Erin S. Brooks

A GIS-based variable source area hydrology model

Effective control of nonpoint source pollution from contaminants transported by runoff requires information about the source areas of surface runoff. Variable source hydrology is widely recognized by hydrologists, yet few methods exist for identifying the saturated areas that generate most runoff in humid regions. The Soil Moisture Routing model is a daily water balance model that simulates the hydrology for watersheds with shallow sloping soils. The model combines elevation, soil, and land use data within the geographic information system GRASS, and predicts the spatial distribution of soil moisture, evapotranspiration, saturation-excess overland flow (i.e., surface runoff), and interflow throughout a watershed. The model was applied to a 170 hectare watershed in the Catskills region of New York State and observed stream flow hydrographs and soil moisture measurements were compared to model predictions. Stream flow prediction during non-winter periods generally agreed with measured flow resulting in an average r2 of 0·73, a standard error of 0·01 m3/s, and an average Nash-Sutcliffe efficiency R2 of 0·62. Soil moisture predictions showed trends similar to observations with errors on the order of the standard error of measurements. The model results were most accurate for non-winter conditions. The model is currently used for making management decisions for reducing non-point source pollution from manure spread fields in the Catskill watersheds which supply New York Citys drinking water. Copyright

Application of SMR to modeling watersheds in the Catskill Mountains

Very few hydrological models commonly used in watershed management are appropriate for simulating the saturation excess runoff. The Soil Moisture Routing model (SMR) was developed specifically to predict saturation excess runoff from variable source areas, especially for areas where shallow interflow controls saturation. A recent version of SMR was applied to two rural catchments in the Catskill Mountains to evaluate its ability to simulate the hydrology of these systems. Only readily available meteorological, topographical, and landuse information from published literature and governmental agencies was used. Measured and predicted streamflows showed relatively good agreement; the average Nash–Sutcliffe efficiency for the two watersheds were R2=72% and R2=63%. Distributed soil moisture contents and the locations of hydrologically sensitive areas were also predicted well.

Model for Prioritizing Best Management Practice Implementation: Sediment Load Reduction

Understanding the best way to allocate limited resources is a constant challenge for water quality improvement efforts. The synoptic approach is a tool for geographic prioritization of these efforts. It uses a benefit-cost framework to calculate indices for functional criteria in subunits (watersheds, counties) of a region and then rank the subunits. The synoptic approach was specifically designed to incorporate best professional judgment in cases where information and resources are limited. To date, the synoptic approach has been applied primarily to local or regional wetland restoration prioritization projects. The goal of this work was to develop a synoptic model for prioritizing watersheds within which suites of agricultural best management practices (BMPs) can be implemented to reduce sediment load at the watershed outlets. The model ranks candidate watersheds within an ecoregion or river basin so that BMP implementation within the highest ranked watersheds will result in the most sediment load reduction per conservation dollar invested. The model can be applied anywhere and at many scales provided that the selected suite of BMPs is appropriate for the evaluation area’s biophysical and climatic conditions. The model was specifically developed as a tool for prioritizing BMP implementation efforts in ecoregions containing watersheds associated with the USDA-NRCS conservation effects assessment project (CEAP). This paper presents the testing of the model in the little river experimental watershed (LREW) which is located near Tifton, Georgia, USA and is the CEAP watershed representing the southeastern coastal plain. The application of the model to the LREW demonstrated that the model represents the physical drivers of erosion and sediment loading well. The application also showed that the model is quite responsive to social and economic drivers and is, therefore, best applied at a scale large enough to ensure differences in social and economic drivers across the candidate watersheds. The prioritization model will be used for planning purposes. Its results are visualized as maps which enable resource managers to identify watersheds within which BMP implementation would result in the most water quality improvement per conservation dollar invested.

Agricultural BMP Effectiveness and Dominant Hydrological Flow Paths: Concepts and a Review

We present a conceptual framework that relates agricultural best management practice (BMP) effectiveness with dominant hydrological flow paths to improve nonpoint source (NPS) pollution management. We use the framework to analyze plot, field and watershed scale published studies on BMP effectiveness to develop transferable recommendations for BMP selection and placement at the watershed scale. The framework is based on the location of the restrictive layer in the soil profile and distinguishes three hydrologic land types. Hydrologic land type A has the restrictive layer at the surface and BMPs that increase infiltration are effective. In land type B1, the surface soil has an infiltration rate greater than the prevailing precipitation intensity, but there is a shallow restrictive layer causing lateral flow and saturation excess overland flow. Few structural practices are effective for these land types, but pollutant source management plans can significantly reduce pollutant loading. Hydrologic land type B2 has deep, well-draining soils without restrictive layers that transport pollutants to groundwater via percolation. Practices that increased pollutant residence time in the mixing layer or increased plant water uptake were found as the most effective BMPs in B2 land types. Matching BMPs to the appropriate land type allows for better targeting of hydrologically sensitive areas within a watershed, and potentially more significant reductions of NPS pollutant loading.

