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Agricultural nonpoint source pollution is the main source of nitrogen (N) and phosphorus (P) in the intensely row-cropped Upper Mississippi River Basin (UMRB) stream system and is considered the primary cause of the northern Gulf of Mexico hypoxic zone according to the US Environmental Protection Agency. A point of crucial importance in this region is therefore how intensive corn (Zea maysL.)-based cropping systems for food and fuel production can be sustainable and coexist with a healthy water environment, not only under existing climate conditions but also under a changed climate in the future. To address this issue, a UMRB integrated modeling system has been built with a greatly refined 12-digit subbasin structure based on the Soil and Water Assessment Tool (SWAT) water quality model, which is capable of estimating landscape and in-stream water and pollutant yields in response to a wide array of alternative cropping and/or management strategies and climatic conditions. The effects of the following four agricultural management scenarios on crop production and pollutant loads exported from the cropland of the UMRB to streams and rivers were evaluated: (1) expansion of continuous corn across the entire basin, (2) adoption of no-till on all corn and soybean (Glycine maxL.) fields in the region, (3) substitution of the traditional continuous corn and corn–soybean rotations with an extended five-year rotation consisting of corn, soybean, and three years of alfalfa (Medicago sativaL.), and (4) implementation of a winter cover crop within the baseline rotations. The effects of each management scenario were evaluated both for current climate and a projected midcentury (2046 to 2065) climate from a General Circulation Model (GCM). All four scenarios behaved similarly under the historical and future climate, generally resulting in reduced erosion and nutrient loadings to surface water bodies compared to the baseline agricultural management. Continuous corn was the only scenario which resulted in increased N pollution while no-till was the most environmentally effective and able to sustain production at almost the same levels. Rye (Secale cerealeL.) cover crop within the fallow period was also effective in reducing erosion and both sediment-bound and soluble forms of nutrients. The results indicated that alternative management practices could reduce sediment, N, and P exports from UMRB cropland by up to 50% without significantly affecting yields. Results for the climate change scenario showed that the effectiveness of the management scenarios was strongly linked to the reduced water availability predicted under the future climate, which assisted in mitigating pollutant transport, although with a small loss of production.

AbstractExcessive nutrients transported from the Mississippi River Basin (MRB) have created a hypoxic zone within the Gulf of Mexico, with numerous negative ecological effects. Furthermore, federal expenditures on agricultural conservation practices have received intense scrutiny in recent years. Partly driven by these factors, the USDA Conservation Effects Assessment Project (CEAP) recently completed a comprehensive evaluation of nutrient sources and delivery to the Gulf. The modeling framework used in the CEAP Cropland National Assessment, or Cropland CEAP, consists of the Agricultural Policy/Environmental eXtender (APEX) and Soil and Water Assessment Tool (SWAT) models. This CEAP modeling framework was successfully calibrated for flow, sediment, and nutrients at 38 sites and validated at an additional 17. Simulation results indicated that cultivated cropland was the dominant source of nitrogen (N) and phosphorus (P) to both local waters and the Gulf, but this was not true for each water resource region within the MRB. In addition, the results showed that point sources remain significant contributors of P loads, especially in the Tennessee and Arkansas/Red River basins where point source P loads exceeded those from cultivated cropland. Similarly, urban nonpoint sources were significant nutrient sources. The Upper Mississippi, Lower Mississippi, and Ohio basins contributed the largest amounts of nutrients delivered to the Gulf. The high delivery areas near the Mississippi River main stem, from which 87% of N and 90% of P was predicted to reach the Gulf, also coincided with elevated nutrient yields to local waters. Conservation practices established on agricultural lands within the MRB were predicted to have reduced nutrient loads to the Gulf by 20% as compared with a no conservation condition. The results indicate the importance of targeted implementation of conservation practices and consideration of local water and/or Gulf impacts depending on program goal(s). The present application illustrates the value of the Cropland CEAP modeling framework as a useful, science-based tool to evaluate pollutant sources and delivery and effects of agricultural conservation practices.

AbstractAlthough subsurface drainage is a water management system widely used to maximize crop production in regions with seasonal high water tables, such as the midwestern United States, it is also a major source of nutrients into water bodies. Recently, physically based Hooghoudt and Kirkham tile drain equations were incorporated into the Soil and Water Assessment Tool (SWAT) model (herein referred to as Modified SWAT) as alternative tile flow simulation methods and a tool to design cost-effective and environment-friendly tile drain water management systems. The goal of this study was to determine a range of values for the new tile drain parameters and to use measured streamflow data from the South Fork Watershed (SFW) in Iowa to evaluate the capability of the Modified SWAT to simulate water balance components for this tile-drained watershed. This was accomplished by reviewing literature of tile drainage studies and by comparing measured streamflow with that predicted by the Modified SWAT using the Nash-Sutcliffe efficiency (NSE) and percent bias (PBIAS [%]) statistical methods in addition to hydrographs. During the calibration period, the Modified SWAT simulated streamflow very well (monthly NSE = 0.85 and PBIAS = ±2.3%). During the validation period, the Modified SWAT model simulated streamflow well (monthly NSE = 0.70 and PBIAS = ±2.5%). Simulated water balance results indicated that the soil water with tile drainage (260 mm [10 in]) was significantly (p-value = 0.00) lower than soil water without tile drainage (355 mm [14 in]), while streamflow with (205 mm [8 in]) tile drainage was significantly (p-value = 0.03) greater than streamflow without (128 mm[5 in]) tile drainage. This shows that the Hooghoudt steady-state and Kirkham tile drain equations are potential alternative tile flow simulation methods and tile drainage design tools in SWAT.

