate that the datasets produced by photogrammetric techniques provide an excellent representation of micro-landform features on moraine dams, lake water surfaces and the changes in both over time, thereby allowing highly accurate pre- and post-GLOF (volumetric) change analysis of glacial lakes. Furthermore, it enables precise measurement of several predictive variables of GLOFs that can be useful for identifying potentially dangerous glacial lakes or prioritizing them for detailed field investigations. This article is protected by copyright. All rights reserved.

Beschreibung: Although the importance to account for microrelief in the calculation of specific yields for shallow groundwater systems is well recognized, the microrelief influence is often treated very simplified, which can cause considerable errors. We provide a general one-dimensional expression that correctly represents the effect of a microrelief on the total specific yield that is composed of the soil and surface specific yield. The one-dimensional expression can be applied for different soil hydraulic parameterizations and soil surface elevation frequency distributions. Applying different van Genuchten parameters and a simple linear microrelief model, we demonstrate that the specific yield is influenced by the microrelief not only when surface storage directly contributes to specific yield by (partial) inundation but also when water levels are lower than the minimum surface elevation. Compared to a simplified representation of the soil specific yield, in which a mean soil surface is assumed for the calculation of soil specific yield, the correct representation can lead to lower as well as higher soil specific yields depending on the specific interaction of the soil water retention characteristics and the microrelief. The new equation can be used to obtain more accurate evapotranspiration estimates from water level fluctuations and to account for the effect of microtopographic subgrid variability on simulated water levels of spatially-distributed hydrological models. This article is protected by copyright. All rights reserved.

Beschreibung: Globally, Dissolved Inorganic Carbon (DIC) accounts for more than half the annual flux of carbon exported from terrestrial ecosystems via rivers. Here we assess the relative influences of biogeochemical and hydrological processes on DIC fluxes exported from a tropical river catchment characterized by distinct land cover, climate and geology transition from the wet tropical mountains to the low lying savanna plains. Processes controlling changes in river DIC were investigated using dissolved organic carbon (DOC), particulate organic carbon (POC) and DIC concentrations and stable isotope ratios of DIC (δ 13 C DIC ) at two time scales; seasonal and diel. The recently developed Isotopic Continuous Dissolved Inorganic Carbon Analyser (ISO-CADICA) was used to measure diel DIC concentration and δ 13 C DIC changes at a 15 minute temporal resolution. Results highlight the predominance of biologically mediated processes (photosynthesis and respiration) controlling diel changes in DIC. These resulted in DIC concentrations varying between 3.55-3.82 mg/L, and δ 13 C DIC values ranging from -19.7 ± 0.31 to -17.1 ± 0.08 ‰. In contrast, at the seasonal scale we observe wet season DIC variations predominantly from mixing processes, and dry season DIC variations due to both mixing processes and biological processes. The observed wet season increases in DIC concentrations (by 6.81 mg/L) and δ 13 C DIC values of river water (by 5.4 ‰) largely result from proportional increases in subsurface inflows from the savanna plains (C 4 vegetation) region relative to inflows from the rainforest (C 3 vegetation) highlands. The high DIC river load during the wet season results in the transfer of 97% of the annual river carbon load. Therefore, in this gaining river there are significant seasonal variations in both the hydrological and carbon cycles, and there is evidence of substantial coupling between the carbon cycles of the terrestrial and the fluvial environments. Recent identification of a substantial savanna carbon sink in wetter years in the recent past does not take into account the possibility of a substantial, rapid, lateral flux of carbon to rivers and back to the atmosphere. This article is protected by copyright. All rights reserved.

Beschreibung: Various remote-sensing methods are available to estimate soil moisture, but few address the fine spatial resolutions (e.g., 30 m grid cells) and root-zone depth requirements of agricultural and other similar applications. One approach that has been previously proposed to estimate fine-resolution soil moisture is to first estimate the evaporative fraction from an energy balance that is inferred from optical and thermal remote-sensing images (e.g., using the ReSET algorithm) and then estimate soil moisture through an empirical relationship to evaporative fraction. A similar approach has also been proposed to estimate the degree of saturation. The primary objective of this study is to evaluate these methods for estimating soil moisture and degree of saturation, particularly for a semiarid grassland with relatively dry conditions. Soil moisture was monitored at twenty-eight field locations in southeastern Colorado with herbaceous vegetation during the summer months of three years. In-situ soil moisture and degree of saturation observations are compared with estimates calculated from Landsat imagery using the ReSET algorithm. The in-situ observations suggest that the empirical relationships with evaporative fraction that have been proposed in previous studies typically provide overestimates of soil moisture and degree of saturation in this region. However, calibrated functions produce estimates with an accuracy that may be adequate for various applications. The estimates produced by this approach are more reliable for degree of saturation than for soil moisture, and the method is more successful at identifying temporal variability than spatial variability in degree of saturation for this region. This article is protected by copyright. All rights reserved.

Beschreibung: Groundwater movements in volcanic mountains and their effects on streamflow discharge and representative elementary area (REA) have remained largely unclear. We surveyed the discharge and chemical composition of spring and stream water in two catchments: the Hontani river (NR) catchment (6.6 km 2 ) and the Hosotani river (SR) catchment (4.0 km 2 ) at the southern part of Daisen volcano, Japan. Daisen volcano is a young volcano (17 × 10 3  years) at an early stage of erosion. Our study indicated that deep groundwater that moved through thick lava and pyroclastic flows and that could not be explained by shallow movements controlled by surface topography contributed dominantly to streamflow at larger catchment areas. At the NR catchment, the deep groundwater contribution clearly increased at a catchment boundary defined by an area of 3.0 km 2 and an elevation of 800 m. At the SR catchment, the contribution deep groundwater to the stream also increased suddenly at a boundary threshold of 2.0 km 2 and 700 m. Beyond these thresholds, the contributions of deep bedrock groundwater remained constant, indicating that the REA is between 2 and 3 km 2 at the observed area. These results indicate that the hydrological conditions of base flow were controlled mainly by the deep bedrock groundwater that moved through thick lava and pyroclastic flows in the undissected volcanic body of the upper part of the catchment. Our study demonstrates that deep and long groundwater movements via a deep bedrock layer including thick deposits of volcanic materials at the two catchments on Daisen volcano strongly determined streamflow discharge instead of the mixing of small-scale hydrological conditions. This article is protected by copyright. All rights reserved.

Beschreibung: We propose a novel technique for improving a long-term multi-step-ahead streamflow forecasts. A model based on wavelet decomposition and a multivariate Bayesian machine learning approach is developed for forecasting the streamflow three, six, nine and twelve months ahead simultaneously. The inputs of the model utilize only the past monthly streamflow records. They are decomposed into components formulated in terms of wavelet multiresolution analysis. It is shown that the model accuracy can be increased by using the wavelet boundary rule introduced in this study. A simulation study is performed to evaluate the effects of different wavelet boundary rules using synthetic and real streamflow data from the Yellowstone River in the Uinta Basin in Utah. The model based on the combination of wavelet and Bayesian machine learning regression techniques is compared to the wavelet and artificial neural networks based model. The robustness of the models is evaluated. This article is protected by copyright. All rights reserved.

Beschreibung: The combined use of water erosion models and geographic information systems (GIS) has facilitated soil loss estimation at the watershed scale. Tools such as the Geo-spatial interface for the Water Erosion Prediction Project (GeoWEPP) model provide a convenient spatially-distributed soil loss estimate but require discretization to identify hillslopes and channels. In GeoWEPP, the TOpographic PArameteriZation (TOPAZ) model is used as an automated procedure to extract a watershed boundary, hillslopes and channels from a digital elevation model (DEM). Previous studies in small watersheds have shown that the size of the hillslopes and the channel distribution affect the model estimates, but in large watersheds the effects on the soil loss estimates have yet to be tested. Therefore, the objective of this study was to evaluate the effect of discretization on the hillslope sediment yield estimates using GeoWEPP in two large watersheds (〉10 km 2 ). The watersheds were selected and discretized varying the TOPAZ parameters (CSA – Critical Source Area, and MSCL – Minimum Source Channel Length) in a 30-m resolution DEM. The drainage networks built with TOPAZ were compared among each other using the drainage density index. The results showed that the discretization affected hillslope sediment yield estimates and their spatial distribution more than total runoff. The drainage density index and the hillslope sediment yield were proportional but inversely related, thus soil loss estimates were highly affected by the spatial discretization. As a result of this analysis, a method to choose the CSA and MSCL values that generates the greatest fraction of hillslopes having profile lengths less than 200 m was developed. This slope length condition is particularly crucial when using the WEPP and GeoWEPP models, in order for them to produce realistic estimates of sheet and rill erosion. Finally, and as a result of this analysis, a more reliable method was developed for selecting the TOPAZ channel network parameters (CSA and MSCL).

Beschreibung: Linkages between the controls on surface storage and catchment streamflow response were examined in a wetland dominated basin in the Canadian Prairie Pothole region. Snowmelt, surface storage, water table elevation, atmospheric fluxes, and streamflow were monitored during spring snowmelt and summer in a 1 km 2 sub-catchment containing a semi-permanent pond complex connected via an intermittent stream. Snow accumulation in the basin in spring of the 2013 study year was the largest in the 24-year record. Rainfall totals in 2013 were close to the long term average, though June was an anomalously wet month. The water budget of the pond complex indicates that there was a significant subsurface contribution to surface storage. Activation of an effective transmission zone occurred between uplands and the stream network where the water table was located near the ground surface, which allowed significant lateral movement of subsurface water into the stream network. This was also important for maintaining and re-establishing surface connectivity and streamflow during rainfall events. The observed period of surface-water connectivity was one of the longest on record in the catchment due to unusually wet conditions; nevertheless, the results of this study have implications for how contributing area and runoff should be considered in monitoring and modelling studies in the region, as inclusion of more frequent and varied runoff processes will be essential to understanding changing streamflow regimes. This article is protected by copyright. All rights reserved.

Beschreibung: Stream-subsurface exchange plays a significant role in the fate and transport of contaminants in streams. It has been modeled explicitly by considering fundamental processes such as hydraulic exchange, colloid filtration, and contaminant interactions with streambed sediments and colloids. The models have been successfully applied to simulate the transport of inorganic metals and nutrients. In this study, laboratory experiments were conducted in a recirculating flume to investigate the exchange of a hydrophobic organic contaminant (HOC), p,p′ -DDE, between a stream and a quartz sand bed. A previously developed process-based multiphase exchange model was modified by accounting for the p,p′ -DDE kinetic adsorption to and desorption from the bed sediments/colloids and was applied to interpret the experimental results. Model input parameters were obtained by conducting independent small-scale batch experiments. Results indicate that the immobilization of p,p′ -DDE in the quartz sand bed can occur under representative natural stream conditions. The observed p,p′ -DDE exchange was successfully simulated by the process-based model. The model sensitivity analysis results show that the exchange of p,p′ -DDE can be sensitive to either the sediment sorption/desorption parameters or colloidal parameters depending on the experimental conditions tested. For the experimental conditions employed here, the effect of colloids on contaminant transport is expected to be minimal and the stream-subsurface exchange of p,p′ -DDE is dominated by the interaction of p,p′ -DDE with bed sediment. The work presented here contributes to a better mechanistic understanding of the complex transport process that HOCs undergo in natural streams, and to the development of reliable, predictive models for the assessment of impacted streams. This article is protected by copyright. All rights reserved.

Beschreibung: The physical and hydrological conditions in extracted peatlands often act as barriers to the regeneration of the keystone peat-forming genus Sphagnum . Although previous work has suggested that Sphagnum mosses regenerating on cutover peat surfaces quickly become vulnerable to water stress as the thickness of the regenerated layer increases, uncertainties regarding the storage and transmission properties of this layer and how these might evolve over time have made this assertion difficult to evaluate. This study investigates the hydrophysical properties and hydrological behaviour of regenerating Sphagnum layers ranging from 3-43 years in age using both field and laboratory methods. The 〉40 year old regenerated layers had significantly (p 〈 0.001) higher bulk density and retention capacity in the 5 cm thick basal layer directly overlying the cutover peat than the newer (〈10 year old) regenerated layers. Capillarity was a much stronger control on surficial water content (θ) than precipitation, which was poorly retained in the Sphagnum canopy, suggesting that regulation of water table position is an effective method of controlling θ as a means of optimizing productivity. In general, the θ sustained at a given water table position decreased as regenerated layer thickness increased. Analysis of water table position relative to the former cutover peat surface in different areas of the site suggests that the soil water dynamics of the 〉40 year old regenerated layers may be becoming increasingly similar to those of a natural bog peatland. This article is protected by copyright. All rights reserved.

Beschreibung: Nutrient concentrations in stream water, rainfall, throughfall, stem flow, surface flow and ground water were compared before, during, and after strip thinning (intensive 50%) in plantation forested watersheds in Tochigi, Japan. Influences were evaluated comparing four thinning-applied and two reference basins for one year before, six months during and one year after the thinning. Results show that this strip thinning significantly increased DTP, TP and DOC (DTP: 0.01 mg l -1 , TP: 0.04 mg l -1 , DOC: 0.53 mg l -1 ) during the thinning period and DTN and TN (DTN: 0.34 mg l -1 , TN: 0.46 mg l -1 ) after the thinning in stream waters relative to the unthinned basins. The increased phosphorus during thinning indicated ground disturbances by the strip thinning, with a concomitant increase in DOC. Changes in biotic and abiotic conditions resulted in increased nitrogen after the thinning, particularly in the dissolved pool. Changes in hydrological processes due to thinning, e.g. a change in flow distributions (less high nutrient stem flow and more low nutrient throughfall) and an increase in water discharge in stream water, possibly weakened the direct influences of thinning on nutrient concentrations. This article is protected by copyright. All rights reserved.

Beschreibung: Soil water repellency can impact soil hydrology, overland flow generation and associated soil losses. However, current hydrological models do not take it into account, which creates a challenge in repellency-prone regions. This work focused on the adaptation for soil water repellency of a daily water balance model. Repellency is estimated from soil moisture content using site-specific empirical relations, and used to limit maximum soil moisture. This model was developed and tested using c. 2 years of data from one long-unburned and two recently burned eucalypt plantations in northern Portugal, all of which showed strong seasonal soil water repellency cycles. Results indicated important improvements for the burned plantations, with the Nash-Sutcliffe efficiency increasing from -0.55 and -0.49 to 0.55 and 0.65. For the unburned site, model performance was already good without the modification and efficiency only improved slightly from 0.71 to 0.74, mostly due to the better simulation of delayed soil wetting after dry periods. Results suggested that even a simple approach to simulate soil water repellency can markedly improve the performance of hydrological models in eucalypt forests, especially after fire. This article is protected by copyright. All rights reserved.

