ALBERT

Description: A spatially distributed representation of basin hydrology and transport processes in hydrologic models facilitates the identification of critical source areas and the placement of management and conservation measures. Floodplains are critical landscape features that differ from neighboring uplands in terms of their hydrological processes and functions. Accordingly, an important step in watershed modeling is the representation of floodplain and upland areas within a watershed. The aim of this study is (1) to evaluate four floodplain-upland delineation methods that use readily available topographic data (topographic wetness index, slope position, uniform flood stage, and variable flood stage) with regard to their suitability for hydrological models and (2) to introduce an evaluation scheme for the delineated landscape units. The methods are tested in three U.S. watersheds ranging in size from 334 to 629 km 2 with different climatic, hydrological and geomorphological characteristics. Evaluation of the landscape delineation methods includes visual comparisons, error matrices (i.e. cross-tabulations of delineated versus reference data), and geometric accuracy metrics. Reference data was obtained from SSURGO (Soil Survey Geographic database) and FEMA (Federal Emergency Management Agency) flood maps. Results suggest that the slope position and the variable flood stage method work very well in all three watersheds. Overall percentages of floodplain and upland areas allocated correctly were obtained by comparing delineated and reference data. Values range from 83 to 93 % for the slope position and from 80 to 95 % for the variable flood stage method. Future studies will incorporate these two floodplain-upland delineation methods into the subwatershed-based hydrologic model SWAT to improve the representation of hydrological processes within floodplain and upland areas. This article is protected by copyright. All rights reserved.

Description: Residue removal for biofuel production may have unintended consequences for N 2 O emissions from soils, and it is not clear how N 2 O emissions are influenced by crop residue removal from different tillage systems. Thus, we measured field-scale N 2 O flux over five years (2005-2007, 2010-2011) from an annual crop rotation to evaluate how N 2 O emissions are influenced by no-till (NT) compared to conventional tillage (CV), and how crop residue removal (R-) rather than crop residue return to soil (R+) affects emissions from these two tillage systems. Data from all five years indicated no differences in N 2 O flux between tillage practices at the onset of the growing season, but CT had 1.4 to 6.3 times higher N 2 O flux than NT overwinter. Nitrous oxide emissions were higher due to R- compared to R+, but the effect was more marked under CT than NT and overwinter than during spring. Our results thus challenge the assumption based on IPCC methodology that crop residue removal will result in reduced N 2 O emissions. The potential for higher N 2 O emission with residue removal implies that the benefit of utilizing biomass as biofuels to mitigate greenhouse gas emission may be overestimated. Interestingly, prior to an overwinter thaw event soil dissolved organic C (DOC) was negatively correlated to peak N 2 O flux (r = -0.93). This suggests that lower N 2 O emissions with R+ vs R- may reflect more complete stepwise denitrification to N 2 during winter and possibly related to the heterotrophic microbial capacity for processing crop residue into more soluble C compounds and a shift in the preferential C source utilized by the microbial community overwinter. This article is protected by copyright. All rights reserved.

Description: Cosmic-ray soil moisture sensors have the advantage of a large measurement footprint (approximately 700 m in diameter) and are able to operate continuously to provide area-averaged near-surface (top 10-20 cm) volumetric soil moisture content at the field scale. This paper presents the application of this technique at four sites in southern England over almost 3 years. Results show the soil moisture response to contrasting climatic conditions during 2011-2014, and are the first such field-scale measurements made in the UK. These four sites are prototype stations for a UK COsmic-ray Soil Moisture Observing System (COSMOS-UK), and particular consideration is given to sensor operating conditions in the UK. Comparison of these soil water content observations with the Joint UK Land Environment Simulator (JULES) 10 cm soil moisture layer shows that these data can be used to test and diagnose model performance, and indicates the potential for assimilation of these data into hydro-meteorological models. The application of these large-area soil water content measurements to evaluate remotely-sensed soil moisture products is also demonstrated. Numerous applications and the future development of a national COSMOS-UK network are discussed. This article is protected by copyright. All rights reserved.

Description: Short-lived fallout isotopes, such as beryllium-7 ( 7 Be), are increasingly used as erosion and sediment tracers in watersheds. Beryllium-7 is produced in the atmosphere and delivered to Earth's surface primarily in precipitation. However, relatively little has been published about the variation in 7 Be wet deposition caused by storm type and vegetation cover. Our analysis of precipitation, throughfall, and sediments in two forested, headwater catchments in the mid-Atlantic USA indicates significant variation in isotope deposition with storm type and storm height. Individual summer convective thunderstorms were associated with 7 Be activity concentrations up to 5.0 Bq L −1 in precipitation and 4.7 Bq L −1 in throughfall while single-event wet depositional fluxes reached 168 Bq m −2 in precipitation and 103 Bq m −2 in throughfall. Storms originating from the continental USA were associated with lower 7 Be activity concentrations and single-event wet depositional fluxes for precipitation (0.7 – 1.2 Bq L −1 and 15.8 – 65.0 Bq m −2 ) and throughfall (0.1 – 0.3 Bq L −1 and 13.5 – 98.9 Bq m −2 ). Tropical systems had relatively low activity concentrations, 0.2 – 0.5 Bq L −1 in precipitation and 0.2 – 1.0 Bq L −1 in throughfall, but relatively high single-event depositional fluxes due to large rainfall volumes, 32.8 – 67.6 Bq m −2 in precipitation and 25.7 – 134 Bq m −2 in throughfall. The largest sources of 7 Be depositional variation were attributed to storm characteristics including precipitation amount and maximum storm height. 7 Be activity associated with fluvial suspended sediments also exhibited the highest concentration and variability in summer (175 – 1450 Bq kg −1 ). We conclude the dominant source of variation on event-level 7 Be deposition is storm type. Our results illustrate the complex relationships between 7 Be deposition in precipitation and throughfall and demonstrate event-scale relationships between the 7 Be in precipitation and on suspended sediment. This article is protected by copyright. All rights reserved.

