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Subirrigation through subsurface tile drains has potential to increase crop yields and improve water quality in tile-drained landscapes, but it has not been widely implemented. Identifying locations with high potential suitability for subirrigation may help the planning and implementation of this practice. In this study, we developed a fuzzy rating system for subirrigation suitability using the Gridded Soil Survey Geographic Database (gSSURGO). Maps of the fuzzy rating system identified locations of high potential suitability for subirrigation and highlighted physiographic regions highly conducive to the practice. We identified 78,500 km2, about 9%, of agricultural land in the Midwest with high potential suitability for subirrigation where onsite investigation may be targeted. The largest areas of high potential suitability were found in Minnesota, Illinois, and Indiana. Results from the fuzzy rating analysis are provided to the public through three channels: a downloadable data repository, map service, and web map tool. Ultimately, this study can facilitate the adoption of subirrigation by highlighting areas where subirrigation may potentially be a viable practice. [ABSTRACT FROM AUTHOR]

A survey of the literature and in particular an examination of the terminology use in a previous special collection of modeling calibration and validation articles were conducted to arrive at a list of consistent terminology recommended for writing about hydrologic and water quality model calibration and validation. The terminology list includes rudimentary terms necessary for proper understanding of modeling literature for the novice modeler. This article also provides discussions regarding confusing or conflicting terminology found in the literature, alternative terms to those recommended herein, and alternative definitions for those terms that may be used by some authors. [ABSTRACT FROM AUTHOR]

Subsurface drainage is an important flow pathway in the poorly drained soils of the Midwestern U.S. and therefore needs to be included in modeling studies. The SWAT2012 tile drainage simulation method, based on the Hooghoudt and Kirkham equations with a drainage coefficient, shows promise for predicting drainage impacts in drained watersheds but has had limited application. In this study, SWAT2012 was implemented in a small agricultural watershed in Indiana using parameters based on knowledge of typical drainage systems and drainage design theory. Monthly Nash-Sutcliffe efficiency (NSE) values for streamflow exceeded 0.70 in all calibration and validation years but were lower for nutrients during some years, with values ranging between 0.14 and 0.88. Simulated tile flow compared well with measured values from field-scale studies in similar locations, ranging from 8.5% to 16.2% of annual precipitation when tile drains were implemented on 50.9% of the watershed. The depth to impermeable layer (DEP_IMP) parameter was found to be the most important calibration parameter, as it also controls seepage through the restrictive layer. Curve number values need to be substantially decreased for tile flow prediction, while two new tile drain parameters that control the ratio of lateral to vertical hydraulic conductivity (LATKSATF) and the static maximum surface roughness (SSTMAXD) likely need to be calibrated, as they could not be assigned values based on real-world properties. This study provides appropriate parameter values for the SWAT2012 tile drainage routine and demonstrates its ability to improve watershed-scale predictions in tile-drained watersheds. [ABSTRACT FROM AUTHOR]

The Soil and Water Assessment Tool (SWAT) uses hydrologic response units (HRUs) as the basic unit of all model calculations. ArcSWAT, the ArcGIS interface for SWAT, allows users to specify thresholds of land cover, soil, and slope in defining HRUs to improve the computational efficiency of simulations while keeping key landscape features of a watershed in the hydrologic modeling. However, this study found that applying commonly used thresholds in defining HRUs may lead to considerable loss of information about the watershed landscape, emphasizing larger soil types on smaller land covers once the land covers meet a threshold for land cover, and potentially changing average slopes. These changes often have a minor effect on water yield and streamflow simulations by SWAT but a larger effect on sediment and nutrient load simulations, which are more sensitive to slope and soil type and are more influential on outputs at the sub-watershed than at the watershed outlet. Study results can help modelers improve their understanding of the HRU strategy for simplifying watershed representation while maintaining major landscape features and make decisions in the HRU delineation process to minimize the chance of biased simulations. [ABSTRACT FROM AUTHOR]