Beschreibung: This study describes the use of linearly modulated optically stimulated luminescence (LM-OSL) to distinguish surface-soil derived sediments from those derived from channel bank erosion. LM-OSL signals from quartz extracted from fifteen surface-soil and five channel bank samples were analysed and compared to signals from samples collected from two downstream river sites. Discriminant analysis showed that the detrapping probabilities of fast, first slow and second slow components of the LM-OSL signal can be used to differentiate between the samples collected from the channel bank and surface-soil sources. We show that for each of these source end members these components are all normally distributed. These distributions are then used to estimate the relative contribution of surface-soil derived and channel bank derived sediment to the river bed sediments. The results indicate that channel bank derived sediments dominate the sediment sources at both sites, with 90.1 ± 3% and 91.9 ± 1.9% contributions. These results are in agreement with a previous study which used measurements of 137 Cs and 210 Pb ex fallout radionuclides to estimate the relative contribution from these two sources. This result shows that LM-OSL may be a useful method, at least in the studied catchment, to estimate the relative contribution of surface soil and channel erosion to river sediments. However, further research in different settings is required to test the difference of OSL signals in distinguishing these sediment sources. And if generally acceptable, this technique may provide an alternative to the use of fallout radionuclides for source tracing. This article is protected by copyright. All rights reserved.

Beschreibung: The vadose zone plays a crucial role in the water cycle for storing water, providing water to vegetation and transporting solutes or degrading contaminants. Earth scientists have long acknowledged the importance of the vadose zone and numerous methods have been developed to better understand and predict hydrological processes within this “critical zone”. For several decades, stable isotopes ( 18 O and 2 H) of pore water are used as environmental tracers to gain insights into vadose zone water movement and other processes. To determine the pore water stable isotopic composition various sampling procedures have been developed. We present the procedure and the accompanied advantages and drawbacks of each method. We further discuss possible opportunities and limitations regarding the scale of interest and the pore space that is sampled. The methodological review reveals that the choice of the sampling method is crucial for the interpretation of pore water stable isotopes in the vadose zone, but a thorough comparison between the different methods is yet missing. Spiking experiments, where water of known isotopic composition is added to oven-dried soil, have shown to be questionable, since the extracted water is usually depleted compared to the standard water. A comparative study analyzing soil samples with the recently developed direct water-vapor equilibration method and the widely used cryogenic extraction shows deviations, which can only be partly explained, but discloses the need for a more thorough experimental comparative study. Especially promising are developments of continuous isotope measurements based on laser-based spectrometry that will open up new opportunities of analyzing pore water isotopes with higher temporal and spatial resolutions, revealing new insights into hydrological processes across various temporal and spatial scales. This article is protected by copyright. All rights reserved.

Beschreibung: In the Vietnamese Mekong Delta (VMD), water levels at some stations have increased. However, the factors that cause this rise in the VMD have not been identified. We considered four factors that may have contributed to the water level rise: (1) increased runoff from upstream, (2) sea-level rise, (3) land subsidence, and (4) decrease in flood mitigation function due to construction of high dykes. We analyzed daily maximum and minimum water levels, and mean daily water levels from 24 monitoring stations from 1987 to 2006. Using daily and annual water level differences, we classified the delta into two groups; one is dominated by flows from upstream, while the other is tide-dominated. We then tested the trends of annual maximum and minimum water levels using the Mann-Kendall test, and identified the slope of the trend using the method of Sen. The areas of dyke construction were estimated using the Enhanced Vegetation Index (EVI) and Land Surface Water Index (LSWI) from the Moderate Resolution Imaging Spectroradiometer (MODIS) data. Results show (1) river inflow has little impact on rising water levels in the VMD, (2) the influence of high dykes on water level rise could not be quantified in this study, (3) both maximum and minimum water levels significantly increased in the tide-dominated area. Trend of annual minimum water level can be considered as the sum sea-level rise and land subsidence. Therefore, we attribute 6.05 mm year −1 (80%) to land subsidence and 1.42 mm year −1 (20%) to sea level rise, indicating inundations have been severe in the VMD, caused primarily by land subsidence. This article is protected by copyright. All rights reserved.

Beschreibung: Groundwater is the principal water resource in semi-arid and arid environments. Therefore, quantitative estimates of its replenishment rate are important for managing groundwater systems. In dry regions, karst outcrops often show enhanced recharge rates compared to other surface and sub-surface conditions. Areas with exposed karst features like sinkholes or open shafts allow point recharge, even from single rainfall events. Using the example of the As Sulb plateau in Saudi Arabia, this study introduces a cost-effective and robust method for recharge monitoring and modelling in karst outcrops. The measurement of discharge of a representative small catchment (4.0·10 4  m 2 ) into a sinkhole, and hence the direct recharge into the aquifer, was carried out with a time lapse camera. During the monitoring period of two rainy seasons (autumn 2012 to spring 2014) four recharge events were recorded. Afterwards, recharge data as well as proxy data about the drying of the sediment cover are used to set up a conceptual water balance model. The model was run for 17 years (1971 to 1986 and 2012 to 2014). Simulation results show highly variable seasonal recharge-precipitation ratios between zero and 0.27. In addition to the amount of seasonal precipitation, this ratio is influenced by the interannual distribution of rainfall events. Overall, an average annual groundwater recharge for the doline (sinkhole) catchment on As Sulb plateau of 5.1 mm a −1 has estimated for the simulation period. This article is protected by copyright. All rights reserved.

Beschreibung: This paper presents an analytical model for describing the tidal effects in a two-dimensional leaky confined aquifer system in an estuarine delta where ocean and river meet. This system has an unconfined aquifer on top, a confined aquifer on the bottom with an aquitard in between the two. The unconfined and confined aquifers interact with each other through leakage. It was assumed that the aquitard storage was negligible, and that the leakage was linearly proportional to the head difference between the unconfined and confined aquifers. This model's solution was based on the separation of variables method. Two existing solutions that deal with the head fluctuation in one-dimensional or two-dimensional leaky confined aquifers are shown as special cases in the present solution. Based on this new solution, the dynamic effect of the water table's fluctuations can be clearly explored, as well as the influence of leakage on the behavior of fluctuations in groundwater levels in the leaky aquifer system. This article is protected by copyright. All rights reserved.

Beschreibung: A portable Wavelength Scanned-Cavity Ring-Down Spectrometer (Picarro L2120) fitted with a diffusion sampler (DS-CRDS) was used for the first time to continuously measure δ 18 O and δ 2 H of stream water. The experiment took place during a storm event in a wet tropical agricultural catchment in north-eastern Australia. At a temporal resolution of one minute, the DS-CRDS measured 2160 δ 18 O and δ 2 H values continuously over a period of 36 hours with a precision of ± 0.08 and 0.5 ‰ for δ 18 O and δ 2 H, respectively. Four main advantages in using high temporal resolution stream δ 18 O and δ 2 H data during a storm event are highlighted from this study. Firstly, they enabled us to separate components of the hydrograph, which was not possible using high temporal resolution electrical conductivity data that represented changes in solute transfers during the storm event rather than physical hydrological processes. The results from the hydrograph separation confirm fast groundwater contribution to the stream, with the first 5 hours of increases in stream discharge comprising over 70% pre-event water. Secondly, the high temporal resolution stream δ 18 O and δ 2 H data allowed us to detect a short-lived reversal in stream isotopic values (δ 18 O increase by 0.4 ‰ over 9 minutes), which was observed immediately after the heavy rainfall period. Thirdly, δ 18 O values were used to calculate a time lag of 20 minutes between the physical and chemical stream responses during the storm event. Finally, the hydrograph separation highlights the role of event waters in the runoff transfers of herbicides and nutrients from this heavily cultivated catchment to the Great Barrier Reef. This article is protected by copyright. All rights reserved.

Beschreibung: Quantifying the linkages between vegetation disturbance by fire and the changes in hydrologic processes leading to post-fire erosional response remains a challenge. We measured the influence of fire severity, defined as vegetation disturbance (using a satellite derived vegetation disturbance index, VDI), landscape features that organize hydrologic flow pathways (relief and elongation ratios), and pre-fire vegetation type on the probability of the occurrence of post-fire gully rejuvenation (GR). We combined field surveys across 270 burned low-order catchments (112 occurrences of GR) and geospatial analysis to generate a probabilistic model through logistic regression. VDI alone discriminated well between catchments where GR did and did not occur (area under curve = 0.78, model accuracy = 0.72). The strong effect of vegetation disturbance on GR suggests that vegetation exerts a primary influence on the occurrence of infiltration excess runoff and post-fire erosion and that major gully erosion will not occur until fire consumes above ground biomass. Other topographic and local factors also influenced GR response, including catchment elongation, percent pre-fire shrub, mid-slope riparian vegetation, armored headwaters, firehose effects, and concentration of severe burn in source areas. These factors highlight the need to consider vegetation effects in concert with local topography and site conditions to understand the propensity for flow accumulation leading to GR. We present a process-based conceptual hydrologic model where vegetation loss from fire decreases rainfall attenuation and surface roughness, leading to accelerated flow accumulation and erosion; these effects are also influenced by interactions between fire severity and landscape structure. The VDI metric provides a continuous measure of vegetation disturbance and, when placed in a hydrologic context, may improve quantitative analysis of burned-area susceptibility to erosive rainfall, hazard prediction, ecological effects of fire, landform evolution, and sensitivity to climate change. This article is protected by copyright. All rights reserved.

Beschreibung: There is considerable interest in naturalizing flow regime on managed rivers to slow the spread of saltcedar ( Tamarix ramosissima ) invasion in southwestern United States or to preserve riparian forests dominated by saltcedar and other species in northwestern China. However, little is known about the responses of established saltcedar in water sources to frequent intra-annual fluctuation of water table resulting from this new, more dynamic flow regime. This study investigates how saltcedar at a riparian site in the middle reaches of the Heihe River, northwest China responds in water sources use to intra-annual water table fluctuations. Stable oxygen isotope was employed to determine accurate depth at which saltcedar obtains its water supply, and soil moisture monitoring was used to determine sources of plant-available soil water. We found that the primary zone of water uptake by saltcedar was stable at 25-60 cm depth, but the water sources used by saltcedar switched between groundwater and soil moisture with the water table fluctuations. Saltcedar derived its water from groundwater when water table was at depth less than 60 cm, but switched to soil moisture at 25-60 cm depth when water table declined. It is supposed that the well-developed clay layer at 60–80 cm depth constrained lateral roots of saltcedar to the soil layers above 60 cm, while the fine-textured soils at this site, which was periodically re-saturated by rising groundwater before the stored soil moisture had become depleted, provided an important water reservoir for saltcedar when groundwater dropped below the primary zone of fine roots. The root distribution of saltcedar may also be related to local groundwater history. The quick decline in water table in the early 1980s when the riparian saltcedar had established may strand its roots in the shallow unsaturated zone. We suggested that raising the water table periodically instead of maintaining it invariably above the rooting depth could sustain desired facultative phreatophytes while maximizing water deliveries. This article is protected by copyright. All rights reserved.

Beschreibung: Secondary circulation is the component of three-dimensional (3-D) flow in river channels perpendicular to the primary flow direction. Secondary circulation calculated from ADCP transects is sensitive to the calculation method, and is affected by the transect angle relative to the mean flow direction and variations in the flow direction along a transect. To quantify bounds on transect alignment relative to river flow for field data collection and examine tidal time-scale variability in secondary circulation, the 3-D hydrodynamic model UnTRIM was applied to simulate the hydrodynamics in the lower reach of the Sacramento River (CA, USA). Secondary circulation was calculated using the Rozovskii and the zero net discharge methods on repeated transects extracted from the model results in regions of both relatively uniform and complex flows. When the depth-averaged flow direction along a transect varied by more than about 5 degrees, occurring when the transect was as little as 10 to 20 degrees out of normal to the mean flow direction, the Rozovskii method produced more realistic secondary circulation than the zero net discharge method. Analysis indicated that ADCP transects should be within 20 degrees of perpendicular to the mean flow direction when calculating secondary circulation. Secondary circulation strength around two tidally influenced bends generally increased with increasing flow and broke down near slack water. However, the strength of the secondary circulation was not only a function of the flow magnitude, but also depended on the direction of the water flow, and the transect location relative to the river curvature, which varied with the tidal flow direction. This article is protected by copyright. All rights reserved.

Beschreibung: Flooding associated with tropical storms can cause extreme perturbations in riverine and coastal ecosystems. Measuring isotope variability of tropical storm events can help investigate the impacts of flooding. We measured the water isotope composition (δD and δ 18 O) of rain and associated floodwater collected during two storms and subsequent major and minor flooding events in the subtropical coast of eastern Australia. Compared to baseline regional rainfall isotope values of -15.0 ± 1.9‰ for δD and -3.3 ± 0.2‰ for δ 18 O, floodwater had lower values with -33.8 ± 2.5‰ δD and -5.1 ± 0.4‰ δ 18 O for the major flood and -29.4 ± 1.0‰ δD and -4.6 ± 0.1‰ δ 18 O for the minor flood. The low isotope composition of the floodwater was associated with the transport of large quantities of suspended sediments, with sediment loads 30 to 70 times larger than during base flow conditions. Floods carried up to 35% of the annual phosphorus and up to 208% of the currently calculated average annual nitrogen load of the Brisbane River. The dramatic changes caused by a rapid increase in discharge from 2 to 2,015 m 3 s -1 over two days in the major flood would have major consequences in riverine and coastal ecosystems of the region. These changes could potentially be traced using the isotope composition of the floodwaters. This article is protected by copyright. All rights reserved.

Beschreibung: Large river floods are a key water source for many lakes in fluvial periglacial settings. Where permeable sediments occur, the distribution of permafrost may play an important role in the routing of floodwaters across a floodplain. This relationship is explored for lakes in the discontinuous permafrost of Yukon Flats, interior Alaska, using an analysis that integrates satellite-derived gradients in water surface elevation, knowledge of hydrogeology, and hydrologic modeling. We observed gradients in water surface elevation between neighboring lakes ranging from 0.001 to 0.004. These high gradients, despite a ubiquitous layer of continuous shallow gravel across the flats, are consistent with limited groundwater flow across lake basins resulting from the presence of permafrost. Permafrost impedes the propagation of floodwaters in the shallow subsurface and constrains transmission to “fill-and-spill” over topographic depressions (surface sills), as we observed for the Twelvemile-Buddy Lake pair following a May 2013 ice-jam flood on the Yukon River. Model results indicate that permafrost table deepening of 1–11 m in gravel, depending on watershed geometry and subsurface properties, could shift important routing of floodwater to lakes from overland flow (fill-and-spill) to shallow groundwater flow (“fill-and-seep”). Such a shift is possible in the next several hundred years of ground surface warming, and may bring about more synchronous water level changes between neighboring lakes following large flood events. This relationship offers a potentially useful tool, well-suited to remote sensing, for identifying long-term changes in shallow groundwater flow resulting from thawing of permafrost. This article is protected by copyright. All rights reserved.