Description: With increasing demands on limited water resources, regulation of larger river systems continues to increase and so too does the need for accurate water accounting and prediction in these systems. River system models are either calibrated manually or automatically on a reach–by–reach basis, i.e. each reach is calibrated as a separate entity with little or no consideration of fluxes at other locations within the river system. While this is a practical approach, simulation errors can propagate downstream to make calibration or prediction difficult at those locations. Likewise parameters may suffer from over-fitting especially where observations are erroneous. We developed and implemented a system calibration strategy in a portion of the Murrumbidgee River, Australia, where parameters for 11 gauges (36 parameters) were calibrated together. Parameter values, model states and model goodness of fit were compared to reach–by–reach calibration. The system calibration produced a better goodness of fit across the whole system relative to reach-by-reach calibration. Additionally, model system states were more realistic than reach-by-reach optimized models. Over-fitting was obvious using the reach-by-reach method for one reach/gauge in particular. This was avoided with system calibration method, with improved goodness of fit at all gauges downstream of the problem gauge. The results here suggest that the system calibration approach provides more hydrologically consistent states, improved overall fit and avoids over–fitting at problem gauges. This article is protected by copyright. All rights reserved.

Description: Municipalities may alter their storm water management focus depending on the most relevant processes (left); the analytical framework developed in this study can be used with measured soil properties to estimate the propensity of urban versus predeveloped reference soil profiles towards saturation‐excess overland flow (SEOF) or infiltration‐excess overland flow (IEOF), with 11 cities in the United States analysed (right). Abstract Uncontrolled overland flow drives flooding, erosion, and contaminant transport, with the severity of these outcomes often amplified in urban areas. In pervious media such as urban soils, overland flow is initiated via either infiltration‐excess (where precipitation rate exceeds infiltration capacity) or saturation‐excess (when precipitation volume exceeds soil profile storage) mechanisms. These processes call for different management strategies, making it important for municipalities to discern between them. In this study, we derived a generalized one‐dimensional model that distinguishes between infiltration‐excess overland flow (IEOF) and saturation‐excess overland flow (SEOF) using Green–Ampt infiltration concepts. Next, we applied this model to estimate overland flow generation from pervious areas in 11 U.S. cities. We used rainfall forcing that represented low‐ and high‐intensity events and compared responses among measured urban versus predevelopment reference soil hydraulic properties. The derivation showed that the propensity for IEOF versus SEOF is related to the equivalence between two nondimensional ratios: (a) precipitation rate to depth‐weighted hydraulic conductivity and (b) depth of soil profile restrictive layer to soil capillary potential. Across all cities, reference soil profiles were associated with greater IEOF for the high‐intensity set of storms, and urbanized soil profiles tended towards production of SEOF during the lower intensity set of storms. Urban soils produced more cumulative overland flow as a fraction of cumulative precipitation than did reference soils, particularly under conditions associated with SEOF. These results will assist cities in identifying the type and extent of interventions needed to manage storm water produced from pervious areas.

Description: Abstract In the Hanford Reach of the Columbia River, a thin layer of recent alluvium overlies the sedimentary formations that comprise the unconfined groundwater aquifer. Experimental and modelling studies have demonstrated that this alluvial layer exerts significant control on the exchange of groundwater and surface water (hydrologic exchange flux), and is associated with elevated levels of biogeochemical activity. This layer is also observed to be strongly heterogeneous, and quantifying the spatial distribution of properties over the range of scales of interest is challenging. Facies are elements of a sediment classification scheme that groups complex geologic materials into a set of discrete classes according to distinguishing features. Facies classifications have been used as a framework for assigning heterogeneous material properties to grid cells of numerical models of flow and reactive transport in subsurface media. The usefulness of such an approach hinges on being able to relate facies to quantitative properties needed for flow and reactive transport modelling, and on being able to map facies over the domain of interest using readily available information. Although aquifer facies have been used in various modelling contexts, application of this concept to riverbed sediments is relatively new. Here, we describe an approach for categorizing and mapping recent alluvial (riverbed) sediments based on the integration of diverse observations with numerical simulations of river hydrodynamics. The facies have distinct distributions of sediment texture that correspond to variations in hydraulic properties, and therefore provide a useful framework for assigning heterogeneous properties in numerical simulations of hydrologic exchange flows and biogeochemical processes.

