Application of SWAT and APEX Models Using SWAP (SWAT/APEX Program) for Upper North Bosque River Watershed in Texas

Abstract

In response to the Clean Water Act in early 1970s, the Agricultural Research Service (ARS) branchof United States Department of Agriculture (USDA) initiated the development of several processed-basednonpoint source models. These models are used to assess and evaluate various BMPs (best managementpractices) at field (using Agricultural Policy/Environmental eXtender, APEX) and watershed (using Soil WaterAssessment Tool, SWAT) levels. However, these models are capable of simulating a limited number ofscenarios individually. During this study the SWAP (SWAT/APEX Programs) program is developed to facilitatethe simultaneous use of these two models. The SWAT (version 2000) and APEX (version 1310) models areapplied using the SWAP program to the upper North Bosque River (UNBR) watershed located in central Texas.Flow and loadings (sediment, and nutrient) from various land uses, such as cropland and pasture, aresimulated by APEX and then are routed by SWAT within the SWAP program. SWAT alone (SWAT-A) andcombined SWAT and APEX models within SWAP program are calibrated and verified against historicalmonitoring data collected within UNBR watershed. The UNBR watershed is simulated from 1988 through 1999;model output is calibrated for flow, sediment, and nutrients measured at the outlet of UNBR watershed for theperiod of January 1994 through June 1995 and verified for the period of July 1995 to December 1998. Theresults of this study show that output from SWAT-A and SWAP are similarly close to measured values, whichindicates that the simulated field conditions by APEX can be routed by SWAT at the watershed level using theSWAP program.