Beschreibung: Imperviousness, considered as a critical indicator of the hydrologic impacts of urbanization, has gained increasing attention both in the research field and in practice. However, the effectiveness of imperviousness on rainfall-runoff dynamics has not been fully determined in a fine spatiotemporal scale. In this study, 69 drainage subareas 〈 1 ha of a typical residential catchment in Beijing were selected to evaluate the hydrologic impacts of imperviousness, under a typical storm event with a three-year return period. Two metrics, total impervious area (TIA) and effective impervious area (EIA), were identified to represent the impervious characteristics of the selected subareas. Three runoff variables, total runoff depth (TR), peak runoff depth (PR), and lag time (LT), were simulated by using a validated hydrologic model. Regression analyses were developed to explore the quantitative associations between imperviousness and runoff variables. Then, three scenarios were established to test the applicability of the results in considering the different infiltration conditions. Our results showed that runoff variables are significantly related to imperviousness. However, the hydrologic performances of TIA and EIA were scale-dependent. Specifically, with finer spatial scale and the condition heavy rainfall, TIA rather than EIA was found to contribute more to TR and PR. EIA tended to have a greater impact on LT and showed a negative relationship. Moreover, the relative significance of TIA and EIA was maintained under the different infiltration conditions. These findings may provide potential implications for landscape and drainage design in urban areas, which help to mitigate the runoff risk. This article is protected by copyright. All rights reserved.

Beschreibung: The presence or absence of snow and its seasonal variability have profound effects on the climate, energy balance, and ecology in high-latitude and high-altitude regions of the world. While the timing, amount and duration of snow and its water content is of fundamental importance for several natural processes, it is also a convincing indicator of climate change in seasonally snow covered regions. The reduction in the amount and duration of the snow pack that already may have begun to happen across large geographic areas risks affecting several of the important ecosystem services that snow provides. Despite its fundamental role, snow and especially its water content is one of the least monitored climate variables in many seasonally snow-covered regions. Therefore we add complimentary long-term snow depth data (1980–2015) and snow water equivalent (SWE) data (1986–2015) to an already well established field research infrastructure – the Krycklan Catchment Study in northern Sweden. By combining digitized snow data to the already available long-term daily precipitation, runoff, and water quality information the ambition is to provide a more holistic database. While the primary purpose of this data release is to make the data readily available to any interested user, it also shows that the length of seasonal winter snow cover has shortened, especially because of its later arrival in the autumn, at the study site over the 35 year monitoring period. This article is protected by copyright. All rights reserved.

Beschreibung: Tree-ring-based reconstructions of paleo-hydrology have proved useful for better understanding the irregularities and extent of past climate changes, and therefore, for more effective water resources management. Despite considerable advances in the field, there still exist challenges that introduce significant uncertainties into paleo-reconstructions. This study outlines these challenges and address them by developing two themes: (1) the effect of temporal scaling on the strength of the relationship between the hydrologic variables, streamflow in this study, and tree growth rates, and (2) the reconstruction uncertainty of streamflow due to the dissimilarity or inconsistency in the pool of tree-ring chronologies (predictors in reconstruction) in a basin. Based on the insight gained, a methodology is developed to move beyond only relying on the annual hydrology-growth correlations, and to utilize additional information embedded in the annual time series at longer time scales (e.g, multi-year to decadal time scales). This methodology also generates an ensemble of streamflow reconstructions to formally account for uncertainty in the pool of chronology sites. The major headwater tributaries of the Saskatchewan River basin, the main source of surface water in the Canadian Prairie Provinces, are used as the case study. It is shown that the developed methodology explains the variance of streamflows to a larger extent than the conventional approach, and better preserves the persistence and variability of streamflows across time scales (Hurst-type behaviour). The resulting ensemble of paleo-hydrologic time series is able to more credibly pinpoint the timing and extent of past dry and wet periods, and provides a dynamic range of uncertainty in reconstruction. This range varies with time over the course of the reconstruction period, indicating that the utility of tree-ring chronologies for paleo-reconstruction differs for different time periods over the past several centuries in the history of the region. The proposed ensemble approach provides a credible range of multiple-century-long water availability scenarios that can be used for vulnerability assessment of the existing water infrastructure and improving water resources management. This article is protected by copyright. All rights reserved.

Beschreibung: Discharge in mountain streams may be a mixture of snowmelt, water from surface runoff, and deep return flow through valley bottom alluvia. We used δ 18 O and δ 2 H, solute concentrations, and 222 Rn to determine water sources of a headwater stream located at the McDonald Creek watershed, Glacier National Park, USA during summer recession flow period. We analyzed minimal water isotope ranges of -17.6 to -16.5‰ and -133 to -121‰ for δ 18 O and δ 2 H, respectively, potentially due to dominance of snow-derived water in the stream. Likewise, solute concentrations measured in the stream through the watershed showed minimal variation with little indication of sub-surface water input into the stream. However, we observed 222 Rn activities in the stream that ranged from 39 to 2,646 Bq/m 3 with the highest value measured in middle of the watershed associated with channel constriction corresponding to changes in local orientation of underlying rocks. Downstream from this point, 222 Rn activity decreased from 581 to 117 Bq/m 3 in a series of punctuated steps associated with small rapids and waterfalls that we hypothesized to cause radon degassing with a maximum predicted loss of 427 Bq/m 3 along a 400 m distance. Based on mass balance calculations using 222 Rn activity values, streamflow, and channel characteristics, we estimated that groundwater contributed between 0.3 and 29% of total flow. Overall, we estimated a 5.9% of groundwater contribution integrated for stream reach measured at McDonald Creek during recession flow period. Finally, a lower mean hyporheic flux of 14 m 3 /day was estimated compared to the groundwater flux of 70,710 m 3 /day. These assessments highlight the potential for radon as a conservative tracer that can be used to estimate sub-surface water contribution in mountain streams within a complex geologic setting. This article is protected by copyright. All rights reserved.

Beschreibung: Vegetative filter strips (VFS) can effectively trap sediment in overland flow, but little information is available on its performance in controlling high-concentration sediment and the runoff hydraulics in VFS. Flume experiments were conducted to investigate the sediment deposition, hydraulics of overland flow and their relationships in simulating VFS under a great range of sediment concentrations with four levels of vegetation cover (bare slope and 4, 11 and 17%) and two flow rates (15 and 30 L min −1 ). Sediment concentrations varied from 30 to 400 kg m −3 and slope gradient was 9°. Both the deposited sediment load and deposition efficiency in VFS increased as the vegetation cover increased. Sediment concentration had a positive effect on the deposited load but no effect on deposition efficiency. A lower flow rate corresponded to greater deposition efficiency but had little effect on deposited load. Flow velocities decreased as vegetation cover increased. Sediment concentration had a negative effect on the mean velocity but no effect on surface velocity. Hydraulic resistance increased as the vegetation cover and sediment concentration increased. Sediment deposition efficiency had a much more pronounced relationship with overland flow hydraulics compared with deposited load, especially with the mean flow velocity, and there was a power relationship between them. Flow regime also affected the sediment deposition efficiency, and the efficiency was much higher under subcritical than supercritical flow. The results will be useful for the design of VFS and the control of sediment flowing into rivers in areas with serious soil erosion.

Beschreibung: Recent changes in land use and new industrial developments in river basins around the world have raised serious concerns regarding future water quality. Assessment of regional nutrient export from river basins are needed in order to identify main nutrient sources and hydrological variables involved. The Red Deer River catchment in Alberta, Canada has been chosen as a test case to assess total phosphorus export regionally. Agricultural and livestock activities in the catchment have increased manure production and the supply of fertilizer to crops. Oil and gas exploitation has also increased the risk of surface water and groundwater contamination. The rapid population growth has not only lead to increases in water consumption and wastewater discharges, but also to further construction of transportation infrastructure and the expansion of new urban developments. This has imposed hydraulic controls on waterways, affecting the catchment hydrology and changing the dynamics of sediment and nutrient transport. River ecosystems are not exempt from the negative effects of water quality deterioration. Downstream from the city of Red Deer, the physiology and reproduction habits of native fish species have changed due to high riverine productivity. Although improvements have been made to surface water quality standards by Alberta Environment, further research is needed in order to identify major nutrient sources and quantify nutrient export. The SPAtially Referenced Regression On Watershed (SPARROW) model has therefore been used in this study of the Red Deer River catchment to assess regional water quality, in order to describe the spatial and temporal patterns of the processes that affect water quality. The model is suitable for interpreting monitoring data sets that suffer from network sparseness, bias and basin heterogeneity. Ultimately, the model could provide improved information to environmental agencies to guide future water quality management practices and policies.

Beschreibung: The White method is a simple but the most frequently applied approach to estimate groundwater evapotranspiration ( ET g ) from groundwater level diurnal signals. Due to a lack of direct measurements of ET g , it is difficult to evaluate the performance of the White method, particularly in field environments with variable groundwater fluctuations. A two-year field observation in a hyper-arid riparian tamarisk ( Tamarix spp.) stand with deep groundwater depth in the lower Tarim River basin of China was conducted to measure the surface evapotranspiration ( ET s ) and groundwater table. The performance of the White method and the influences of the variable groundwater fluctuations on the determinations of the specific yield ( S y ) and recharge rate of groundwater ( r ) in the White method were investigated. The results showed that the readily available S y determined by Loheide's method was feasible, but must be finely determined based on the soil textures in the layers in which the groundwater level fluctuated. A constant S y value for a defined porous medium could be assumed regardless of the discharge or recharge processes of groundwater. The time span of 0 A.M. to 6 A.M. for r estimation for the White method worked best than other time spans. A two-day moving average of r values further enhance ET g estimation . Slight effects of environmental or anthropogenic disturbances on the diurnal fluctuations of groundwater level did not influence the ET g estimations by the White method. Our results provide valuable references to the application of the White method for estimating daily ET g in desert riparian forests with deep groundwater depth. This article is protected by copyright. All rights reserved.

Beschreibung: This study analyzed age-related water use dynamics across three temperate conifer forest plantations (aged 11-, 39-, and 74-years old, as of 2013, henceforth referred to as TP02, TP74, and TP39, where the last two digits represent the year of planting) in southern Ontario, Canada from 2008 to 2013. Eddy covariance-measured mean evapotranspiration over the growing season (April-October) was 438 ± 19, 392 ± 19 and 323 ± 25 mm at TP39, TP74 and TP02 respectively. Daytime bulk surface conductance was highest and most variable at the TP39 site (8.5 ± 4.0 mm s −1 ), followed by the TP74 (7.0 ± 2.8 mm s −1 ) and TP02 (5.4 ± 2.5 mm s −1 ) sites. Evapotranspiration at all the forests was sensitive to air temperature and also tended to decrease with increasing atmospheric dryness. The youngest forest's evapotranspiration was most conservative, which led to an increase in water use efficiency throughout the study period, in particular during drought events. The oldest forest was the least restrictive in its water use during drought, which led to lower water use efficiency during such events as compared to the younger forests. The oldest forest was thinned in early 2012, where about 1/3 of trees were commercially harvested. No significant change in evapotranspiration or water use efficiency was observed at this site following thinning, however daytime bulk surface conductance declined. Our results suggested that changes in stand structure with forest ageing, such as reduction in stem density and increase in sapwood area, were responsible for differences in soil water demand during drought and non-drought periods, leading to differences in forest water use. Hence, forest age, due to its structural implications, is an important control on the stand-level water use efficiency and forests’ response to drought events. Our study suggested that younger forests may be best suited to maximize growth and carbon uptake efficiency under rising air temperatures and increasing precipitation variability as predicted by climate models for eastern North America. This article is protected by copyright. All rights reserved.

Beschreibung: Physically-based distributed and semi-distributed hydrological models have become some of the primary tools for water resources studies and management over the past decades owing to increased computational capabilities and advances in data measurements. Representation of the existing heterogeneity in nature still remains one of the main challenges in these models, and is accomplished primarily via watershed discretization, subdividing watersheds into hydrologically similar land parcels. Discretization decisions in distributed modeling studies are often ad hoc and determined with little or no quantitative analysis to support these decisions. In this work, we present a quantitative methodology for assessing alternative watershed discretization schemes in terms of their corresponding model performance in ungauged basins. The effect of the computational time spent for calibrating each scheme (calibration budget) is considered as part of the assessment. Here, these schemes differ in how they represent landscape attributes, and range from a simple lumped scheme to more complex ones by adding spatial land cover and then soil information. The methodology was demonstrated using the Modélisation Environmentale–Surface et Hydrologie (MESH) model as applied to the Nottawasaga river basin in Ontario, Canada. Results reveal that model performance in ungauged basins depends upon the location of the validation sub-basin (i.e. upstream or downstream) with respect to the calibration sub-basins. Also, using a more complex scheme did not necessarily lead to improved performance in validation, when constrained by calibration budget. Therefore, the calibration budget also should be considered as a factor in the assessment process. This methodology was also implemented using a shorter sub-period for calibration, which leads to substantial computational saving. Results of the sub-period test were promising and consistent particularly when sufficient budget is spent to calibrate the model. Other strategies utilized for reducing the computational burden of the proposed analyses are also discussed in this study. This article is protected by copyright. All rights reserved.

Beschreibung: Floods are the most frequent natural disaster, causing more loss of life and property than any other in the United States. Floods also strongly influence the structure and function of watersheds, stream channels, and aquatic ecosystems. The Pacific Northwest is particularly vulnerable to climatically-driven changes in flood frequency and magnitude, because snowpacks that strongly influence flood generation are near the freezing point and thus sensitive to small changes in temperature. To improve predictions of future flooding potential and inform strategies to adapt to these changes, we mapped the sensitivity of landscapes to changes in peak flows due to climate warming across Oregon and Washington. We first developed principal component based models for predicting peak flows across a range of recurrence intervals (2-, 10-, 25-, 50-, and 100-years) based on historical instantaneous peak flow data from 1000 gaged watersheds in Oregon and Washington. Key predictors of peak flows included drainage area and principal component scores for climate, land cover, soil, and topographic metrics. We then used these regression models to predict future peak flows by perturbing the climate variables based on future climate projections (2020s, 2040s, and 2080s) for the A1B emission scenario. For each recurrence interval, peak flow sensitivities were computed as the ratio of future to current peak flow magnitudes. Our analysis suggests that temperature induced changes in snowpack dynamics will result in large (〉30-40%) increases in peak flow magnitude in some areas, principally the Cascades, Olympics, and Blue Mountains and parts of the western edge of the Rocky Mountains. Flood generation processes in lower elevation areas are less likely to be affected, but some of these areas may be impacted by floodwaters from upstream. These results can assist land, water, and infrastructure managers in identifying watersheds and resources that are particularly vulnerable to increased peak flows and developing plans to increase their resilience. This article is protected by copyright. All rights reserved.

