Aleksey Y. Sheshukov

Field-Level Targeting Using SWAT: Mapping Output from HRUs to Fields and Assessing Limitations of GIS Input Data

Soil erosion from agricultural fields is a fundamental water quality and quantity concern throughout the U.S. Watershed models can help target general areas where soil conservation measures are needed, but they have been less effective at making field-level recommendations. The objectives of this study were to demonstrate a method of field-scale targeting using ArcSWAT and to assess the impact of topography, soil, land use, and land management source data on field-scale targeting results. The study was implemented in Black Kettle Creek watershed (7,818 ha) in south-central Kansas. An ArcGIS toolbar was developed to post-process SWAT hydrologic response unit (HRU) output to generate sediment yields for individual fields. The relative impact of each input data source on field-level targeting was assessed by comparing ranked lists of fields on the basis of modeled sediment-yield density (Mg ha-1) from each data-source scenario. Baseline data of field-reconnaissance land use and management were compared to NASS and NLCD data, 10 m DEM topography were compared to 30 m, and SSURGO soil data were compared to STATSGO. Misclassification of cropland as pasture by NASS and aggregation of all cropland types to a single category by NLCD led to as much as 75% and 82% disagreement, respectively, in fields identified as having the greatest sediment-yield densities. Neither NASS nor NLCD data include land management data (such as terraces, contour farming, or no-till), but such inclusion changed targeted fields by as much as 71%. Impacts of 10 m versus 30 m DEM topographic data and STATSGO versus SSURGO soil data altered the fields targeted as having the highest sediment-yield densities to a lesser extent (about 10% to 25%). SWAT results post-processed to field boundaries were demonstrated to be useful for field-scale targeting. However, use of incorrect source data directly translated into incorrect field-level sediment-yield ranking, and thus incorrect field targeting. Sensitivity was greatest for land use data source, followed closely by inclusion of land management practices, with less sensitivity to topographic and soil data sources.

Opinion: Endogenizing culture in sustainability science research and policy

Integrating the analysis of natural and social systems to achieve sustainability has been an international scientific goal for years (1, 2). However, full integration has proven challenging, especially in regard to the role of culture (3), which is often missing from the complex sustainability equation. To enact policies and practices that can achieve sustainability, researchers and policymakers must do a better job of accounting for culture, difficult though this task may be.

Reservoir Sedimentation and Upstream Sediment Sources: Perspectives and Future Research Needs on Streambank and Gully Erosion

The future reliance on water supply and flood control reservoirs across the globe will continue to expand, especially under a variable climate. As the inventory of new potential dam sites is shrinking, construction of additional reservoirs is less likely compared to simultaneous flow and sediment management in existing reservoirs. One aspect of this sediment management is related to the control of upstream sediment sources. However, key research questions remain regarding upstream sediment loading rates. Highlighted in this article are research needs relative to measuring and predicting sediment transport rates and loading due to streambank and gully erosion within a watershed. For example, additional instream sediment transport and reservoir sedimentation rate measurements are needed across a range of watershed conditions, reservoir sizes, and geographical locations. More research is needed to understand the intricate linkage between upland practices and instream response. A need still exists to clarify the benefit of restoration or stabilization of a small reach within a channel system or maturing gully on total watershed sediment load. We need to better understand the intricate interactions between hydrological and erosion processes to improve prediction, location, and timing of streambank erosion and failure and gully formation. Also, improved process-based measurement and prediction techniques are needed that balance data requirements regarding cohesive soil erodibility and stability as compared to simpler topographic indices for gullies or stream classification systems. Such techniques will allow the research community to address the benefit of various conservation and/or stabilization practices at targeted locations within watersheds.

Seasonal and Annual Impacts of Climate Change on Watershed Response Using an Ensemble of Global Climate Models