At the hillslope profile and/or field scale, a simple Windows graphical user interface (GUI) is available to easily specify the slope, soil, and management inputs for application of the USDA Water Erosion Prediction Project (WEPP) model. Likewise, basic small watershed configurations of a few hillslopes and channels can be created and simulated with this GUI. However, as the catchment size increases, the complexity of developing and organizing all WEPP model inputs greatly increases due to the multitude of potential variations in topography, soils, and land management practices. For these types of situations, numerical approaches and special user interfaces have been developed to allow for easier WEPP model setup, utilizing either publicly available or user-specific geospatial information, e.g., digital elevation models (DEMs), geographic information system (GIS) soil data layers, and GIS land use/land cover data layers. We utilize the Topographic Parameterization (TOPAZ) digital landscape analysis tool for channel, watershed, and sub-catchment delineation and to derive slope inputs for each of the subcatchment hillslope profiles and channels. A user has the option of specifying a single soil and land management for each subcatchment or utilizing the information in soils and land use/land cover GIS data layers to automatically assign those values for each grid cell. Once WEPP model runs are completed, the output data are analyzed, results interpreted, and maps of spatial soil loss and sediment yields are generated and visualized in a GIS. These procedures have been used within a number of GIS platforms including GeoWEPP, an ArcView/ArcGIS extension that was the first geospatial interface to be developed in 2001. GeoWEPP allows experienced GIS users the ability to import and utilize their own detailed DEM, soil, and/or land use/land cover information or to access publicly available spatial datasets. A web-based GIS system that used MapServer web GIS software for handling and displaying the spatial data and model results was initially released in 2004. Most recently, Google Maps and OpenLayers technologies have been integrated into the web WEPP GIS software to provide significant enhancements. This article discusses in detail the logic and procedures for developing the WEPP model inputs, the various WEPP GIS interfaces, and provides example real-world geospatial WEPP applications. Further work is ongoing in order to expand these tools to allow users to customize their own inputs via the internet and to link the desktop GeoWEPP with the web-based GIS system. [ABSTRACT FROM AUTHOR]

A new version of the online Water Erosion Prediction Project (WEPP) GIS interface has been developed to assist in evaluating sediment sources associated with forests and forest management within the Great Lakes basin. WEPP watershed structure and topographical inputs for each watershed element are generated from the USGS 30 m National Elevation Dataset (NED), soil inputs are automatically retrieved from the USDA-NRCS SSURGO database, and land use and management inputs are selected from the WEPP database based on the USGS National Land Cover Database 2001 (NLCD2001). Additionally, ground cover and soil properties of the WEPP management and soil input files can be customized to represent site-specific conditions. Daily climate inputs are generated from long-term climate parameters using CLIGEN, a stochastic climate generator embedded in the online interface. Alternatively, a registered user can upload and use observed daily climate data for online WEPP simulation. Long-term observational data, including runoff and water chemistry, from two mature forest watersheds of the Fernow Experimental Forest in West Virginia were used to assess the online WEPP GIS interface. Online WEPP simulations were carried out using both observed and CLIGEN-generated climate inputs, and model performance was examined by comparing simulated and observed runoff and simulated and estimated (from measured water chemistry data) sediment yield. The online WEPP reasonably simulated average annual runoff and the annual maximum runoff series for both watersheds, but overpredicted sediment yield for the annual average and annual maximums. The online WEPP simulation results accurately reflected the differences between the two watersheds in their hydrological characteristics. The online WEPP GIS interface is a user-friendly, web-based computer package that can be used by scientists, researchers, and practitioners as a cost-effective simulation tool for watershed management. [ABSTRACT FROM AUTHOR]

Erosion models play an important role in assessing the influence of human activities on the environment. For cold areas, adequate frost simulation is crucial for predicting surface runoff and water erosion. The Water Erosion Prediction Project (WEPP) model is a physically based erosion prediction software program developed by the USDA. One of the major components of WEPP is the simulation of winter processes, which include snow accumulation and melt as well as soil freeze and thaw. WEPP is successfully used in the evaluation of important natural resource issues throughout the U.S. and in a number of other countries. However, previous studies revealed problems in the winter component of the WEPP model, especially the routine for frost simulation. The main purpose of this study was to improve the WEPP model (v2006.5) by changing the soil profile discretization and computation of key thermal and hydraulic parameters in the frost simulation routines so that the model can adequately simulate soil freeze-thaw and winter runoff and erosion. WEPP v2006.5 and the modified version (v2010.1) were applied to experimental plots in Pullman, Washington, and Morris, Minnesota. The simulated snow and frost depths as well as runoff and sediment yield were contrasted and compared with field observations, the results from v2010.1 showed substantial improvement compared to those from v2006.5. [ABSTRACT FROM AUTHOR]