Beschreibung: Numerous studies investigated the influence of abiotic (meteorological conditions) and biotic factors (tree characteristics) on stemflow generation. Though these studies identified the variables that influence stemflow volumes in simply structured forests, the combination of tree characteristics that allows a robust prediction of stemflow volumes in species-rich forests is not well known. Many hydrological applications, however, require at least a rough estimate of stemflow volumes based on the characteristics of a forest stand. The need for robust predictions of stemflow motivated us to investigate the relationships between tree characteristics and stemflow volumes in a species-rich tropical forest located in central Panama. Based on a sampling setup consisting of 10 rainfall collectors, 300 throughfall samplers, and 60 stemflow collectors and cumulated data comprising 26 rain events, we derive three main findings. First, stemflow represents a minor hydrological component in the studied 1 ha forest patch (1.0 % of cumulated rainfall). Second, in the studied species-rich forest, single tree characteristics are only weakly related to stemflow volumes. The influence of multiple tree parameters (e.g. crown diameter, presence of large epiphytes, and inclination of branches) and the dependencies among these parameters require a multivariate approach to understand the generation of stemflow. Third, predicting stemflow in species-rich forests based on tree parameters is a difficult task. Although our best model can capture the variation in stemflow to some degree, a critical validation reveals that the model cannot provide robust predictions of stemflow. A reanalysis of data from previous studies in species-rich forests corroborates this finding. Based on these results and considering that for most hydrological applications stemflow is only one parameter among others to estimate, we advocate using the base model, i.e. the mean of the stemflow data, to quantify stemflow volumes for a given study area. Studies in species-rich forests that wish to obtain predictions of stemflow based on tree parameters probably need to conduct a much more extensive sampling than currently implemented by most studies. This article is protected by copyright. All rights reserved.

Beschreibung: This study analyzes the stable isotopic compositions of hydrogen and oxygen ( δ 2 H, δ 18 O) in montane meteoric waters including precipitation and stream water of central Taiwan to identify hydrological processes in montane catchments. Results of precipitation demonstrate that monsoon and altitude effects are two principal processes affecting δ and deuterium excess ( d E ) values of inland precipitation in central Taiwan. Furthermore, slope and intercept values of summer and winter local Meteoric Water Line (MWL) are modified by secondary evaporation effects such as moisture recycling and raindrop evaporation. Additionally, stream water's results indicate that differences in δ values among stream waters reflect isotopic altitude effect whereby lower values are more evident in stream water originating from high-elevation catchments than low-elevation catchments. Comparison of the isotopic results between precipitation and stream water indicates that summer precipitation containing recycled moisture is the most important water source for the studied stream waters and indicates that catchment effect and base-flow contribution are the two major hydrological processes affecting mountain stream hydrology. The hydrological processes identified by the isotopic study re-stress the important role of forests in mountain hydrology. This article is protected by copyright. All rights reserved.

Beschreibung: Groundwater ridging is the rapid rise of a shallow water table during a rainfall event, in an environment where, in the pre-event period, the capillary fringe extends to the ground surface. Groundwater ridging is widely cited to account for the observed significant appearance of pre-event water in a stream stormflow hydrograph. Various hypotheses have been advanced to explain the groundwater ridging mechanism; and most recently, from a field study site in South Africa, an energy hypothesis was proposed, which explains that groundwater ridging water table rise is as a result of rapid introduction and transmission of additional pressure head into the capillary fringe from an intense rainfall at the ground surface. However, there is need for further analysis and evidence from other field study sites to confirm and support this newly proposed energy hypothesis. The objectives of this paper are, therefore, to: review previous observations on groundwater ridging, from other study sites, in order to deduce evidence of the newly proposed energy hypothesis; present and evaluate a one-dimensional diffusion mathematical model that can simulate groundwater ridging water table rise, based on the newly proposed energy hypothesis; and evaluate the importance of a capillary fringe in streamflow generation. Analysis of previous observations from other study sites generally indicated that the rate of groundwater ridging water table rise is directly related to the rainfall intensity, hence, confirming and agreeing with the newly proposed energy hypothesis. Additionally, theoretical results by the mathematical model agreed fairly well with the field results observed under natural rainfall, confirming that the rapidly rainfall-induced energy is diffusively transmitted downwards through pore water, elevating the pressure head at every depth. The results in this study also support the concept of a three-end member stream stormflow hydrograph, and contribute to the explanation of how catchments can store water for long periods, but then release it rapidly during storm events. This article is protected by copyright. All rights reserved.

Beschreibung: Nine mid- to high-latitude headwater catchments —part of the North-Watch (Northern Watershed Ecosystem Response to Climate Change) program — were used to analyze threshold response to rainfall and snowmelt-driven events, and link the different responses to the catchment characteristics of the nine sites. The North-Watch data include daily time-series of various lengths of multiple variables such as air temperature, precipitation and discharge. Rainfall and meltwater inputs were differentiated using a degree-day snowmelt approach. Distinct hydrological events were identified, and precipitation-runoff response curves were visually assessed. Results showed that eight of nine catchments showed runoff initiation thresholds and effective precipitation input thresholds. For rainfall-triggered events, catchment hydroclimatic and physical characteristics (e.g., mean annual air temperature, median flow path distance to the stream, median sub-catchment area) were strong predictors of threshold strength. For snowmelt-driven events, however, thresholds and their governing factors controlling precipitation-runoff response were difficult to identify. The variability in catchments responses to snowmelt was not fully explained by runoff initiation thresholds and input magnitude thresholds. The quantification of input intensity thresholds (e.g., snow melting and permafrost thawing rates) is likely required for an adequate characterization of nonlinear spring runoff generation in such northern environments. This article is protected by copyright. All rights reserved.

Beschreibung: Rivers in the Mediterranean region often exhibit an intermittent character. An understanding and classification of the flow regimes of these rivers is needed, since flow patterns control both physicochemical and biological processes. This paper reports an attempt to classify flow regimes in Mediterranean rivers based on hydrological variables extracted from discharge time series. Long-term discharge records from 60 rivers within the Mediterranean region were analysed in order to classify the streams into different flow regime groups. Hydrological indices (HIs) were derived for each stream and principal component analysis (PCA) then applied to these indices to identify subsets of HIs describing the major sources of variations, whilst simultaneously minimising redundancy. PCA was performed for two groups of streams (perennial and temporary) and for all streams combined. The results show that whereas perennial streams are mainly described by high flow indices, temporary streams are described by duration, variability and predictability indices. Agglomerative cluster analysis based on hydrological indices identified six groups of rivers classified according to differences in intermittency and variability. A methodology allowing such a classification for ungauged catchments was also tested. Broad-scale catchment characteristics based on digital elevation, climate, soil and land-use data were derived for each long-term station where these data were available. By using stepwise multiple regression analysis, statistically significant relationships were fitted linking the three selected hydrological variables (mean annual number of zero flow days, predictability and flashiness) to the catchment characteristics. The method provides a means of simplifying the complexity of river systems and is thus useful for river basin management. This article is protected by copyright. All rights reserved.

Beschreibung: This study uses stable isotopes and major ions to examine the seasonal evolution of penitentes on the surface of Tapado Glacier, in the Norte Chico region of the Chilean Andes. A snow pit was sampled in November 2011 and penitentes were sampled during the summer (December 2011 and January 2012). The major ion load of the winter snowpack is dominated by Ca 2+ (60%), SO 4 2− (16%) and NO 3 − (13%) and there is little influence from marine air masses at the site, with most SO 4 2− , Mg 2+ , Ca 2+ , and Na + , derived from non-sea salt sources. During the early ablation season we observe increases in stable isotope ratios and major ion concentrations (particularly lithic ions Na + , Mg 2+ , and Ca 2+ ) in the upper reaches of penitentes, which is attributed to sublimation and the aeolian deposition of dust particles. In the late-summer, melt replaces sublimation as the dominant ablation process, and results in smoothing of the stable isotope profile and the elution of major ions within the penitente snow and ice matrix. This article is protected by copyright. All rights reserved.

Beschreibung: Mathematical modelling is a well-accepted framework to evaluate the effects of wetlands on stream flow and watershed hydrology in general. Although the integration of wetland modules into a distributed hydrological model represents a cost-effective way to make this assessment, the added value brought by landscape-specific modules to a model's ability to replicate basic hydrograph characteristics remains unclear. The objectives of this paper were to: (i) present the adaptation of PHYSITEL (a GIS) to parameterize isolated and riparian wetlands; (ii) describe the integration of specific isolated wetland and riparian wetland modules into HYDROTEL, a distributed hydrological model; and (iii) evaluate the performance of the updated modelling platform with respect to the capacity of replicating various hydrograph characteristics. To achieve this, two sets of simulations were performed (with and without wetland modules) and the added-value was assessed at three river segments of the Becancour River watershed, Quebec, Canada, using six general goodness-of-fit indicators (GOFIs) and fourteen water flow criteria (WFC). A sensitivity analysis of the wetland module parameters was performed to characterize their impact on stream flows of the modelled watershed. Results of this study indicate that: (i) integration of specific wetland modules can slightly increase the capacity of HYDROTEL to replicate basic hydrograph characteristics and (ii) the updated modelling platform allows for the explicit assessment of the impact of wetlands ( e.g ., typology, location) on watershed hydrology. This article is protected by copyright. All rights reserved.

Beschreibung: Hypothesis testing about catchment functioning with conceptual hydrological models is affected by uncertainties in the model representation of reality as well as in the observed data used to drive and evaluate the model. We formulated a learning framework to investigate the role of observational uncertainties in hypothesis testing using conceptual models and applied it to the relatively data-scarce tropical Sarapiqui catchment in Costa Rica. Observational uncertainties were accounted for throughout the framework that incorporated different choices of model structures to test process hypotheses, analyses of parametric uncertainties and effects of likelihood choice, a posterior performance analysis, and (iteratively) formulation of new hypotheses. Estimated uncertainties in precipitation and discharge were linked to likely non-linear near-surface runoff generation and the potentially important role of soils in mediating the hydrological response. Some model-structural inadequacies could be identified in the posterior analyses (supporting the need for an explicit soil-moisture routine to match streamflow dynamics), but the available information about the observational uncertainties prevented conclusions about other process representations. The importance of epistemic data errors, the difficulty in quantifying them and their effect on model simulations was illustrated by an inconsistent event with long-term effects. Finally we discuss the need for new data, new process hypotheses related to deep groundwater losses, and conclude that observational uncertainties need to be accounted for in hypothesis testing to reduce the risk of drawing incorrect conclusions. This article is protected by copyright. All rights reserved.

Beschreibung: This study documents the spatiotemporal variations in the frequency and magnitude of ice jams in the Mistassini River and applies that information to the identification of the hydro-climatic threshold conditions associated with major events. Ice jams cause severe upheavals in water flow, which result in flooding upstream of the ice jam front, and therefore represent a significant geohazard to riparian populations. To analyze the spatiotemporal variations in the magnitude-frequency of ice jams, the Mistassini River was first divided into six different sites representing different geomorphological contexts. A 50-year ice jam chronology was constructed from 85 damaged trees from all of the study sites. This chronology was then coupled with hydro-climatic variables to construct classification trees (CT), which helped identify the conditions and hydro-climatic thresholds favorable to the triggering of ice jams in a predictive model. The results indicate complex interactions between the characteristics of flow, ice cover, and river morphology that affect the frequency and magnitude of ice jam events on the Mistassini River. These factors affect the frequency and magnitude of ice jam events. The triggering of extreme events seems particularly influenced by exceptional ice conditions and sites with high sinuosity and islands. This article is protected by copyright. All rights reserved.

Beschreibung: Mountainous headwaters consist of different landscape units including forests, meadows and wetlands. In these headwaters it is unclear which landscape units contribute what percentage to baseflow. In this study, we analysed spatiotemporal differences in baseflow isotope and hydrochemistry to identify catchment-scale runoff contribution. Three baseflow snapshot sampling campaigns were performed in the Swiss pre-alpine headwater catchment the Zwäckentobel (4.25 km 2 ) and six of its adjacent subcatchments. The spatial and temporal variability of δ 2 H, Ca, DOC, AT, pH, SO 4 , Mg, and H 4 SiO 4 of streamflow, groundwater and spring water samples was analysed and related to catchment area and wetland percentage using bivariate and multivariate methods. Our study found that in the six subcatchments, with variable arrangements of landscape units, the inter- and intra catchment variability of isotopic and hydrochemical compositions was small and generally not significant. Stream samples were distinctly different from shallow groundwater. An upper spring zone located near the water divide above 1400 m and a larger wetland were identified by their distinct spatial isotopic and hydrochemical composition. The upstream wetland percentage was not correlated to the hydrochemical streamflow composition, suggesting that wetlands were less connected and act as passive features with a negligible contribution to baseflow runoff. The isotopic and hydrochemical composition of baseflow changed slightly from the upper spring zone towards the subcatchment outlets and corresponded to the signature of deep groundwater. Our results confirm the need and benefits of spatially distributed snapshot sampling to derive process understanding of heterogeneous headwaters during baseflow. This article is protected by copyright. All rights reserved.

Beschreibung: The processes by which climate change affects streamflow in alpine river basins is not entirely understood. This study evaluated the impacts of temperature and precipitation changes on runoff and streamflow using glacier-enhanced SWAT (Soil and Water Assessment Tool) model. The study used observed and detrended historical meteorological data for recent decades (1961–2005) to analyze individual and combined effects of temperature and precipitation changes on snow and glacier melts and discharges in the Sary-Djaz-Kumaric River Basin (SRB), Tianshan Mountains. The results showed 1.3% increase in annual snowmelt in the basin, due mainly to an increase in precipitation. Snowmelt in the basin varied seasonally, increasing from April through May due to increasing precipitation and decreasing from July through September due to rising temperature. Glacier melt increased by 5.4%, 5.0% of which was due to rising temperature and only 0.4% due to increasing precipitation. Annual streamflow increased by 4.4%, of which temperature and precipitation increases accounted for 2.5% and 1.9%, respectively. The impacts of temperature and precipitation changes on streamflow were especially significant after 1980 and even more so in September. Glacier melt, due to temperature rise, was the dominant driver of increasing streamflow in the glacier-dominated SRB, Tianshan Mountains. This article is protected by copyright. All rights reserved.