Climate change impacts watershed hydrology and contributes to alteration of hydrologic regimes in streams. However, global climate models (GCMs) operate at spatial and temporal scales that are too large to capture important watershed-scale hydrologic shifts. A method of disaggregating monthly ensemble GCM data into temperature and precipitation data series for daily, watershed-specific hydrologic simulations with SWAT was developed and assessed in the Soldier Creek watershed in northeast Kansas. A stochastic weather generator (WINDS) was employed to produce a baseline scenario (no changes from late 20th century conditions) and two scenarios based on ensemble means of 15 GCMs representing future conditions (A2 storyline) referred to as the 2050 and 2100 scenarios. Future hydrologic regimes exhibited non-linear annual and monthly responses in hydrologic budget components, such as surface runoff, baseflow, and soil moisture, to temperature and precipitation changes. For the 2050 scenario, the combination of higher temperatures along with decreased annual precipitation and increased spring precipitation resulted in higher surface runoff, baseflow, and streamflow in May and June with a longer drought season later in summer. The significant decrease in streamflow, runoff, and baseflow for the 2100 scenario reflected an increase in monthly and annual temperature rather than a direct result of precipitation decline. The 2100 scenario also produced a reduction in low-flow duration, an increase in the number of drought occurrences, and a decrease in flood frequency and duration. In retrospect, use of the stochastic weather generator to temporally downscale monthly GCM results while incorporating site-specific climate variability (such as occurs with convective storms often missed in coarse-resolution GCM data) produced more meaningful analysis of hydrological impacts, which is critical to predicting and understanding the impacts of climate change. Although this method allowed simulation of future-climate shifts based on GCM-simulated monthly shifts, it could not simulate potential shifts in climate patterns within a month, such as changes in transitional probabilities that govern the intensity and distribution of storms with months. In future work, translation of regional climate model responses into WINDS stochastic parameter adjustments will allow more accurate and efficient simulation. The severity of the increased drought and decreased flood responses simulated in this study would not be anticipated by review of precipitation trends alone nor by analysis of annual hydrologic responses alone. Similarly, many critical hydrologic responses reflected interactions between climate variables (e.g., precipitation and temperature) at sub-annual temporal scales, which highlights the need to consider climatic interactions in future studies of climate change impacts.

Paying for sediment: Field-scale conservation practice targeting, funding, and assessment using the Soil and Water Assessment Tool

Watershed models have been widely used to estimate soil erosion and evaluate the effectiveness of conservation practices at different temporal and spatial scales; however, little progress has been made in applying these theoretical model results to the practical challenge of allocating conservation practice funding to meet specific soil loss objectives. Black Kettle Creek subwatershed (7,809 ha [19,295 ac]) of Little Arkansas River Watershed (360,000 ha [889,579 ac]) in south central Kansas was the focus of an innovative project to target conservation practice funding and pay directly for modeled sediment reduction. Detailed data (10 m [33 ft] digital elevation model topography, Soil Survey Geographic database soils, and a manually developed land use/land cover layer) were input into the Soil and Water Assessment Tool model, and the calibrated model was used to quantify soil erosion for each field. Effectiveness of locally relevant best management practices (BMPs) was simulated for each field. The simulated field-scale effectiveness for implemented BMPs ranged from 9% to 83% for single BMPs and 67% to 100% for selected combinations of BMPs. An in-field signup sheet was developed with field-specific sediment loss–based payments calculated for each BMP option. BMP implementation was 16.7% of cropland area prior (preinstalled BMPs) to the project, and 30.6% of cropland area (postinstalled BMPs) was added due to project-funded implementation. Postinstalled BMP implementation (47.3% of cropland) resulted in 35.8% sediment yield reduction compared to the no-BMPs scenario and 21.9% reduction compared to preinstalled BMP conditions, which was better than initially projected for this project. Inclusion of nontargeted fields and less-than-optimal BMPs had no influence on achieving soil loss objectives because payments were based on implemented soil loss rather than implemented area. Targeting of conservation practices based on payments scaled directly by project outcome (in this case, dollars per ton of sediment reduction) using a modeling approach allowed flexibility for both adopters (farmers) and funders (project staff) while assuring the project objective (i.e., sediment reduction) was met.

Non-equilibrium model for gravity-driven fingering in water repellent soils: Formulation and 2D simulations

The instability of infiltrating flow is studied using the mass balance equation coupled with a first-order relaxation equation relating the rate of change of saturation to the difference between the dynamic water pressure and the saturation-dependent equilibrium water pressure. A numerical solution of the mass balance equation, based on a mass conservative scheme, is applied to the simulation of infiltrating flows in a vertical, two-dimensional plane region. Both water wettable and water repellent soils are considered in the analysis. The effect of water repellency is introduced by modification of the equilibrium saturation pressure relationship, in which water repellency causes the relation to become flatter. Conditions of even slight water repellency are found to be sufficient to cause infiltrating flows to become unstable. A sensitivity analysis related to the width of the surface source shows that the number of fingers generated increases with increasing source width. The sensitivity analysis also indicates that the non-equilibrium model approach can provide a physically plausible reason for flows becoming stable when the surface flux becomes vanishingly small.

Simulation of two-dimensional gravity-driven unstable flow

The coupled equations for flow in unsaturated soil as proposed by Beliaev and Hassanizadeh [6] are described. These equations account for mechanism of dynamic capillary pressure via a first order relaxation function. A form of the relaxation function for the dynamic capillary pressure-saturation relation is proposed based on physical reasoning and a semi-analytical solution to the flow equations. A mass conservative and computational efficient numerical solution to the coupled equations in two space dimensions is derived and applied to the simulation of gravity-driven unstable flow. Simulated fingers have all the morphological features of fingers observed in laboratory experiments. The results demonstrate that the dynamic capillary pressure mechanism causes initial destabilization of the flow, while the mechanism of capillary hysteresis leads to finger persistence.