Many soils in the Midwest need artificial drainage for economical crop production, but nitrate loss from subsurface tile drains can cause environmental problems downstream. The impact of tile drains on nitrate loss at the watershed scale is not well quantified. SWAT2005, with its modified tile drain component, was used to evaluate nitrate loss through tile drains in a heavily drained watershed, Sugar Creek watershed in Indiana. Monthly streamflow predictions resulted in Nash-Sutcliffe efficiency values of 0.88 for both the 5-year calibration period and the 6-year validation period. Monthly nitrate-N loads were also predicted well, with Nash-Sutcliffe efficiency values of 0.63 in the calibration period and 0.67 in the validation period. Nitrate-N load from tile drains was added as an output, and the simulated nitrate loss through tile drains was compared with nitrate losses from surface flow, groundwater, and lateral flow. The estimated median percentage of monthly nitrate loss entering Sugar Creek that flows through tile drains ranged from 0% to 37% and was more than 30% for three months, April to June. Results like these could be used in estimating potential nitrate reductions at the watershed level that would be achieved by implementing drainage-related conservation practices throughout the watershed. [ABSTRACT FROM AUTHOR]

Structural best management practices (BMPs) that reduce soil erosion and nutrient losses have been recommended and installed on agricultural land for years. A structural BMP is expected to be fully functional only for a limited period after installation, after which degradation of the BMP is likely to lead to a reduction in the water quality improvement provided by the BMP. Assessing the impact of BMPs on water quality is of widespread interest, but no standard methods exist to determine the water quality impact of structural BMPs, particularly as the impact changes through time. The objective of this study was to determine the long-term (∼20 year) impact of structural BMPs in two subwatersheds of Black Creek on sediment and phosphorus loads using the Soil and Water Assessment Tool (SWAT) model The BMPs were represented by modifying SWAT parameters to reflect the impact the practice has on the processes simulated within SWAT, both when practices are fully functional and as their condition deteriorates. The current condition of the BMPs was determined using field evaluation results from a previously developed BMP condition evaluation tool. Based on simulations in the two subwatersheds, BMPs in good condition reduced the average annual sediment yield by 16% to 32% and the average annual phosphorus yield by 10% to 24%. BMPs in their current condition reduced sediment yield by only 7% to 10% and phosphorus yield by 7% to 17%. [ABSTRACT FROM AUTHOR]

Although the U.S. Congress established the Total Maximum Daily Load (TMDL) program in the original Clean Water Act of 1972, Section 303(d), it did not receive attention until the 1990s. Currently, two methods are available for tracking pollution in the environment and assessing the effectiveness of the TMDL process on improving the quality of impaired water bodies: field monitoring and mathematical/computer modeling. Field monitoring may be the most appropriate method, but its use is limited due to high costs and extreme spatial and temporal ecosystem variability. Mathematical models provide an alternative to field monitoring that can potentially save time, reduce cost, and minimize the need for testing management alternatives. However, the uncertainty of the model results is a major concern. Uncertainty is defined as the estimated amount by which an observed or calculated value may depart from the true value, and it has important policy, regulatory, and management implications. The source and magnitude of uncertainty and its impact on TMDL assessment has not been studied in depth. This article describes the collective experience of scientists and engineers in the assessment of uncertainty associated with TMDL models. It reviews sources of uncertainty (e.g., input variability, model algorithms, model calibration data, and scale), methods of uncertainty evaluation (e.g., first-order approximation, mean value first-order reliability method, Monte Carlo, Latin hypercube sampling with constrained Monte Carlo, and generalized likelihood uncertainty estimation), and strategies for communicating uncertainty in TMDL models to users. Four case studies are presented to highlight uncertainty quantification in TMDL models. Results indicate that uncertainty in TMDL models is a real issue and should be taken into consideration not only during the TMDL assessment phase, but also in the design of BMPs during the TMDL implementation phase. First-order error (FOE) analysis and Monte Carlo simulation (MCS) or any modified versions of these two basic methods may be used to assess uncertainty. This collective study concludes that a more scientific method to account for uncertainty would be to develop uncertainty probability distribution functions and transfer such uncertainties to TMDL load allocation through the margin of safety component, which is selected arbitrarily at the present time. It is proposed that explicit quantification of uncertainty be made an integral part of the TMDL process. This will benefit private industry, the scientific community, regulatory agencies, and action agencies involved with TMDL development and implementation. [ABSTRACT FROM AUTHOR]