Beschreibung: Watershed simulation models are used extensively to investigate hydrologic processes, landuse and climate change impacts, pollutant load assessments and best management practices (BMPs). Developing, calibrating and validating these models require a number of critical decisions that will influence the ability of the model to represent real world conditions. Understanding how these decisions influence model performance is crucial, especially when making science based policy decisions. This study used the Soil and Water Assessment Tool (SWAT) model in West Lake Erie Basin (WLEB) to examine the influence of several of these decisions on hydrological processes and streamflow simulations. Specifically, this study addressed the following objectives (1) Demonstrate the importance of considering intra-watershed processes during model development (2) Compare and evaluated spatial calibration versus calibration at outlet and (3) Evaluate parameter transfers across temporal and spatial scales. A coarser resolution (HUC-12) model and a finer resolution model (NHDPlus model) were used to support the objectives. Results showed that knowledge of watershed characteristics and intra-watershed processes are critical to produced accurate and realistic hydrologic simulations. The spatial calibration strategy produced better results compared to outlet calibration strategy and provided more confidence. Transferring parameter values across spatial scales (i.e. from coarser resolution model to finer resolution model) needs additional fine tuning to produce realistic results. Transferring parameters across temporal scales (i.e. from monthly to yearly and daily time-steps) performed well with a similar spatial resolution model. Furthermore, this study shows that relying solely on quantitative statistics without considering additional information can produce good but unrealistic simulations. This article is protected by copyright. All rights reserved.

Beschreibung: Evapotranspiration is an important component of the hydrological cycle, which integrates atmospheric demands and surface conditions. Research on spatial and temporal variations of reference evapotranspiration (ET o ) enable understanding of climate change and its effects on hydrological processes and water resources. In this study, ET o was estimated by the FAO-56 Penman–Monteith method in the Jing River Basin in China, based on daily data from 37 meteorological stations from 1960 to 2005. ET o trends were detected by the Mann–Kendall test in annual, seasonal and monthly timescales. Sensitivity coefficients were used to examine the contribution of important meteorological variables to ET o . The influence of agricultural activities, especially irrigation on ET o was also analyzed. We found that ET o showed a decreasing trend in most of the basin in all seasons, except for autumn, which showed an increasing trend. Mean maximum temperature was generally the most sensitive parameter for ET o , followed by relative humidity, solar radiation, mean minimum temperature, and wind speed. Wind speed was the most dominant factor for the declining trend in ET o . The more significant decrease in ET o for agricultural and irrigation stations was mainly due to the more significant decrease in wind speed and sunshine hours, a mitigation in climate warming, and more significant increase in relative humidity compared with natural stations and non-irrigation stations. Changes in ET o and the sensitivity coefficient of meteorological variables in relation to ET o were also affected by topography. Better understanding of ET o response to climate change will enable efficient use of agricultural production and water resources, which could improve the ecological environment in Jing River Basin. This article is protected by copyright. All rights reserved.

Beschreibung: Groundwater catchment boundaries and their associated groundwater catchment areas are typically assumed to be fixed on a seasonal basis. We investigated whether this was true for a highly permeable carbonate aquifer in England, the Berkshire and Marlborough Downs Chalk aquifer, using both borehole hydrograph data and a physics-based distributed regional groundwater model. Borehole hydrograph data time series were used to construct a monthly interpolated water table surface, from which was then derived a monthly groundwater catchment boundary. Results from field data showed that the mean annual variation in groundwater catchment area was about 20% of mean groundwater catchment area, but interannual variation can be very large, with the largest estimated catchment size being approximately 80% greater than the smallest. The flow in the river was also dependent on the groundwater catchment area. Model results corroborated those based on field data. These findings have significant implications for issues such as definition of source protection zones, recharge estimates based on water balance calculations and integrated conceptual modelling of surface water and groundwater systems. This article is protected by copyright. All rights reserved.

Beschreibung: Understanding groundwater-surface water exchange in river banks is crucial for effective water management and a range of scientific disciplines. While there has been much research on bank storage, many studies assume idealised aquifer systems. This paper presents a field-based study of the Tambo Catchment (southeast Australia) where the Tambo River interacts with both an unconfined aquifer containing relatively young and fresh groundwater (〈500 μS/cm and 〈100 years old) and a semi-confined artesian aquifer containing old and saline groundwater (EC 〉 2,500 μS/cm and 〉 10,000 years old). Continuous groundwater elevation and EC monitoring within the different aquifers and the river suggests that the degree of mixing between the two aquifers and the river varies significantly in response to changing hydrological conditions. Numerical modelling using MODFLOW and the solute transport package MT3DMS indicates that saline water in the river bank moves away from the river during flooding as hydraulic gradients reverse. This water then returns during flood recession as baseflow hydraulic gradients are re-established. Modelling also indicates that the concentration of a simulated conservative groundwater solute can increase for up to ~34 days at distances of 20 and 40 meters from the river in response to flood events approximately 10 meters in height. For the same flood event, simulated solute concentrations within 10 m of the river increase for only ~15 days as the infiltrating low salinity river water drives groundwater dilution. Average groundwater fluxes to the river stretch estimated using Darcy's Law were 7 m 3 /m/day compared to 26 and 3 m 3 /m/day for the same periods via mass balance using Radon ( 222 Rn) and Chloride (Cl), respectively. The study shows that by coupling numerical modelling with continuous groundwater-surface water monitoring, the transient nature of bank storage can be evaluated, leading to a better understanding of the hydrological system and better interpretation of hydrochemical data. This article is protected by copyright. All rights reserved.

Beschreibung: In this paper we report on a series of replicated tracer experiments with deuterium conducted under controlled, steady stormflow conditions at the Gårdsjön G1 catchment in south-western Sweden. In five different years, these experiments were carried out in a subcatchment of G1. Deuterium was applied as a narrow pulse so that distributions of water transit times could be directly inferred from the observed tracer breakthrough curves. Significantly different transit times of water were observed under similar experimental conditions. Coefficients of variation for estimated mean transit times were greater than 60%, which can be understood as a measure of the interannual variability for this type of experiments. Implications for water transit times under more natural flow conditions as wells as for future experimentation are discussed. This article is protected by copyright. All rights reserved.

Beschreibung: A novel electronic sensing system (SAS2) was designed to non-invasively and simultaneously measure snow density, liquid water content and temperature of snow using acoustics and obtain images of the snowpack. The system is an updated and more sophisticated version of previous acoustic sensing systems, and this paper presents an update on the development of acoustic sensing methods to measure multiple snowpack properties and obtain images of snowpack stratigraphy. Air-coupled acoustic waves were sent into and reflected from the snowpack by the SAS2 system. An inverse model based on a modified version of the Biot-Stoll theory of sound propagation through porous media was used to obtain acoustic measurements of snow and images of the snowpack. The SAS2 was deployed at ten field sites in the Canadian Rockies. Stationary and portable versions of the system were deployed and tested. This paper reports initial tests of the SAS2 that were conducted at the field sites to compare acoustic model outputs with gravimetric measurements of snow density, dielectric measurements of liquid water content, and thermocouple and thermometer measurements of snow temperature. Snow water equivalent could be estimated more accurately and quickly with the SAS2 than with an ESC30 gravimetric snow tube. Unlike monopulse sonar and radar sensors, the SAS2 utilizes a continuous-wave Maximum Length Sequence (MLS) that is more robust to environmental noise. The SAS2 serves as a proof-of-concept of the acoustic snow measurement technique with potential for further research as identified and discussed in this paper. This article is protected by copyright. All rights reserved.

Beschreibung: Movies taken by witnesses of extreme flood events are increasingly available on video sharing websites. They potentially provide highly valuable information on flow velocities and hydraulic processes that can help improve the post-flood determination of discharges in streams and flooded areas. We investigated the troubles and potential of applying the now mature LSPIV technique to such flood movies that are recorded under non-ideal conditions. Processing was performed using user-friendly, free software only, such as Fudaa-LSPIV. Typical issues related to the image processing and to the hydrological analysis are illustrated using a selected example of a pulsed flash-flood flow filmed in a mountainous torrent. Simple corrections for lens distortion (fisheye) and limited incoherent camera movement (shake) were successfully applied and the related errors were reduced to a few percents. Testing the different image resolution levels offered by YouTube showed that the difference in time-averaged longitudinal velocity was less than 5% compared to full resolution. A limited number of GRPs, typically 10, is required but they must be adequately distributed around the area of interest. The indirect determination of the water level is the main source of uncertainty in the results, usually much more than errors due to the longitudinal slope and waviness of the flow free-surface. The image-based method yielded direct discharge estimates of the base flow between pulses, of the pulse waves, and of the time-averaged flow over a movie sequence including a series of 5 pulses. A comparison with traditional indirect determination methods showed that the critical-depth method may produce significantly biased results for such a fast, unsteady flow, while the slope-area method seems to be more robust but would overestimate the time-averaged flow rate if applied to the high-water marks of a pulsed flow. This article is protected by copyright. All rights reserved.

Beschreibung: In arid region, direct infiltration from rainfall contributes little to groundwater compared with localized recharge from streams. How to quantify riverbed infltration to groundwater systems is an important area of research in hydrology. In this study, saturated permeability coefficient of riverbed in an arid inland river basin located in the northwest of China was obtained by Guelph Permeameter and laboratory analysis methods. The characteristics of riverbed infiltration and its spatial patterns were analysed using geostatistical method and Kriging method. The results showed that the saturated permeability coefficient varied from 0.089 m/d to 2.802 m/d, indicating moderate degree of variability. The Guelph Permeameter and laboratory test methods provided consistent estimates of saturated permeability coefficient. There was a strong spatial correlation for K fs of riverbed in this study area when Range ( A ) was less than 0.276°, suggesting that the maximum sampling distance for saturated permeability coefficient of the riverbed was 0.276° under isotropic conditions. The K fs near the centre of riverbed was higher than the value near riverbank. The K fs values decreased in the direction of upstream to downstream in the Heihe River Basin. The riverbed mechanical composition, initial soil water content and bulk density have significant influence up on the riverbed infiltration. Besides, the topographical factors including the width, the altitude and the distance factors of the riverbed together impacted the riverbed infiltration, the slope of the riverbed also influenced the riverbed infiltration. This article is protected by copyright. All rights reserved.

Beschreibung: Surface wetting properties are an important cause of changing the groundwater and two-phase fluid flows. Various factors affecting the surface wettability were investigated in a parallel-walled glass fracture with NAPL (gasoline, diesel, TCE, and creosote)-wetted surfaces. First, the effect of the duration of NAPL exposure on wettability change was considered at pre-wet fracture surfaces using the various NAPL species, and the result showed that the surface became hydrophobic after the exposure time of NAPL exceeded 2000 min. Second, the initial wetting state of the surface affected the timing when the wettability change begins as well as the extent of the wettability change in a NAPL-wetted rock fractures. Under the dry condition, the wettability change was completed within a very short time of exposure to NAPL (~5 min.), and then it finally reached the intermediate and weakly NAPL wetting (contact angle of 118°). Under the pre-wet condition, relatively long time of exposure (~5000 min) was needed to observe the obvious change of the surface wettability, which was changed up to strongly NAPL wetting (contact angle of 142°). Third, the wettability changed by NAPL exposure was stable and maintained for a long time regardless of water flushing rate and temperature. Finally, the wettability change by the exposure of NAPL on parallel fracture surfaces was evaluated at various groundwater flow velocities. Result showed that groundwater flow velocity has an important impact upon measured contact angle. Although fracture surfaces was exposed to NAPL at the low groundwater flow velocity, the wettability was not changed from hydrophilic to hydrophobic when the contact time between NAPL and mineral surfaces was not sufficient due to the pulse-type movement of NAPL. This implies that the variation of exposure pattern due to groundwater flow on the wettability change can be an important factor affecting the wettability change of fracture surface and migration behavior at natural fractured rock aquifers in case of NAPL spill. This article is protected by copyright. All rights reserved.

Beschreibung: Analysis of land cover changes is fundamental for providing information about watershed land management, monitoring, and planning. This study reveals large-scale land cover transformation under the effects of frequent natural disturbances within the Taimali watershed in eastern Taiwan during 2005–2011. A landscape analysis approach combining landscape metrics and Markov chain model is used to understand land cover changes with regard to natural disturbances. Results of the composition metrics analysis show that the landslide area within the region has expanded 7.5 times (15.5 km 2 ), but forested area has shrunk by 10.6% (20.9 km 2 ). Spatial configuration metrics analysis indicates that patches of forest are becoming small, irregular, and spatially fragmented, while landslide patches are expanding and becoming spatially aggregated. Land cover changes in the area are considered to have occurred mainly through loss and fragmentation of forest cover, and an increase in the number and area of landslides. During 2005–2011, the most noticeable area changes related to transitions from forest cover to landslides (15.8 km 2 ) and channels (6.7 km 2 ). However, the greatest transition probabilities were for human-made patches changing into forest cover and channel corridors over the study period. Through Markov chain analysis, three projected proportions of land cover for the periods 2017–2035 are produced. These three predictive scenarios pose different risks to downstream life and property. This article is protected by copyright. All rights reserved.

Beschreibung: A large number of rivers are frozen annually and the river ice cover has an influence on the geomorphological processes. These processes in cohesive sediment rivers are not fully understood. Therefore, this paper demonstrates the impact of river ice cover on sediment transport, i.e. turbidity, suspended sediment loads and erosion potential, compared with a river with ice-free flow conditions. The present sediment transportation conditions during the annual cycle are analysed, and the implications of climate change on wintertime geomorphological processes are estimated. A one-dimensional hydrodynamic model has been applied to the Kokemäenjoki River in SW Finland. The shear stress forces directed to the river bed are simulated with present and projected hydro-climatic conditions. The results of shear stress simulations indicate that a thermally formed smooth ice cover diminishes river bed erosion, compared with an ice-free river with similar discharges. Based on long-term field data, the river ice cover reduces turbidity statistically significantly. Furthermore, suspended sediment concentrations measured in ice-free and ice-covered river water reveal a diminishing effect of ice cover on riverine sediment load. The hydrodynamic simulations suggest that the influence of rippled ice cover on shear stress is varying. Climate change is projected to increase the winter discharges by 27–77 % on average by 2070–2099. Thus, the increasing winter discharges and possible diminishing ice cover periods both increase the erosion potential of the river bed. Hence, the wintertime sediment load of the river is expected to become larger in the future. This article is protected by copyright. All rights reserved.