Description: Climate change and thawing permafrost in the arctic will significantly alter landscape hydro-geomorphology and the distribution of soil moisture, which will have cascading effects on climate feedbacks (CO 2 and CH 4 ), and plant and microbial communities. Fundamental processes critical to predicting active layer hydrology are not well understood. This study applied water stable isotope techniques (δ 2 H and δ 18 O) to infer sources and mixing of active layer waters in a polygonal tundra landscape in Barrow, Alaska (USA) in August and September of 2012. Results suggested that winter precipitation did not contribute substantially to surface waters or subsurface active layer pore waters measured in August and September. Summer rain was the main source of water to the active layer, with seasonal ice-melt contributing to deeper pore waters later in the season. Surface water evaporation was evident in August from a characteristic isotopic fractionation slope (δ 2 H versus δ 18 O). Freeze-out isotopic fractionation effects in frozen active layer samples and textural permafrost were indistinguishable from evaporation fractionation, emphasizing the importance of considering the most likely processes in water isotope studies, in systems where both evaporation and freeze-out occur in close proximity. The fractionation observed in frozen active layer ice was not observed in liquid active layer pore waters. Such a discrepancy between frozen and liquid active layer samples suggests mixing of melt water, likely due to slow melting of seasonal ice. This research provides insight into fundamental processes relating to sources and mixing of active layer waters, which should be considered in process-based fine and intermediate scale hydrologic models. This article is protected by copyright. All rights reserved.

Description: Agricultural residues are important sources of feedstock for a cellulosic biofuels industry that is being developed to reduce greenhouse gas emissions and improve energy independence. While the US Midwest has been recognized as key to providing maize stover for meeting near-term cellulosic biofuel production goals, there is uncertainty that such feedstocks can produce biofuels that meet federal cellulosic standards. Here we conducted extensive site-level calibration of the Environmental Policy Integrated Climate (EPIC) terrestrial ecosystems model and applied the model at high spatial resolution across the US Midwest to improve estimates of the maximum production potential and greenhouse gas emissions expected from continuous maize-residue-derived biofuels. A comparison of methodologies for calculating the soil carbon impacts of residue harvesting demonstrates the large impact of study duration, depth of soil considered and inclusion of litter carbon in soil carbon change calculations on the estimated greenhouse gas intensity of maize-stover-derived biofuels. Using the most representative methodology for assessing long-term residue harvesting impacts, we estimate that only 5.3 billion liters per year (bly) of ethanol, or 8.7% of the near-term US cellulosic biofuel demand, could be met under common no-till farming practices. However, appreciably more feedstock becomes available at modestly higher emissions levels, with potential for 89.0 bly of ethanol production meeting US advanced biofuel standards. Adjustments to management practices, such as adding cover crops to no-till management, will be required to produce sufficient quantities of residue meeting the greenhouse gas emission reduction standard for cellulosic biofuels. Considering the rapid increase in residue availability with modest relaxations in GHG reduction level, it is expected that management practices with modest benefits to soil carbon would allow considerable expansion of potential cellulosic biofuel production. This article is protected by copyright. All rights reserved.

Description: Municipalities and agencies use green infrastructure to combat pollution and hydrological impacts (e.g., flooding) related to excess stormwater. Bioretention cells are one type of infiltration green infrastructure (GI) intervention that infiltrate and redistribute otherwise uncontrolled stormwater volume. However, the effects of these installations on the rest of the local water cycle is understudied; in particular, impacts on stormwater return flows and groundwater levels are not fully understood. In this study, full water cycle monitoring data was used to construct and calibrate a two-dimensional Richards equation model (HYDRUS-2D/3D) detailing hydrological implications of an unlined bioretention cell (Cleveland, Ohio) that accepts direct runoff from surrounding impervious surfaces. Using both pre- and post-installation data, the model was used to: 1) establish a mass balance to determine reduction in stormwater return flow, 2) evaluate GI effects on subsurface water dynamics, and 3) determine model sensitivity to measured soil properties. Comparisons of modeled versus observed data indicated that the model captured many hydrological aspects of the bioretention cell, including subsurface storage and transient groundwater mounding. Model outputs suggested that the bioretention cell reduced stormwater return flows into the local sewer collection system, though the extent of this benefit was attenuated during high inflow events that may have exhausted detention capacity. The model also demonstrated how, prior to bioretention cell installation, surface and subsurface hydrology were largely decoupled, whereas after installation, exfiltration from the bioretention cell activated a new groundwater dynamic. Still, the extent of groundwater mounding from the cell was limited in spatial extent, and did not threaten other subsurface infrastructure. Finally, the sensitivity analysis demonstrated that the overall hydrological response was regulated by the hydraulics of the bioretention cell fill material, which controlled water entry into the system, and by the water retention parameters of the native soil, which controlled connectivity between the surface and groundwater.