ALBERT

Beschreibung: Western US forest ecosystems and downstream water supplies are reliant on seasonal snowmelt. Complex feedbacks govern forest-snow interactions in which forests influence the distribution of snow and the timing of snowmelt but are also sensitive to snow water availability. Notwithstanding, few studies have investigated the influence of forest structure on snow distribution, snowmelt and soil moisture response. Using a multi-year record from co-located observations of snow depth and soil moisture, we evaluated the influence of forest-canopy position on snow accumulation and snow depth depletion, and associated controls on the timing of soil moisture response at Boulder Creek, Colorado, Jemez River Basin, New Mexico, and the Wolverton Basin, California. Forest-canopy controls on snow accumulation led to 12-42cm greater peak snow depths in open versus under-canopy positions. Differences in accumulation and melt across sites resulted in earlier snow disappearance in open positions at Jemez and earlier snow disappearance in under-canopy positions at Boulder and Wolverton sites. Irrespective of net snow accumulation, we found that peak annual soil moisture was nearly synchronous with the date of snow disappearance at all sites with an average deviation of 12, 3 and 22days at Jemez, Boulder and Wolverton sites, respectively. Interestingly, sites in the Sierra Nevada showed peak soil moisture prior to snow disappearance at both our intensive study site and the nearby snow telemetry stations. Our results imply that the duration of soil water stress may increase as regional warming or forest disturbance lead to earlier snow disappearance and soil moisture recession in subalpine forests. © 2014 John Wiley & Sons, Ltd.

Beschreibung: The synthesis of experimental understanding of catchment behaviour and its translation into qualitative perceptual models is an important objective of hydrological sciences. We explore this challenge by examining the cumulative understanding of the hydrology of three experimental catchments and how it evolves through the application of different investigation techniques. The case study considers the Huewelerbach, Weierbach and Wollefsbach headwater catchments of the Attert basin in Luxembourg. Subsurface investigations including bore holes and pits, analysis of soil samples and Electrical Resistivity Tomography measurements are presented and discussed. Streamflow and tracer data are used to gain further insights into the streamflow dynamics of the catchments, using end-member mixing analysis and hydrograph separation based on dissolved silica and electrical conductivity. We show that the streamflow generating processes in all three catchments are controlled primarily by the subsolum and underlying bedrock. In the Huewelerbach, the permeable sandstone formation supports a stable groundwater component with little seasonality, which reaches the stream through a series of sources at the contact zone with the impermeable marls formation. In the Weierbach, the schist formation is relatively impermeable and supports a 'fill and spill'-type of flow mechanism; during wet conditions, it produces a delayed response dominated by pre-event water. In the Wollefsbach, the impermeable marls formation is responsible for a saturation-excess runoff generating process, producing a fast and highly seasonal response dominated by event water. The distinct streamflow generating processes of the three catchments are represented qualitatively using perceptual models. The perceptual models are in turn translated into quantitative conceptual models, which simulate the hydrological processes using networks of connected reservoirs and transfer functions. More generally, the paper illustrates the evolution of perceptual models based on experimental fieldwork data, the translation of perceptual models into conceptual models and the value of different types of data for processes understanding and model representation. © 2014 John Wiley & Sons, Ltd.

Beschreibung: Laboratory experiments were used to investigate the influence of simulated cracks and roots on soil water repellency (SWR) dynamics with and without basal drainage impedance in wetting-drying cycles. Observations and measurements were taken following water application equivalent to 9.2-mm rainfall and then periodically during 80h of drying. In total, 180 experiments were carried out using 60 samples of three homogeneous, reconstituted soils with different organic matter contents and textures, but of similar initial severity of SWR [18% molarity of an ethanol droplet (MED)]. Water flowing down the cracks and roots left the soil matrix largely dry and water repellent except for vertical zones adjacent to them and a shallow surface layer. A hydrophilic shallow basal layer was produced in experiments where basal drainage was impeded. During drying, changes in SWR were largely confined to the zones that had been wetted. Soil that had remained dry retained the initial severity of SWR, while wetted soil re-established either the same or slightly lower severity of SWR. In organic-rich soil, the scale of recovery to pre-wetting MED levels was much higher, perhaps associated with temporarily raised levels (up to 36% MED) of SWR recorded during drying of these soils. With all three soils, the re-establishment of the original SWR level was less widespread for surface than subsurface soil and with impeded than unimpeded basal drainage. Key findings are that as follows: (1) with unimpeded basal drainage, the soils remained at pre-wetting repellency levels except for a wettable thin surface layer and zones close to roots and cracks, (2) basal drainage impedance produced hydrophilic basal and surface layers, (3) thorough wetting delayed a return to water-repellent conditions on drying, and (4) temporarily enhanced SWR occurred in organic-rich soils at intermediate moisture levels during drying. Hydrological implications are discussed, and the roles of cracks and roots are placed into context with other influences on preferential flow and SWR under field conditions. © 2014 John Wiley & Sons, Ltd.