Beschreibung: Saltwater intrusion is a serious issue in estuarine deltas all over the world because of rapid urban sprawl and water shortage. Therefore, detecting the major flow paths or locations at risk of saltwater intrusion in estuarine ecosystems is important for mitigating saltwater intrusion. In this paper, we introduce a centrality index, the betweenness centrality ( BC ), to address this problem. Using the BC as the weighted attribute of the river network, we identify the critical confluences for saltwater intrusion and detect the preferential flow paths for saltwater intrusion through the least cost path algorithm from graph theory approach. Moreover, we analyze the responses of the BC values of confluences calculated in the river network to salinity. Our results show that the major flow paths and critical confluences for saltwater intrusion in a deltaic river network can be represented by the least cost paths and the BC values of confluences, respectively. In addition, a significant positive correlation between the BC values of confluences and salinity is determined in the Pearl River Delta. Changes in the salinity can produce significant variation in the BC values of confluences. Therefore, freshwater can be diverted into these major flow paths and critical confluences to improve river network management under saltwater intrusion. This article is protected by copyright. All rights reserved.

Beschreibung: Marine overwash events for atoll islands in the Pacific and Indian Oceans, which cause salinization of fresh groundwater due to infiltrating seawater, pose a significant challenge for island community sustainability in regards to water supply. Understanding transient fresh groundwater development during a post-overwash period for a range of island sizes, geologic characteristics, and rainfall patterns is essential for water management. This paper presents a methodology for quantifying this development for an atoll nation, with methods applied to the 32 atolls of the Federated States of Micronesia (FSM) in the western Pacific. Using the numerical groundwater modeling code SUTRA, overwash events and post-overwash freshwater-seawater dynamics are simulated for the range of island widths (200 m to 1100 m), geologic characteristics (hydraulic conductivity corresponding to leeward and windward islands), and rainfall patterns (western, central, and eastern regions) present in the FSM, thereby providing results for each atoll island. Results show that 10-17 months, 8-12 months, and 6-12 months are required to achieve 60% freshwater lens recovery for leeward islands in the western, central, and eastern FSM, respectively, with variation due to rainfall rate and island width. In contrast, 4-9 months are required for 60% recovery for windward islands. However, the natural thinness of the lend on windward islands typically precludes extensive use of groundwater under average rainfall conditions. Overwash characteristics (depth, duration, seasonal timing) did not significantly affect recovery times. For the region of lowest rainfall (western FSM), 6-10 months are required to achieve potable groundwater at the typical depth of hand-dug wells. Results provide water resource managers and atoll island communities with important information regarding timing of potential fresh groundwater use following an overwash event. This article is protected by copyright. All rights reserved.

Beschreibung: The closure problem of representing hydrological boundary fluxes given the state of the system has been described as the scientific ‘Holy Grail’ of hydrology. This relationship between storage state and flux should be hysteretic and scale-dependant due to the differences between velocities and celerities in a hydrological system – effectively velocities are storage controlled, and celerities are controlled by storage deficits. To improve our understanding of the nature of these relationships a new hydrology model is used (the Multiple Interacting Pathways or MIPs model) to explore the influence of catchment scale on storage-flow-transport relationships, and their non-linearities. The MIPs model has been shown to produce acceptable simulations of both flow and tracer, i.e. of both celerities and velocities, at the Gårdsjön catchment in Sweden. In this study the model is used to simulate scaled versions of the Gårdsjön catchment to allow us for the first time to investigate the influence of scale on the non-linearities in storage-flow-transport relationships, and help us steer the quest for the scientific hydrological ‘Holy Grail’. The simulations reveal the influence of scale on flow response in the nature of storage-discharge hysteresis and its links with antecedent storage; fractal-like systematic change of mean output travel times with scale; the effect of scale on input, output and storage residence time distributions; hysteric relations between storage and output travel times; and links between storage and water table level hysteresis. This article is protected by copyright. All rights reserved.

Beschreibung: To obtain a better understanding of water's journey from the hillslope to the stream, water level time series from 26 groundwater wells in a hillslope-riparian study area in the Panola Mountain Research Watershed, Georgia, USA were examined with respect to the occurrence of perched groundwater and groundwater flow directions. A connected groundwater table was taken as an indication of a hydrologic connection between the hillslope and the stream. Flow directions were determined based on observed water levels at different combinations of three neighboring wells. The analyses indicated that the hillslope was disconnected from the stream most of the time but that almost the entire hillslope became connected to the stream during large rainfall events. This coincided with more sustained streamflow during these events. There were clear differences in the timing of the groundwater response between the upper hillslope, lower hillslope and riparian zone, with water levels near the stream and on the upper hillslope rising earlier than on the midslope. The flow directions were generally downslope and were most variable around a bedrock depression on the midslope. On the hillslope, the direction of the water table followed the local bedrock topography when the water level was low at the start and end of an event and the overall surface topography when water levels were high. In the riparian zone, flow directions either did not change significantly during events or turned from a more downstream (i.e. downvalley) direction to a more downslope (i.e. towards the stream) direction. These results highlight the competing influence of surface and bedrock topography on hillslope flow directions and improve our understanding of hillslope-riparian-stream connectivity. This article is protected by copyright. All rights reserved.

Beschreibung: The process of development and application of the HBV hydrological model over a time period of more than 40 years is reviewed and discussed. Emphasis is on the early modelling strategy and physical considerations based on contemporary research on runoff formation processes in the drainage basin. This includes areal considerations on the catchment scale, soil moisture and evapotranspiration and storages and discharge as represented by the response function of the model. The introduction of the concept of dynamic recharge and discharge areas is also addressed as well as the modelling of snow accumulation and melt. Some operational international experiences are also addressed. This article is protected by copyright. All rights reserved.

Beschreibung: Wildfires change the infiltration properties of soil, reduce the amount of interception and result in increased runoff. A wildfire at Northeast Attica, Central Greece, in August 2009, destroyed approximately one third of a study area consisting of a mixture of shrublands, pastures, and pines. The present study simultaneously models multiple semi-arid, shrubland-dominated Mediterranean catchments and assesses the hydrological response (mean annual and monthly runoff and runoff coefficients) during the first few years following wildfires. A physically based, hydrological model (MIKE SHE) was chosen. Calibration and validation results of mean monthly discharge presented very good agreement with the observed data for the pre- and post-wildfires period for two subcatchments (Nash-Sutcliffe Efficiency coefficient of 79.7%). The model was then used to assess the pre- and post-wildfires runoff responses for each of seven catchments in the study area. Mean annual surface runoff increased for the first year and after the second year following the wildfires increased by 112% and 166%, respectively. These values are within the range observed in similar cases of monitored sites. This modelling approach may provide a way of prioritizing catchment selection with respect to post-fire remediation activities. Additionally, this modelling assessment methodology would be valuable to other semi-arid areas, because it provides an important means for comprehensively assessing post-wildfire response over large regions and therefore attempts to address some of the scaled issues in the specific literature field of research. This article is protected by copyright. All rights reserved.

Beschreibung: The present study focuses on the analysis of dryness/wetness conditions in the Danube River catchment area from 1901 to 2013 based on reanalysis data. The spatio-temporal variability of dryness/wetness conditions is analyzed by means of the Standardized Precipitation Index (SPI) for an accumulation periods of 6 months. To characterize the spatial variability of SPI6 at monthly time scales an Empirical Orthogonal Function (EOF) analysis was applied. The leading mode of SPI variability captures in-phase variability of SPI over the entire catchment area of Danube River. The leading mode of dryness/wetness variability was found to be strongly related to the different phases of the Arctic Oscillation. The second and third modes of variability show a more regional character of the dryness/wetness variability over the Danube River catchment area. Based on a composite map analysis, between the time series corresponding to the first three leading modes of dryness/wetness variability and the geopotential height at 850mb and precipitation totals, it is shown that dryness (wetness) conditions over the Danube catchment area are associated with an anticyclonic (cyclonic) circulation, transport of dry (humid) air towards the Danube catchment area and reduced (enhanced) number of rain days. This article is protected by copyright. All rights reserved.

Beschreibung: Tree canopy processes affect the volume and biogeochemistry of inputs to the hydrological cycle in cities. From June, 2012 to November, 2013, we studied stemflow production from 37 isolated deciduous park trees in a semi-arid climate dominated by small precipitation events. To clarify the effects of canopy traits on stemflow metrics, we analyzed branch angles, bark relief (one component of roughness), tree size, canopy and wood cover fraction, median leaf size, and branch and leader counts. High branch angles contributed to stemflow production in both single- and multi-leader trees. While bark relief was negatively correlated with stemflow rates in multi-leader trees, it was positively correlated with rates for single-leader trees, possibly reflecting the conducive role of linear furrows once bark of single-leader trees is saturated. The association between numerous leaders, low stemflow initiation thresholds, and high rates deserves further study. Among meteorological variables, rain depth was strongly correlated with stemflow yields; rainfall inclination angle and wind speed were positively correlated with yields while total intra-storm break duration and vapour pressure deficit were inversely related. For rain depths 〈 3 mm, greater stemflow was generally associated with leafless canopies. In support of integrated stormwater management, our results can inform climate-sensitive selection and siting of urban trees with traits that tend to either promote or minimize stemflow, depending on infiltration potential. This article is protected by copyright. All rights reserved.

Beschreibung: In recent years, several papers contributed to the development and clarification of key theoretical issues underlying the formulation of transport by travel time distributions in catchments. Such a formulation provides a robust description of the temporal evolution of ages contained in the catchment storage and sampled by the output fluxes. In particular, special attention has been devoted to general time-variant dynamics that are likely to occur in complex systems like catchments. In this context, important theoretical and practical implications arise from a proper distinction between backward or forward age distributions, which are based on the definition of diverse reference variables. The ‘age’ of a water particle represents the time elapsed since a previous injection, and as such it is intrinsically a backward time concept. A forward approach, instead, requires the introduction of the particle's ‘life expectancy’, which quantifies the time a water particle will spend within the system before being sampled by one of the outflows (e.g. stream discharge or evapotranspiration). The sum of age and life expectancy is the particle's travel time. Despite forward and backward approaches being different, and that they only coincide in the special case of stationary systems, a proper distinction of these formulations has been sometimes overlooked in the literature. In this contribution, we review recent backward formulations using a unified notation and discuss a novel forward formulation. This paper illustrates how age and life expectancy distributions naturally evolve in response to unsteady hydrologic fluxes and presents numerical applications relevant to catchment-scale solute circulation. In both forward and backward formulations, the mixing of ages, which is modeled through age-selection functions, plays a central role in describing the fate of solutes introduced in the system and measured at the system outlets. This article is protected by copyright. All rights reserved.

Beschreibung: A long-term study of O, H and C stable isotopes has been undertaken on river waters across the 7000 km 2 upper Thames lowland river basin in the southern UK. During the period, flow conditions ranged from drought to flood. A 10-year monthly record (2003–2012) of the main River Thames showed a maximum variation of 3‰ (δ 18 O) and 20‰ (δ 2 H), though inter-annual average values varied little around a mean of –6.5‰ (δ 18 O) and –44‰ (δ 2 H). A δ 2 H/δ 18 O slope of 5.3 suggested a degree of evaporative enrichment, consistent with derivation from local rainfall with a weighted mean of –7.2‰ (δ 18 O) and –48‰ (δ 2 H) for the period. A tendency towards isotopic depletion of the river with increasing flowrate was noted, but at very high flows (〉100 m 3 /s) a reversion to the mean was interpreted as the displacement of bank storage by rising groundwater levels (corroborated by measurements of specific electrical conductivity). A shorter quarterly study (October 2011 – April 2013) of isotope variations in 15 tributaries with varying geology revealed different responses to evaporation, with a well-correlated inverse relationship between Δ 18 O and baseflow index (BFI) for most of the rivers. A comparison with aquifer waters in the basin showed that even at low flow, rivers rarely consist solely of isotopically unmodified groundwater. Long-term monitoring (2003–2007) of carbon stable isotopes in dissolved inorganic carbon (DIC) in the Thames revealed a complex interplay between respiration, photosynthesis and evasion, but with a mean inter-annual δ 13 C-DIC value of –14.8 ± 0.5‰, exchange with atmospheric carbon could be ruled out. Quarterly monitoring of the tributaries (October 2011 – April 2013) indicated that in addition to the above factors, river flow variations and catchment characteristics were likely to affect δ 13 C-DIC. Comparison with basin groundwaters of different alkalinity and δ 13 C-DIC values showed that the origin of river baseflow is usually obscured. The findings show that long-term monitoring of environmental tracers can help to improve the understanding of how lowland river catchments function. This article is protected by copyright. All rights reserved.

Beschreibung: Annual streamflows have decreased across mountain watersheds in the Pacific Northwest of the United States over the last ~70 years, however in some watersheds, observed annual flows have increased. Physically based models are useful tools to reveal the combined effects of climate and vegetation on long-term water balances by explicitly simulating the internal watershed hydrological fluxes that affect discharge. We used the physically based Simultaneous Heat and Water (SHAW) model to simulate the inter-annual hydrological dynamics of a 4 km 2 watershed in northern Idaho. The model simulates seasonal and annual water balance components including evaporation, transpiration, storage changes, deep drainage, and trends in streamflow. Independent measurements were used to parameterize the model, including forest transpiration, stomatal feedback to vapor pressure, forest properties (height, leaf area index, and biomass), soil properties, soil moisture, snow depth, and snow water equivalent. No calibrations were applied to fit the simulated streamflow to observations. The model reasonably simulated the annual runoff variations during the evaluation period from water year 2004 to 2009, which verified the ability of SHAW to simulate the water budget in this small watershed. The simulations indicated that inter-annual variations in streamflow were driven by variations in precipitation and soil water storage. One key parameterization issue was leaf area index, which strongly influenced interception across the catchment. This approach appears promising to help elucidate the mechanisms responsible for hydrological trends and variations resulting from climate and vegetation changes on small watersheds in the region. This article is protected by copyright. All rights reserved.