Residual phosphorus in runoff from successional forest on abandoned agricultural land: 1. Biogeochemical and hydrological processes

Soluble reactive phosphorus (SRP)concentrations measured in runoff fromabandoned agricultural land now in forestsuccession in the northeastern United Stateswere significantly higher than expected fromundisturbed forest land. This finding differsfrom P uptake in hardwood forest successionfollowing natural disturbance. Fieldmonitoring of a 16.6 ha old-field regrowthforest stand in the Catskills Mountains, NewYork, USA demonstrated runoff SRP trendsincluding an early summer flush that could notbe explained by simple dilution. An assay ofoutflow sediment and biomass, flowpath sedimentand biomass, forest floor leaf litter andbiomass, and Bh horizon mineral soil indicatedthat surface litter from the regrowth forestprovided the most significant contribution tothe elevated SRP in runoff. It is posited thatmicrobial mineralization of residual organic Pin surface litter coupled with the transientprocess of SRP mobilization at the soil surfaceresulting from a rising saturated layerfollowed by dissolution in surface runoff mayelevate SRP to the range observed. MeasuredSRP concentrations remain lower than reportedvalues for crop or pastureland. The resultsreported represent an important deviation fromthe prevailing view that forest land does notcontribute to eutrophication (based on enhancedP uptake in forest succession); this is aconsequence of residual P from landabandonment – a widespread practice throughoutthe northeastern US and other regions.

Long-term sediment loading trends in the Paradise Creek watershed

The Northwest Wheat and Range Region is historically known for high soil erosion rates. During the 1920s and 1930s, erosion rates of 200 to 450 t ha−1 (90 to 200 tn ac−1) in a single winter season were observed. Improved soil conservation practices over the last 80 years have significantly reduced soil erosion rates, yet there is scarce evidence of significant reductions in sediment loading delivered by streams in the region. In this paper, detailed monitoring data collected in the Paradise Creek watershed, located in the high precipitation zone of the Northwest Wheat and Range Region in north central Idaho, provided an opportunity to assess the impacts of management practices on sediment loading at the watershed outlet. Both detailed event-based sampling over the last eight years and three day per week grab samples collected over the last 28 years indicate a statistically significant decreasing trend in overall sediment load. This decreasing sediment load can be attributed primarily to conversion from conventional tillage systems to minimum tillage and perennial grasses through the Conservation Reserve Program, practices initiated in the late 1970s and early 1980s. Over the last 10 years (1999 to 2009), management practices have targeted gully erosion and stream bank failures. Upstream and downstream sampling shows a larger than expected increase in sediment load through the urban areas of the watershed. Preliminary modeling results and empirical evidence indicate that delayed reduction in sediment load at the watershed outlet and the increased sediment load through the lower urban portion of the watershed may be caused by sediment storage in the stream channel.

Evaluating opportunities for an increased role of winter crops as adaptation to climate change in dryland cropping systems of the U.S. Inland Pacific Northwest

The long-term sustainability of wheat-based dryland cropping systems in the Inland Pacific Northwest (IPNW) of the United States depends on how these systems adapt to climate change. Climate models project warming with slight increases in winter precipitation but drier summers for the IPNW. These conditions combined with elevated atmospheric CO2, which promote crop growth and improve transpiration-use efficiency, may be beneficial for cropping systems in the IPNW and may provide regional opportunities for agricultural diversification and intensification. Crop modeling simulation under future climatic conditions showed increased wheat productivity for the IPNW for most of the century. Water use by winter wheat was projected to decrease significantly in higher and intermediate precipitation zones and increase slightly in drier locations, but with winter crops utilizing significantly more water overall than spring crops. Crop diversification with inclusion of winter crops other than wheat is a possibility depending on agronomic and economic considerations, while substitution of winter for spring crops appeared feasible only in high precipitation areas. Increased weed pressure, higher pest populations, expanded ranges of biotic stressors, and agronomic, plant breeding, economic, technology, and other factors will influence what production systems eventually prevail under future climatic conditions in the region.