Beschreibung: In regions where aquifers sustain rivers, the location and quantification of groundwater discharge to surface water are important to prevent pollution hazards, to quantify and predict low flows and to manage water supplies. 222Rn is commonly used to determine groundwater discharge to rivers. However, using this isotopic tracer is challenging because of the high diffusion capacity of 222Rn in open water. This study illustrates how a combination of isotopic tracers can contribute to an enhanced understanding of groundwater discharge patterns in small rivers. The aim of this paper is to combine 222Rn and δ13CDIC to better constrain the physical parameters related to the degassing process of these tracers in rivers. The Hallue River (northern France) was targeted for this study because it is sustained almost exclusively by a fractured chalk aquifer. The isotopes 222Rn, δ13CDIC, δ2H and δ18O were analysed along with other natural geochemical tracers. A mass balance model was used to simulate 222Rn and δ13CDIC. The results of δ2H and δ18O analyses prove that evaporation did not occur in the river. The calibration of a numerical model to reproduce 222Rn and δ13CDIC provides a best-fit diffusive layer thickness of 3.21×10-5m. This approach is particularly useful for small rivers flowing over carbonate aquifers with high groundwater DIC where the evolution of river DIC reflects the competing processes of groundwater inflow and CO2 degassing. This approach provides a means to evaluate groundwater discharge in small ungauged rivers. © 2014 John Wiley & Sons, Ltd.

Beschreibung: This work examines future flood risk within the context of integrated climate and hydrologic modelling uncertainty. The research questions investigated are (1) whether hydrologic uncertainties are a significant source of uncertainty relative to other sources such as climate variability and change and (2) whether a statistical characterization of uncertainty from a lumped, conceptual hydrologic model is sufficient to account for hydrologic uncertainties in the modelling process. To investigate these questions, an ensemble of climate simulations are propagated through hydrologic models and then through a reservoir simulation model to delimit the range of flood protection under a wide array of climate conditions. Uncertainty in mean climate changes and internal climate variability are framed using a risk-based methodology and are explored using a stochastic weather generator. To account for hydrologic uncertainty, two hydrologic models are considered, a conceptual, lumped parameter model and a distributed, physically based model. In the conceptual model, parameter and residual error uncertainties are quantified and propagated through the analysis using a Bayesian modelling framework. The approach is demonstrated in a case study for the Coralville Dam on the Iowa River, where recent, intense flooding has raised questions about potential impacts of climate change on flood protection adequacy. Results indicate that the uncertainty surrounding future flood risk from hydrologic modelling and internal climate variability can be of the same order of magnitude as climate change. Furthermore, statistical uncertainty in the conceptual hydrological model can capture the primary structural differences that emerge in flood damage estimates between the two hydrologic models. © 2014 John Wiley & Sons, Ltd.

Beschreibung: In this study, the Hillslope River Routing (HRR) model was modified for arctic river basin applications and used to route surface and subsurface run-off from the Community Land Model (CLM) in the Mackenzie River Basin (MRB) for the period 2000-2004. The HRR modelling framework performs lateral surface and subsurface run-off routing from hillslopes and channel/floodplain routing. The HRR model was modified here to include a variable subsurface active layer thickness (ALT; permafrost) to enable subsurface water to resurface, a distributed surface storage component to store and attenuate the rapid generation of snowmelt water, compound hillslopes to account for the low relief near rivers and floodplains, and reservoir routing to complete the total surface and subsurface water storage accounting. To illustrate the new HRR model components, a case study is presented for the MRB. The basin is discretized into 5077 sub-basins based on a drainage network derived from the global digital elevation model (DEM) developed from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) sensor on board NASA's Terra satellite and river widths extracted from LandSat images. The median hillslope land area is 68.5km2 with a flow length of 2.8km. Gridded CLM surface and subsurface run-offs are remapped to the HRR model's irregular sub-basins. The role of each new model component is quantified in terms of peak annual streamflow (magnitude and timing) at select locations and basin-wide total water storage anomalies. The role of distributed surface storage is shown to attenuate the relatively rapid generation of snowmelt water, impact the annual peak hydrograph (reduced peaks by 〉30% and detailed peak by 〉20days), and account for 20% of the monthly total water storage anomalies averaged over the year and ranging from 14 to 25% (-10 to 30mm) throughout the year. Although additional research is needed to dynamically link spatially distributed ALT to HRR, the role of ALT is shown to be important. A basin-wide, uniform 1m ALT impacts the annual peak hydrograph (reduced peaks by 9% and detailed peak by 8days) and trends in total water storage anomalies. © 2014 John Wiley & Sons, Ltd.