Beschreibung: A correct identification of drought events over vegetated lands can be achieved by detecting those soil moisture conditions that are both unusually dry compared to the “normal” state and causing severe water stress to the vegetation. In this paper we propose a novel drought index that accounts for the mutual occurrence of these two conditions by means of a multiplicative approach of a water deficit factor and a dryness probability factor. The former quantifies the actual level of plant water stress, whereas the latter verifies that the current water deficit condition is unusual for the specific site and period. The methodology was tested over Europe between 1995 and 2012 using soil moisture maps simulated by Lisflood, a distributed hydrological precipitation-runoff model. The proposed drought severity index, DSI, demonstrates to be able to detect the main drought events observed over Europe in the last two decades, as well as to provide a reasonable estimation of both extension and magnitude of these events. It also displays an improved adaptability to the range of possible conditions encountered in the experiment as compared to currently available indices based on the sole magnitude or frequency. The results show that, for the analyzed period, the most extended drought events observed over Europe were the ones in Central Europe in 2003 and in southern Europe in 2011/2012, while the events affecting the Iberian Peninsula in 1995 and 2005 and Eastern Europe in 2000 were among the most severe ones. This article is protected by copyright. All rights reserved.

Beschreibung: The measurement of discharge is fundamental in nutrient load estimation. Due to our ability to monitor discharge routinely, it is generally assumed that the associated uncertainty is low. This paper challenges this preconception, arguing that discharge uncertainty should be explicitly taken into account to produce robust statistical analyses. In many studies, paired discharge and chemical datasets are used to calculate ‘true’ loads and used as the benchmark to compare with other load estimates. This paper uses two years of high frequency (daily and sub-hourly) discharge and nutrient concentration data (nitrate-N and total phosphorus (TP)) collected at four field sites as part of the Hampshire Avon Demonstration Test Catchment (DTC) programme. A framework for estimating observational nutrient load uncertainty was used which combined a flexible non-parametric approach to characterising discharge uncertainty, with error modelling that allowed the incorporation of errors which were heteroscedastic and temporally correlated. The results showed that the stage-discharge relationships were non-stationary, and observational uncertainties from ±2-25% were recorded when the velocity-area method was used. The variability in nutrient load estimates ranged from 1.1-9.9% for nitrate-N and from 3.3-10% for TP when daily laboratory data were used, rising to a maximum of 9% for nitrate-N and 83% for TP when the sensor data were used. However, the sensor data provided a better representation of the ‘true’ load as storm events are better represented temporally, posing the question: is it more beneficial to have high frequency, lower precision data or lower frequency but higher precision data streams to estimate nutrient flux responses in headwater catchments?

Beschreibung: Information is scarce on the spatial scale effect on sediment concentrations in runoff. This study addressed this issue within an agricultural subwatershed of the Chinese Loess Plateau, using data observed at a hilltop plot, three nested hillslope plots, two entire slope plots (a combination of hillslope and valley side slope) and the subwatershed outlet. Dominated by the splash and sheet erosions, the hilltop plot has a minimum C ae (mean sediment concentration for all recorded events) of 45 kg m -3 . Unexpectedly, the high sediment concentrations at the hilltop do not occur at high rainfall intensities or large runoff events due to the protection of surface soils by relatively thick sheet flows. Due to the emergency of rills, C ae is as high as 310 kg m -3 even on the most upper hillslope . Downslope, both C ae and ESC (extreme large values of recorded sediment concentrations) increase; such a slope length effect attenuates with increasing slope length and event magnitude as a result of insufficient sediment availability associated with rill development. Active mass wastings ensure sufficient sediment supply and thus a spatially invariant C ae (approximately 700 kg m -3 ) and ESC (approximately 1000 kg m -3 ) at the scale of entire slope and subwatershed . Detailed examination shows that most small events experience a decrease in sediment concentrations when moving from the entire slope to the subwatershed, indicating that the spatially invariant sediment concentration is valid only for large runoff events. This study highlights the control of spatial scale, which determines the dominant erosional process, on erosional regime. This article is protected by copyright. All rights reserved.

Beschreibung: Hydrologic interaction between surface and subsurface water systems has a significant impact on water quality, ecosystems and biogeochemistry cycling of both systems. Distributed models have been developed to simulate this function, but they require detailed spatial inputs and extensive computation time. The SWAT model is a semi-distributed model that has been successfully applied around the world. However it has not been able to simulate the two way exchanges between surface water and groundwater. In this study, the SWAT-LU model – based on a catena method that routes flow across three landscape units (the divide, the hillslope and the valley) – was modified and applied in the floodplain of the Garonne River. The modified model was called SWAT-LUD. Darcy's equation was applied to simulate groundwater flow. The algorithm for surface water level simulation during flooding periods was modified and the influence of flooding on groundwater levels was added to the model. Chloride was chosen as a conservative tracer to test simulated water exchanges. The simulated water exchange quantity from SWAT-LUD was compared with the output of a 2D distributed model, 2SWEM. The results showed that simulated groundwater levels in the LU adjoining the river matched the observed data very well. Additionally, SWAT-LUD model was able to reflect the actual water exchange between the river and the aquifer. It showed that river water discharge has a significant influence on the surface-groundwater exchanges. The main water flow direction in the river/groundwater interface was from groundwater to river, water flowed in this direction accounted for 65 % of the total exchanged water volume. The water mixing occurs mainly during high hydraulic periods. Flooded water was important for the SW-GW exchange process, it accounted for 69 % of total water flowed from the river to the aquifer. The new module also provides the option of simulating pollution transfer occurring at the river/groundwater interface at the catchment scale. This article is protected by copyright. All rights reserved.

Beschreibung: We analysed contributions to runoff using hourly stream water samples from seven individual melt-induced runoff events (plus one rainfall event) during 2011, 2012 and 2013 in two nested glacierized catchments in the Eastern Italian Alps. Electrical conductivity and stable isotopes of water were used for mixing analysis and two- and three-component hydrograph separation. High-elevation snowmelt, glacier melt and autumn groundwater were identified as major end-members. Discharge and tracers in the stream followed the diurnal variations of air temperature but markedly reacted to rainfall inputs. Hysteresis patterns between discharge and electrical conductivity during the melt-induced runoff events revealed contrasting loop directions at the two monitored stream sections. Snowmelt contribution to runoff was highest in June and July (up to 33%) whereas the maximum contribution of glacier melt was reached in August (up to 65%). The maximum groundwater and rainfall contributions were 62% and 11%, respectively. Runoff events were generally characterised by decreasing snowmelt and increasing glacier melt fractions from the beginning to the end of the summer 2012, while runoff events in 2013 showed less variable snowmelt and lower glacier melt contributions than in 2012. The results provided essential insights into the complex dynamics of melt-induced runoff events and may be of further use in the context of water resource management in Alpine catchments. This article is protected by copyright. All rights reserved.

Beschreibung: A two-dimensional variable-density groundwater flow and transport model was developed to provide a conceptual understanding of past and future conditions of nitrate (NO 3 ) transport and estimate groundwater nitrate flux to the Gulf of Mexico. Simulation results show that contaminant discharge to the coast decreases as the extent of saltwater intrusion increases. Other natural and/or artificial surface waters such as navigation channels may serve as major sinks for contaminant loading and act to alter expected transport pathways discharging contaminants to other areas. Concentrations of NO 3 in the saturated zone were estimated to range between 30 and 160 mg · L -1 as NO 3 . Relatively high hydraulic vertical gradients and mixing likely play a significant role in the transport processes, enhancing dilution and contaminant migration to depth. Residence times of NO 3 in the deeper aquifers vary from 100 (locally) to about 300 years through the investigated aquifer system. NO 3 mass fluxes from the shallow aquifers (0 to 5.7 x 10 4  mg · m -2  · d -1 ) were primarily directed toward the navigation channel, which intersects and captures a portion of the shallow groundwater flow/discharge. Direct NO 3 discharge to the sea (i.e. Gulf of Mexico) from the shallow aquifer was very low (0 to 9.0 x 10 1  mg · m -2  · d -1 ) compared to discharge from the deeper aquifer system (0 to 8.2 x 10 3  mg · m -2  · d -1 ). Both model-calibrated and radiocarbon tracer-determined contaminant flux estimates reveal similar discharge trends, validating the use of the model for density-dependent flow conditions. The modeling approach shows promise to evaluate contaminant and nutrient loading for similar coastal regions worldwide. This article is protected by copyright. All rights reserved.

Beschreibung: Using conservative tracers to aid conceptual rainfall-runoff modelling has gained momentum over the past decade. Tracer data has been invaluable in providing rich insights into runoff sources, flow paths and water age that cannot be established by simple rainfall-runoff dynamics alone. Accordingly, this has provided a focus for fertile dialogue between field hydrologists and modellers in joint efforts to understand catchment function and incorporate this in runoff models. Central to this has been the utility of tracers in establishing the differences between the timescales of the celerity of the rainfall-runoff response, and timescales of the pore velocity of water. The literature now has numerous examples of using tracers to aid modelling as a learning framework. Despite this progress, utilisation of tracer-aided models and exploitation of their evident advantages by the wider modelling community has been slow. This in part reflects lack of suitable data sets at many sites and the fact that studies to date have highlighted various problems and challenges when trying to integrate tracers into rainfall-runoff models (e.g. increased parameterisation). Nevertheless, interest in tracer-aided modelling has continued to build as there have been marked improvements in the reliability and economics of field and laboratory methods for collecting spatially distributed and high temporal resolution tracer data sets. Consequently, we stand on the threshold of unprecedented advances in applications of this area. Here, we critically evaluate progress to date and assess the challenges that remain. The key current research frontiers with the greatest potential for rapid advancement are 1) to go beyond hydrograph simulation alone and build more realistic models of catchment functioning based on tracer data, 2) investigations into the nonlinear, threshold-type, non-stationary, and hysteresis-driven nature of how catchments process water and solutes, 3) detailed eco-hydrological studies of connectivity patterns and the role of vegetation on water partitioning and 4) the assessment of anthropogenic influences on the catchment hydrological cycle. This article is protected by copyright. All rights reserved.

Beschreibung: Soil moisture is a critical variable in the water and energy cycles. The prediction of soil moisture patterns, especially at high spatial resolution, is challenging. This study tests the ability of a land surface hydrologic model (Flux-PIHM) to simulate high-resolution soil moisture patterns in the Shale Hills watershed (0.08 km 2 ) in central Pennsylvania. Locally measured variables including a soil map, soil parameters, a tree map, and lidar topographic data, all have been synthesized into Flux-PIHM to provide model inputs. The predicted 10-cm soil moisture patterns for fifteen individual days encompassing seven months in 2009 are compared with the observations from 61 soil moisture monitoring sites. Calibrated using only watershed-scale and a few point-based measurements, and driven by spatially uniform meteorological forcing, Flux-PIHM is able to simulate the observed macro spatial pattern of soil moisture at ~10 m resolution (spatial correlation coefficient ~ 0.6) and the day-to-day variation of this soil moisture pattern, although it underestimates the amplitude of the spatial variability and the mean soil moisture. Results show that the spatial distribution of soil hydraulic parameters has the dominant effect on the soil moisture spatial pattern. The surface topography and depth to bedrock also affect the soil moisture patterns in this watershed. Using the National Land Cover Database (NLCD) in place of a local tree survey map makes a negligible difference. Field measured soil type maps and soil type-specific hydraulic parameters significantly improve the predicted soil moisture pattern as compared to the most detailed national soils database (Soil Survey Geographic Database, or SSURGO, 30-m resolution). This article is protected by copyright. All rights reserved.

Beschreibung: Hydrologic research has increasingly recognized the importance of mesoscale studies to provide a fundamental understanding of how processes combine at scales relevant to water resource management. At the mesoscale, the influence of landscape heterogeneity including both natural and human-altered conditions on streamflow generation is an open question; one to which analysis of stable water isotopes (SWI) are increasingly being applied to help address. In this study, SWI surveys are used to better understand spatial and temporal patterns of source water contributions to streamflow in the Wistiwasing watershed (235 km 2 ) located near Callander Bay, Ontario, Canada, a Precambrian Shield headwater with mixed landuse (e.g. agriculture, forest). Bi-weekly surveys of surface water, groundwater and precipitation were conducted during May to September 2012 and samples were analyzed for SWI (δ 18 O and δ 2 H) using a Picarro L2120- i . Maps of point-scale surface water SWI were generated for each of the nine surveys and an SWI isoscape, an interpolated contour map, was generated from groundwater observations. Based on a comparison of surface and groundwater SWI maps, regions of strong groundwater influence on streamflow were particularly identifiable during low-flow, late summer conditions and corresponded with coarse-textured glaciolacustrine deposits. Higher-flow, early period conditions featured small SWI variation with values resembling long-term groundwater recharge, a mix of snowmelt and spring/fall rains. Late period low-flow conditions indicated large spatial variability due to changing influences of groundwater and upstream surface water undergoing summertime evaporative enrichment of heavier isotopes. In this case study, SWI observations provide important insight into source water dynamics across a mesoscale watershed. This article is protected by copyright. All rights reserved.

Beschreibung: Wind redistribution, radiation and turbulent heat fluxes determine seasonal snow accumulation and melt patterns in alpine environments. Mathematical representations of windflow vary in complexity and introduce uncertainty to snow modelling. To characterize this uncertainty, a spatially distributed snow model that considers the physics of blowing snow transport and sublimation and the energy fluxes contributing to snowpack ablation was evaluated for its ability to simulate seasonal snow patterns around a windy alpine ridge in the Canadian Rockies. The model was forced with output from three windflow models of varying computational complexity and physical realism: i) a terrain-based empirical interpolation of station observations, ii) a simple turbulence model, and iii) a computational fluid dynamics model. Compared to wind measurements, the windflow simulations produced similar and relatively accurate (biases lower than ±1.1 m s -1 ) wind speed estimates. However, the snow mass budget simulated by the snow model was highly sensitive to the windflow simulation used. Compared to measurements, distributed snow model depth and water equivalent errors were smallest using either of the two turbulence models, with the best representation of downwind drifts by the computational fluid dynamics model. Sublimation was an important mass loss from the ridge and windflow model choice resulted in cumulative seasonal sublimation differences ranging from 10.5% to 19.0% of seasonal snowfall. When aggregated to larger scales, differences in cumulative snowmelt and snow transport were negligible but persistent differences in sublimation and snow-covered area suggest that windflow model choice can have significant implications at multiple scales. Uncertainty can be reduced by using physically based windflow models to drive distributed snow models. This article is protected by copyright. All rights reserved.