Variable Source Area Hydrology Modeling with the Water Erosion Prediction Project Model

In nondegraded watersheds of humid climates, subsurface flow patterns determine where the soil saturates and where surface runoff is occurring. Most models necessarily use infiltration-excess (i.e., Hortonian) runoff for predicting runoff and associated constituents because subsurface flow algorithms are not included in the model. In this article, we modify the Water Erosion Prediction Project (WEPP) model to simulate subsurface flow correctly and to predict the spatial and temporal location of saturation, the associated lateral flow and surface runoff, and the location where the water can re-infiltrate. The modified model, called WEPP-UI, correctly simulated the hillslope drainage data from the Coweeta Hydrologic Laboratory hillslope plot. We applied WEPP-UI to convex, concave, and S-shaped hillslope profiles, and found that multiple overland flow elements are needed to simulate distributed lateral flow and runoff well. Concave slopes had the greatest runoff, while convex slopes had the least. Our findings concur with observations in watersheds with saturation-excess overland flow that most surface runoff is generated on lower concave slopes, whereas on convex slopes runoff infiltrates before reaching the stream. Since the WEPP model is capable of simulating both saturation-excess and infiltration-excess runoff, we expect that this model will be a powerful tool in the future for managing water quality.

Isotope hydrology and baseflow geochemistry in natural and human-altered watersheds in the Inland Pacific Northwest, USA.

This study presents a stable isotope hydrology and geochemical analysis in the inland Pacific Northwest (PNW) of the USA. Isotope ratios were used to estimate mean transit times (MTTs) in natural and human-altered watersheds using the FLOWPC program. Isotope ratios in precipitation resulted in a regional meteoric water line of δ2H = 7.42·δ18O + 0.88 (n = 316; r2 = 0.97). Isotope compositions exhibited a strong temperature-dependent seasonality. Despite this seasonal variation, the stream δ18O variation was small. A significant regression (τ = 0.11D−1.09; r2 = 0.83) between baseflow MTTs and the damping ratio was found. Baseflow MTTs ranged from 0.4 to 0.6 years (human-altered), 0.7 to 1.7 years (mining-altered), and 0.7 to 3.2 years (forested). Greater MTTs were represented by more homogenous aqueous chemistry whereas smaller MTTs resulted in more dynamic compositions. The isotope and geochemical data presented provide a baseline for future hydrological modelling in the inland PNW.

Hydrologic processes in valley soilscapes of the eastern Palouse basin in northern Idaho

Vadose zone hydrology in the eastern Palouse Basin of northern Idaho is poorly understood because loess deposits often contain multiple hydraulically restrictive horizons that impede water flow. Valley soilscapes are of particular interest from a hydrologic perspective, because during the winter months, most of the precipitation is redistributed as runoff and throughflow into these landscape positions. Understanding the relationship between near-surface perched water table dynamics and vadose zone hydraulic processes in valley soilscapes is necessary to assess the sustainability of the groundwater resource. We implemented a combined approach to assess hydrologic processes in valley positions using hydrometric measurements, natural tracers, and stratigraphic observations. Hydrographs of near-surface monitoring wells indicate that valley positions maintain a thicker zone of saturation for longer duration compared with adjacent upland positions. Deep tensiometers demonstrate that multiple zones of seasonal saturation develop within the vadose zone of valley soilscapes in response to paleosol fragipan horizons and sediments of contrasting hydraulic conductivity. In some instances, the saturated thickness of vadose zone water tables was greater than 2.0 m and, because they are confined, displayed a positive pressure head. On adjacent uplands, seasonal saturation occurs only above the uppermost fragipan. Eluvial horizons having low Mnd correspond to zones of saturation, whereas aquitards reflect Mnd maxima. The recharge rate calculated using natural Cl− mass balance was 2.4 mm y−1 and did not correspond to measurements of saturated thickness by tensiometers. In addition, natural chloride profiles of other valley soilscapes display differences in recharge rates according to regional patterns in soil development. Together, deep tensiometer readings, secondary Mn distributions, and Cl− profiles suggest that groundwater recharge does not occur via piston flow. Detailed stratigraphic analysis illustrates that preferential flow is a possible recharge mechanism. Results suggest that valley soilscapes play an important role in both surficial and deep regolith hydrological processes in the Palouse Basin.