Beschreibung: The temporal variability in nitrogen (N) transport in the Corbeira agroforestry catchment (NW Spain) was analysed from October 2004 to September 2008. Nitrate (NO〈inf〉3〈/inf〉-N) and total Kjeldahl nitrogen (TKN) loads and concentrations were determined at various timescales (annual, seasonal and event). The results revealed a strong intra-annual and inter-annual variability in N transport influenced by weather patterns and consequently by the hydrological regime. Mean annual export of total N in the catchment was 5.5kgha-1year-1, with NO〈inf〉3〈/inf〉-N being the dominant form. Runoff events comprised 10% of the study period but contributed 40 and 61% of the total NO〈inf〉3〈/inf〉-N and TKN loads, respectively. The NO〈inf〉3〈/inf〉-N and TKN concentrations were higher during runoff events than under baseflow conditions, pointing to diffuse sources of N. The mobilization of TKN during runoff events was attributed to surface runoff, while NO〈inf〉3〈/inf〉-N might be related to subsurface and groundwater flow. Runoff events were characterized by high variability in N loads and concentrations. Higher variability was observed in N loads than in N concentrations, indicating that event magnitude plays an important role in N transport in this catchment; event magnitude explained approximately 96% of the NO〈inf〉3〈/inf〉-N load. However, a combination of variables related to runoff event intensity (rainfall, discharge increase and kinetic energy) explained only 66% of the TKN load. © 2014 John Wiley & Sons, Ltd.

Beschreibung: There is global concern about headwater management and associated impacts on river flow. In many wet temperate zones peatlands can be found covering headwater catchments. In the UK there is major concern about how environmental change, driven by human interventions, has altered the surface cover of headwater blanket peatlands. However, the impact of such land-cover changes on river flow is poorly understood. In particular, there is poor understanding of the impacts of different spatial configurations of bare peat or well-vegetated, restored peat on river flow peaks in upland catchments. In this paper, a physically based, distributed and continuous catchment hydrological model was developed to explore such impacts. The original TOPMODEL, with its process representation being suitable for blanket peat catchments, was utilized as a prototype acting as the basis for the new model. The equations were downscaled from the catchment level to the cell level. The runoff produced by each cell is divided into subsurface flow and saturation-excess overland flow before an overland flow calculation takes place. A new overland flow module with a set of detailed stochastic algorithms representing overland flow routing and re-infiltration mechanisms was created to simulate saturation-excess overland flow movement. The new model was tested in the Trout Beck catchment of the North Pennines of England and found to work well in this catchment. The influence of land cover on surface roughness could be explicitly represented in the model and the model was found to be sensitive to land cover. © 2014 John Wiley & Sons, Ltd.

Beschreibung: The traditional hydrological time series methods tend to focus on the mean of whichever variable is analysed but neglect its time-varying variance (i.e. assuming the variance remains constant). The variances of hydrological time series vary with time under anthropogenic influence. There is evidence that extensive well drilling and groundwater pumping can intercept groundwater run-off and consequently induce spring discharge volatility or variance varying with time (i.e. heteroskedasticity). To investigate the time-varying variance or heteroskedasticity of spring discharge, this paper presents a seasonal autoregressive integrated moving average with general autoregressive conditional heteroskedasticity (SARIMA-GARCH) model, whose the SARIMA model is used to estimate the mean of hydrological time series, and the GARCH model estimates its time-varying variance. The SARIMA-GARCH model was then applied to the Xin'an Springs Basin, China, where extensive groundwater development has occurred since 1978 (e.g. the average annual groundwater pumping rates were less than 0.20m3/s in the 1970s, reached 1.20m3/s at the end of the 1980s, surpassed 2.0m3/s in the 1990s and exceeded 3.0m3/s by 2007). To identify whether human activities or natural stressors caused the heteroskedasticity of Xin'an Springs discharge, we segmented the spring discharge sequence into two periods: a predevelopment stage (i.e. 1956-1977) and a developed stage (i.e. 1978-2012), and set up the SARIMA-GARCH model for the two stages, respectively. By comparing the models, we detected the role of human activities in spring discharge volatility. The results showed that human activities caused the heteroskedasticity of the Xin'an Spring discharge. The predicted Xin'an Springs discharge by the SARIMA-GARCH model showed that the mean monthly spring discharge is predicted to continue to decline to 0.93m3/s in 2013, 0.67m3/s in 2014 and 0.73m3/s in 2015. © 2014 John Wiley & Sons, Ltd.

Beschreibung: We examine how tracer studies have enhanced our understanding of flow paths, residence times and sources of stream flow in northern catchments. We define northern catchments as non-glacial sites in the temperate conifer/boreal/permafrost zone, focussing our review mainly on sites in North America and Europe. Improved empirical and theoretical understanding of hydrological functioning has advanced the analytical tools available for tracer-based hydrograph separations, derivation of transit time distributions and tracer-aided rainfall-runoff models that are better able to link hydrological response to storage changes. However, the lack of comprehensive tracer data sets still hinders development of a generalized understanding of how northern catchments will respond to change. This paucity of empirical data leads to many outstanding research needs, particularly in rapidly changing areas that are already responding to climatic warming and economic development. To continually improve our understanding of hydrological processes in these regions our knowledge needs to be advanced using a range of techniques and approaches. Recent technological developments for improved monitoring, distributed hydrological sensor systems, more economic analysis of large sample numbers in conjunction with novel, tracer-aided modelling approaches and the use of remote sensing have the potential to help the understanding of the northern hydrological systems as well as inform policy at a time of rapid environmental change. © 2014 John Wiley & Sons, Ltd.