Beschreibung: Global climate change and diverse human activities have resulted in distinct temporal-spatial variability of watershed hydrological regimes, especially in water-limited areas. This study presented a comprehensive investigation of streamflow and sediment load changes on multi-temporal scales (annual, flood season, monthly and daily scales) during 1952–2011 in the Yanhe watershed, Loess Plateau. The results indicated that the decreasing trend of precipitation and increasing trend of potential evapotranspiration and aridity index were not significant. Significant decreasing trends ( p 〈0.01) were detected for both the annual and flood season streamflow, sediment load, sediment concentration and sediment coefficient. The runoff coefficient exhibited a significantly negative trend ( p 〈0.01) on the flood season scale, whereas the decreasing trend on the annual scale was not significant. The streamflow and sediment load during July-August contributed 46.7% and 86.2% to the annual total, respectively. The maximum daily streamflow and sediment load had the median occurrence date of July 31st, and they accounted for 9.7% and 29.2% of annual total, respectively. All of these monthly and daily hydrological characteristics exhibited remarkable decreasing trends ( p 〈0.01). However, the contribution of the maximum daily streamflow to the annual total progressively decreased (−0.07% yr −1 ), while that of maximum daily sediment load increased over the last 60 years (0.08% yr −1 ). The transfer of sloping cropland for afforestation and construction of check-dams represented the dominant causes of streamflow and sediment load reductions, which also made the sediment grain finer.

Beschreibung: In many catchments the geographical demarcation does not coincide with the limits of the aquifers, so groundwater may be exchanged beyond their topographic boundaries. By studying groundwater exchanges, the natural resources of a catchment can be better assessed, and the divergences between hydrological models and measurements can be explained. The aim of this work is to reveal the importance of including groundwater exchanges in the hydrological modelling of some catchments, using a water balance model. For this purpose, a simple example is conducted. The so-called model Parent Model Scheme is modified to only allow groundwater exchanges, and it is applied to the headwater of the Segura River Basin District, located in the southeast of Spain. This area is selected because groundwater plays an important role in surface hydrology. The results reveal that groundwater exchanges cannot be neglected in some catchments when assessing water resources, since their integration in the hydrological model corrects errors in the water balance. Moreover, this paper proves that water balance models are a useful tool for estimating groundwater exchanges between catchments, which can be contrasted with more complex distributed models or isotopic tracers if there is enough information available. This article is protected by copyright. All rights reserved.

Beschreibung: Knowledge on groundwater-surface water interaction and especially on exchange fluxes between streams and aquifers is an important prerequisite for the study of transport and fate of contaminants and nutrients in the hyporheic zone. One possibility to quantify groundwater-surface water exchange fluxes is by using heat as an environmental tracer. Modern field equipment including multilevel temperature sticks and the novel open-source analysis tool LPML make this technique ever more attractive. The recently developed LPML method solves the 1D fluid flow and heat transport equation by combining a local polynomial (LP) method with a maximum likelihood (ML) estimator. In this study we apply the LPML method on field data to quantify the spatial and temporal variability of vertical fluxes and their uncertainties from temperature-time series measured in a Belgian lowland stream. Over several months temperature data were collected with multilevel temperature sticks (MLTS) at the streambed top and at six depths for a small stream section. Long-term estimates show a range from gaining fluxes of -291 mmd -1 to loosing fluxes of 12 mmd -1 ; average seasonal fluxes ranged from -138 mmd -1 in winter to -16 mmd -1 in summer. With our analyses we could determine a high spatial and temporal variability of vertical exchange fluxes for the investigated stream section. Such spatial and temporal variability should be taken into account in biogeochemical cycling of carbon, nutrients and metals and in fate analysis of contaminant plumes. In general, the stream section was gaining during most of the observation period. Two short-term high stream-stage events seemingly caused by blockage of the stream outlet, led to a change in flow direction from gaining to losing conditions. We also found more discharge occurring at the outer stream bank than at the inner one indicating a local flow-through system. With the conducted analyses, we were able to advance our understanding of the regional groundwater flow system. This article is protected by copyright. All rights reserved.

Beschreibung: Incorporating the influence of soil structure and horizons into parameterizations of distributed surface water / groundwater models remains a challenge. Often only a single soil unit is employed and soil-hydraulic properties are assigned based on textural classification, without evaluating the potential impact of these simplifications. This study uses a distributed physics-based model to assess the influence of soil horizons and structure on effective parameterization. This paper tests the viability of two established and widely used hydrogeologic methods for simulating runoff and variably-saturated flow through layered soils: (1) accounting for vertical heterogeneity by combining hydrostratigraphic units with contrasting hydraulic properties into homogeneous, anisotropic units, and (2) use of established pedotransfer functions based on soil texture alone to estimate water retention and conductivity, without accounting for the influence of pedon structures and hysteresis. The viability of this latter method for capturing the seasonal transition from runoff dominated to evapotranspiration dominated regimes is also tested here. For cases tested here, event-based simulations using simplified vertical heterogeneity did not capture the state-dependent anisotropy and complex combinations of runoff generation mechanisms resulting from permeability contrasts in layered hillslopes with complex topography. Continuous simulations using pedotransfer functions that do not account for the influence of soil structure and hysteresis generally over-predicted runoff, leading to propagation of substantial water balance errors. Analysis suggests that identifying a dominant hydropedological unit provides the most acceptable simplification of subsurface layering, and that modified pedotransfer functions with steeper soil-water retention curves might adequately capture the influence of soil structure and hysteresis on hydrologic response in headwater catchments. This article is protected by copyright. All rights reserved.

Beschreibung: This study presents a versatile index for the quantification of hysteretic loops between hydrological variables at the runoff event timescale. The conceptual development of the index is based on a normalisation of the input data and the computation of definite integrals at fixed intervals of the independent variable. The sum, the minimum and the maximum of the differences between integrals computed on the rising and falling curve provide information on the direction, the shape and the extent of the loop. The index was tested with synthetic data and field data from experimental catchments in Northern Italy. Hysteretic relations between streamflow (the independent variable) and soil moisture, depth to water table, isotopic composition and electrical conductivity of stream water (dependent variables) were correctly identified and quantified by the index. The objective quantification of hysteresis by the index allows for the automatic classification of hysteretic loops and thus the determination of differences in hydrological responses during different events. The index was used to examine the seasonal dynamics in the relation between streamflow and soil moisture and captured the switch in the direction of the loop with changes in event size and antecedent wetness conditions. The sensitivity of the index to the temporal resolution of the measurements and measurement errors was also tested. The index can successfully quantify hysteresis, except for very noisy data or when the temporal resolution of the measurements is not well suited to study hysteresis between the variables. This article is protected by copyright. All rights reserved.

Beschreibung: Compaction of upper soil layers by intensive sheep grazing has been connected with increased local flood risk in silvopastoral systems. A 12 week field study was conducted at the Henfaes Research Station near Bangor, Wales to compare canopy interception, soil water infiltration, and bulk density between a control pasture and two silvopastoral tree planting configurations: trees clumped in fenced-off ungrazed plots and trees planted evenly and grazed. The study's aim was to characterize the potential of these tree planting configurations to reduce local flood risk. Automated throughfall gauges were installed in each silvopastoral treatment along with a similarly designed control gauge located in the grazed control pasture. Soil water infiltration and bulk density were measured 20 times in a stratified random design for each treatment and control. Results showed that the mean soil infiltration capacity in clumped configuration was 504% greater than the control pasture and 454% greater than the even spaced configuration. Average canopy interception was higher in the clumped configuration (34%) than in the even spaced configuration (28%). Soil bulk density was lower in the clumped configuration (0.87 Mg/m 3 ) than in the control pasture (0.93 Mg/m 3 ) and even spaced configuration (1 Mg/m 3 ). Results suggest that silvopastoral systems are more likely to benefit from clumped and ungrazed tree configurations than from evenly-spaced and grazed tree configurations due to enhanced infiltration, lower soil compaction, and increased canopy interception. Our findings support the growing evidence base that fenced-off tree areas in silvopastoral settings can increase infiltration and thus reduce local flood risk. This article is protected by copyright. All rights reserved.

Beschreibung: Stable water isotopes provide a means of tracing many hydrologic processes, including poorly understood dynamics like soil water interactions with the atmosphere. We present a four-year dataset of biweekly water isotope samples from eight fluxes and stores in a headwater catchment at the Hubbard Brook Experimental Forest, New Hampshire, U.S.A. We use Dansgaard's deuterium excess ( d ) parameter to infer hydrologic processes that cause stable water isotope fractionation. Although we expected to observe a decrease in d from precipitation to soil water due to evaporation, instead we observed an increase, which suggests sub-canopy water vapor recycling (evapotranspiration and then re-condensation). However, the underlying mechanisms and spatial dynamics remain uncertain. The apparent recycling is most evident in the growing season; weak evidence suggests a similar process in the dormant season. Sub-canopy water recycling is a novel hydrologic process that should have implications for micro-meteorology and habitat provided by the forest sub-canopy environment. This article is protected by copyright. All rights reserved.

Beschreibung: Recent studies have shown that boreal peatlands exhibit considerable chemical variability but without clear spatial pattern. This chemical heterogeneity illustrates the complex hydrological behaviour of peatlands, particularly patterned fen. Isotopic, chemical and physical tracers were used to describe the hydrological behaviour of a small boreal headwater catchment (13 ha) during the snow-free period with a special emphasis on the downstream patterned fen. Results showed that shallow pools were mixed every day during the summer, particularly during nights or discharge periods. Despite large water storage capacities in pools, which should induce large buffer effect, hydrological behaviour of patterned fen is more similar to a piston flow process. This is probably due to the division of the fen into successive small cascading streamflow reservoirs. The consequences were a rapid change of the chemical signature throughout the fen, particularly upstream. A spatial pattern was observed downstream in early summer. The isotopic signature passed from an upstream depleted and homogeneous signature to a progressively enriched downstream signature. However, this pattern was not identified during the wetter period (late summer), probably because the discharge, which dominated the water budget, decreased the surface water residence time and flushed a large proportion of stored surface water. We developed for this patterned fen a conceptual model of the surface flow to explain these particular mixing effects and the implications on the dynamics of the chemical signature. To further our understanding of similar boreal headwater catchments, future work should include the development of a multiple mixed-reservoir model. This article is protected by copyright. All rights reserved.

Beschreibung: To improve quantitative understanding of mixed-land-use impacts on nutrient yields, a nested-scale experimental watershed study design ( n  = 5) was applied in a 303(d), clean water act impaired urbanizing watershed of the lower Missouri River Basin, USA. From 2010–2013, water samples ( n  = 858 sample days per site) were analyzed for total inorganic nitrogen (TIN-N), nitrite (NO 2 -N) nitrate (NO 3 -N), ammonia (NH 3 -N), and total phosphorus (TP-P). Annual, seasonal, and monthly flow-weighted concentrations (FWCs) and nutrient yields were estimated. Mean nutrient concentrations were highest where agricultural land use comprised 58% of the drainage area (NH 3  = 0.111 mg/L; NO 2  = 0.045 mg/L; NO 3  = 0.684 mg/L, TIN = 0.840 mg/L; TP = 0.127 mg/L). Average TP-P increased by 15% with 20% increased urban land use area. Highly variable annual precipitation was observed during the study with highest nutrient yields during 2010 (record setting wet year) and lowest nutrient yields during 2012 (extreme drought year). Annual TIN-N and TP-P yields exceeded 10.3 and 2.04 kg ha −1 yr −1 from the agricultural dominated headwaters. Mean annual NH 3 -N, NO 2 -N, NO 3 -N, TIN-N, and TP-P yields were 0.742, 0.400, 4.24, 5.38, and 0.979 kg ha −1 yr −1 , respectively near the watershed outlet. Precipitation accounted for the majority of the explained variance in nutrient yields ( R 2 values from 0.68 to 0.85). Nutrient yields were also dependent on annual precipitation of the preceding year ( R 2 values from 0.87 to 0.91) thus enforcing the great complexity of variable mixed-land-use mediated source-sink nutrient yield relationships. Study results better inform land managers and best management practices designed to mitigate nutrient pollution issues in mixed-land-use freshwater ecosystems. This article is protected by copyright. All rights reserved.

Beschreibung: Much debate has occurred in catchment hydrology regarding the connectivity of flow paths from upslope areas to catchment outlets. This study was conducted in two catchments, one with three upper branches, in a loess soil with a fragipan that fosters lateral flow and exhibits an extensive distribution of soil pipe collapse features. The study aimed to determine the connectivity of multiple soil pipe networks as well as determine pipe flow velocities during storm events. Fluorescein dye was injected directly into soil pipes at the upper most pipe collapse feature of four different hillslopes. Breakthrough curves (BTC) were determined by sampling multiple pipe collapse features downslope. The BTCs were used to determine the “average” (center of mass) and “maximum” (first arrival) flow velocities. This study confirmed that these catchments contain individual continuous soil pipe networks that extend over 190 m and connect the upper most hillslopes areas with the catchment outlet. While the flow paths are continuous, the individual pipe networks consist of alternating reaches of subsurface flow through soil pipes and reaches of surface flow through gullies formed by pipe collapses. In addition, flow can be occurring both through the subsurface soil pipes simultaneous with surface flow generated by artesian flow from the soil pipes. The pipe flow velocities were as high as 0.3 m/s which was in the range of streamflow velocities. These pipe flow velocities were also in the range of velocities observed in pinhole erosion tests suggesting that these large, mature soil pipes are still actively eroding. This article is protected by copyright. All rights reserved.

Beschreibung: Soil pipes are important subsurface flow pathways in many soil erosion phenomena. However, limited research has been performed on quantifying and characterizing their flow and transport characteristics. The objectives of this research were to determine the applicability of a streamflow model with transient storage in deriving flow and transport characteristics of soil pipes. Tracer data from pulse inputs were collected in four different soil pipes after a fluorescein dye was injected in the upstream end of each soil pipe network in three Branches (West, Middle, and East) of a Main Catchment and a Back Catchment in Goodwin Creek Experimental Watershed in Mississippi. Multiple sampling stations were positioned along each soil pipe network. The transient storage zone model OTIS-P was executed inversely to estimate transport parameters by soil pipe reach such as the soil pipe cross-sectional area ( A ), soil storage zone cross-sectional area ( A s ), and exchange rate between the soil pipe and the soil storage zone ( α s ). Model convergence was achieved, and simulated breakthrough curves of the reaches were in good agreement with actual tracer data for eight of the nine reaches of the three Branches of the Main Catchment and five of the seven reaches of the Back Catchment soil pipe. Simulation parameters for the soil pipe networks were similar to the range of values reported for flow and transport characteristics commonly observed in streams. Inversely estimated soil pipe flow velocities were higher with increased tortuosity which led to a smaller cross-sectional areas predicted for the soil pipe flowpaths while other parameters were not sensitive to tortuosity. In general, application of OTIS-P to this unique soil pipe condition suggested larger transient storage ( A s and α s ) compared to most stream systems. This was hypothesized to be due to relatively higher ratio of the wetted perimeter to flow area in the soil pipe, the hydraulic roughness of the soil pipe, potential retention in collapsed portions of the pipe, and interaction with smaller preferential flow systems. This article is protected by copyright. All rights reserved.