Transactions of the ASABE Vol. 50(4): 1177-1187 E 2007 American Society of Agricultural and Biological Engineers ISSN 0001-2351 1177 APPLICATION OF SWAT AND APEX USING THE SWAPP (SWAT ‐APEX) PROGRAM FOR THE UPPER NORTH BOSQUE RIVER W ATERSHED IN TEXAS A. Saleh, O. Gallego ABSTRACT. Environmental models are used to assess and evaluate various best management practices (BMPs) at field (e.g., Agricultural Policy/Environmental eXtender, APEX) and watershed (e.g., Soil and Water Assessment Tool, SWAT) levels. However, models such as SWAT and APEX are only capable of simulating, mechanistically, a limited number of BMP scenarios individually. Therefore, this study was conducted to: (1) develop an automated program referred to as SWAPP to convert SWAT files to and from APEX format and simulate SWAT and APEX simultaneously, and (2) to evaluate this program using measured data from the upper North Bosque River watershed in central Texas. In this study, SWAT (version 2000) and APEX (version 2110) were utilized within SWAPP. Simulations from SWAT alone (SWAT‐A) and the combined SWAT and APEX models in SWAPP were compared with the historical measured data collected in the upper North Bosque River (UNBR) watershed. In the first phase of this study, SWAT‐A was applied to simulate all land uses in the UNBR watershed. In the second phase, stream flow and losses of sediment and nutrients from all crop and pasture lands within the UNBR watershed were simulated by APEX. The results obtained from APEX, and remaining land uses in the UNBR watershed (e.g., urban) simulated by SWAT, were then routed to the outlet of the watershed using the SWAT routing function. The measured flow, sediment, and nutrients at three sites within the UNBR watershed were simulated by SWAT‐A and the combined SWAT and APEX within SWAPP for the period of January 1994 through July 1999. The results obtained from the three sites within the UNBR watershed indicate that the pattern (Nash‐Sutcliffe efficiencies, E) and average monthly values of flow and loadings predicted by the combined SWAT and APEX within SWAPP were similar, and in some cases (mainly for sediment and nutrients loadings) closer to the measured values, as compared to SWAT‐A. For instance, the E values for flow obtained from SWAPP ranged from 0.65 to 0.74, while those from SWAT‐A ranged from 0.55 to 0.75. The E value for sediment loading at three sampling sites ranged from 0.55 to 0.74 for SWAPP, as compared to 0.41 to 0.72 for SWAT‐A, and the E values for nutrient loadings ranged from -0.04 to 0.88 for SWAPP, compared to -0.74 to 0.82 for SWAT‐A. The results obtained from this study indicated that the SWAPP program not only improved the modeling results, but also provides an opportunity to utilize APEX within the SWAPP program for simulating management practices, such as multicropping or filter strips, at the field level, whereas SWAT alone currently has limited capability to simulate these practices. In addition, SWAPP can be used to rapidly convert SWAT data files, generated from Geographical Information System (GIS) layers, to APEX data file format. Keywords. APEX, GIS, Modeling, Nutrient, Sediment, SWAT, Water quality. n response to the Clean Water Act of the early 1970s, the USDA Agricultural Research Service (ARS) initiated the development of several process‐based, nonpoint‐ source (NPS) models. At the field scale, Knisel (1980) developed the Chemical, Runoff, and Erosion from Agricultural Management Systems (CREAMS) model, to simulate the impact of land management on water, sediment, nutrients, and pesticides leaving the edge‐of‐field. The Submitted for review in May 2006 as manuscript number SW 6460; approved for publication by the Soil & Water Division of ASABE in April 2007. The authors acknowledge support from the U.S. Environmental Protection Agency (USEPA) for providing funding for the project under Contract No. R‐82680701. The views expressed in this article are not necessarily those of the USEPA. The authors are Ali Saleh, Research Scientist, and Oscar Gallego, Assistant Research Scientist, Texas Institute for Applied Environmental Research, Tarleton State University, Stephenville, Texas. Corresponding author: Ali Saleh, Texas Institute for Applied Environmental Research, Tarleton State University, P.O. Box T410, Stephenville, TX 76401; phone: 254‐968‐9799; fax: 254‐968‐9790; e‐mail: saleh@tiaer.tarleton.edu. Erosion‐Productivity Impact Calculator (EPIC) model (Williams, 1990), also a field‐scale model, was initially developed to simulate the impact of erosion on crop productivity and has since evolved into a comprehensive agricultural management and NPS source loading model. EPIC can be used to compare management systems and their effects on nitrogen, phosphorus, carbon, pesticides, and sediment. The management components that can be changed in EPIC are crop rotations, tillage operations, irrigation scheduling, drainage, furrow diking, liming, grazing, tree pruning, harvest, manure handling, nutrient, and pesticide application rates and timing. The APEX (Agricultural Policy/Environmental eXtender) model was developed to extend the EPIC model capabilities to whole farms and small watersheds (Williams et al., 2000). The individual field simulation components of EPIC were used for APEX development. In addition to the EPIC functions, APEX has components for routing water, sediment, nutrients, and pesticides across complex landscapes and channel systems to the watershed outlet. Recently, the carbon fate and transport functions of the I