AbstractThe Leon River Watershed (LRW) in central Texas is a Benchmark and Special Emphasis watershed within the Conservation Effects Assessment Project located in central Texas. Model simulations from 1977 through 2006 were used to evaluate six manure nutrient management scenarios that reflect realistic strategies that could be employed to reduce nutrient and sediment loadings in the LRW. Due to the presence of several dairies and a relatively large number of cows, special attention within this watershed is necessary to determine beneficial adjustments that can be made to protect water quality. The nutrient management scenarios analyzed reduced total nitrogen (N) and total phosphorus (P) loading in nearly all of the subbasins. Total N and total P loading was also reduced at the watershed scale; sediment load reduction was minor due to effective management measures already existing within the LRW. The percentage change in total N and total P loadings varied from an increase of 3% to a decrease of 11% and an increase of 5% to a decrease of 12%, respectively. The percentage change in sediment loadings varied from an increase of 22% to a decrease of 12% per subbasin. Model simulations conducted over a 30-year period for six manure management scenarios indicated that both total N and total P can be significantly reduced by employing additional nutrient strategies. This is the case whether the nutrients are removed from the immediate vicinity of the dairy or are transported outside of the watershed.

The Leon River watershed which drains into Lake Belton, a primary drinking water supply for central Texas residents, is being affected by high-density dairy production and manure management. Our objective was to apply the Soil and Water Assessment Tool (SWAT) model to evaluate its ability to simulate the hydrology of the Leon River watershed including water discharge from treatment facilities, reservoirs, and point sources. The 2005 version of SWAT (SWAT2005) was calibrated and verified using hydrologic data from the watershed. Runoff was simulated well (0.65 < ENS≤ 0.75 [good]) to very well (ENS> 0.75 [very good]) based on the Nash-Sutcliffe efficiency (ENS) value. Average streamflow simulations agreed well with observed values during the calibration phase (PBIAS < ±10 [very good]), but the validation period agreement (PBIAS ≥ ±25 [unsatisfactory]) was less than desired because one of the five validated stream gauges fell into the unsatisfactory range. These results demonstrate the rigor needed to calibrate and validate simulation models for the Conservation Effects Assessment Project, and although additional studies are needed, they also confirm that SWAT2005 can be an effective tool for evaluating the hydrology within the Leon River watershed.

AbstractComputer based frameworks, including geographical information systems (GIS), associated digital data sets and integrated hydrological models, have become essential tools for the practical deployment of watershed assessment projects and ultimately for providing support for water quality protection and conservation. Within these frameworks digital soil information and data sets play a key role in defining the spatial distribution of important hydraulic variables and consequently of fundamental hydrologic functions connected with NPSs (Nonpoint Sources) and their modeling. This paper illustrates background and results from the development of a GIS based tool required for the use of Soil Survey Geographic (SSURGO) data sets (an undergoing development and delivery effort of digital soil maps by the Natural Resources Conservation Service) and primarily designed in specific support of the Soil and Water Assessment Tool (SWAT) model and its applications in the United States.The tool is developed as an addition or extension to the model companion, the ArcView GIS interface in its latest version (AVSWAT-X) which is designed to define watershed hydrologic features; store, organize, and manipulate the related spatial and tabular data; and analyze management scenarios. Within this framework the tool expedites the otherwise complex inclusion operations of the SSURGO data, such as: (1) downloading, via the Internet, up-to-date SSURGO data sets (version I or II); (2) processing and managing variously formatted data sets in order to create the needed digital soil maps; (3) generating and/or storing the required soil physical and hydraulic model input parameters derived from pedo-transfer functions; and (4) seamlessly including them in any watershed modeling framework. Besides the operational advantages, the tool addresses the trend of employing soil maps with increasing detail in order to improve the landscape representation and reduce the modeling drawbacks involved with the aggregation of the spatial input. Soil map features with unprecedented detail along with the elaborated attributes have a potential for enhancing modeling assessments in support of water quality programs (i.e. Total Maximum Daily Load) at the watershed as well as stream reach scale, as indicated in basic yet important highlights in the paper.

Groundwater recharge and discharge (base flow) estimates from two methods were compared in the Upper Mississippi River basin (USGS hydrologic cataloging unit 07). The Upper Mississippi basin drains 491,700km2in Illinois, Iowa, Missouri, Minnesota, and Wisconsin and outlets in the Mississippi River north of Cairo, Illinois. The first method uses the water balance components from the soil and water assessment tool model (SWAT). The model was used to simulate the daily water balance of approximately 16 soil/land use hydrologic response units (HRU) within each of the 131 USGS 8-digit watersheds. The water balance of each HRU is simulated with four storages: snow, soil (up to ten layers), shallow aquifer, and deep aquifer. Groundwater recharge is defined as water that percolates past the bottom soil layer into the shallow aquifer. Recharge is lagged to become base flow and can also be lost to ET. The second method consists of two procedures to estimate base flow and recharge from daily stream flow: (1) a digital recursive filter to separate base flow from daily flow and (2) a modified hydrograph recession curve displacement technique to estimate groundwater recharge. These procedures were applied to 283 USGS stations ranging in area from 50 to 1200km2. A smoothed surface was obtained using a thin plate spline technique and estimates were averaged for each 8-digit basin. Simulated flow was calibrated against average annual flow for each 8-digit. Without further calibration, simulated monthly stream flow was compared against measured flow at Alton, Illinois (445,000km2) from 1961–1980. To validate the model, measured and simulated monthly stream flow at Alton from 1981–1985 were compared with an R2of 0.65. No attempt was made to calibrate base flow and recharge independent of total stream flow. Base flow and recharge from both methods were shown to be in general agreement. The filter and recession methods have the potential to provide realistic estimates of base flow and recharge for input into regional groundwater models and as a check for surface hydrologic models.