Stability analysis of traveling wave solution for gravity-driven flow

A linear stability analysis was performed for three models of flow in unsaturated porous media to determine the conditions for growth of small perturbations. The models considered include the conventional Richards equation (RE), a sharp front Richards equation (SFRE) and an extended Richards equation (RRE). The first two models are based on the use of an equilibrium capillary pressure-saturation function, while the third model is derived using a dynamic capillary pressure-saturation function represented by a relaxation coefficient. A traveling wave solution was formulated for each of the governing equations and used as the basic solution of each model. The stability analysis was based on imposing a small perturbation to the basic solution. The RE model yields only the well-known monotonically decreasing saturation profile toward the wetting front, and the wetting front is unconditionally stable. The SFRE model by its nature has a monotonically increasing saturation profile toward the front and an abrupt drop to the initial saturation. This flow is unconditionally unstable. The RRE model is distinct from the others in that it is the only model that is able to produce truly non-monotonic saturation profiles. The wetting front for the RRE model is conditionally stable, i.e. stable for high frequency perturbations, and unstable otherwise. This leads to the existence of a wave-number for maximum amplification, which should relate to the dimensions of fingers in unstable flow.

Pasture BMP effectiveness using an HRU-based subarea approach in SWAT

Many conservation programs have been established to motivate producers to adopt best management practices (BMP) to minimize pasture runoff and nutrient loads, but a process is needed to assess BMP effectiveness to help target implementation efforts. A study was conducted to develop and demonstrate a method to evaluate water-quality impacts and the effectiveness of two widely used BMPs on a livestock pasture: off-stream watering site and stream fencing. The Soil and Water Assessment Tool (SWAT) model was built for the Pottawatomie Creek Watershed in eastern Kansas, independently calibrated at the watershed outlet for streamflow and at a pasture site for nutrients and sediment runoff, and also employed to simulate pollutant loads in a synthetic pasture. The pasture was divided into several subareas including stream, riparian zone, and two grazing zones. Five scenarios applied to both a synthetic pasture and a whole watershed were simulated to assess various combinations of widely used pasture BMPs: (1) baseline conditions with an open stream access, (2) an off-stream watering site installed in individual subareas in the pasture, and (3) stream or riparian zone fencing with an off-stream watering site. Results indicated that pollutant loads increase with increasing stocking rates whereas off-stream watering site and/or stream fencing reduce time cattle spend in the stream and nutrient loads. These two BMPs lowered organic P and N loads by more than 59% and nitrate loads by 19%, but TSS and sediment-attached P loads remained practically unchanged. An effectiveness index (EI) quantified impacts from the various combinations of off-stream watering sites and fencing in all scenarios. Stream bank contribution to pollutant loads was not accounted in the methodology due to limitations of the SWAT model, but can be incorporated in the approach if an amount of bank soil loss is known for various stocking rates. The proposed methodology provides an adaptable framework for pasture BMP assessment and was utilized to represent a consistent, defensible process to quantify the effectiveness of BMP proposals in a BMP auction in eastern Kansas.

Impacts of Climate Change on Hydrologic Indices in a Northeast Kansas Watershed

Great Plains watersheds contribute to the diminishing North American unpolluted surface water supply as well as provide habitat for a number of threatened or endangered species. The number and size of these important systems has been greatly reduced by agriculture and urbanization. Climate change could pose an even greater threat to these endangered systems. There are many climate change scenarios that predict varying changes in future temperature and precipitation amounts for the Great Plains Region. In this study, two Global Climate Model (GCM) scenarios were analyzed to determine monthly rainfall and precipitation trends from 2000 to 2100. The trends were applied to the actual monthly precipitation and temperature distributions in a Northeast Kansas watershed. Daily weather data were simulated for 100 years using the WINDS weather generator. These simulations were input into the Soil and Water Assessment Tool (SWAT) hydrologic model. The streamflow output from these simulations was input into the Indicators of Hydrologic Alteration (IHA) software, which calculated multiple hydrologic indices that were compared back to a baseline scenario. The analysis showed that these climate change scenarios caused a varying increase in mean monthly streamflow patterns as well as a reduction in low flow occurrences and durations. Flood frequency and duration showed varying changes based on the individual scenario. This analysis shows that climate change scenarios have an effect on terrestrial and aquatic ecosystems in the Great Plains Region.