Rangeland watershed study using the Agricultural Policy/Environmental eXtender

Abstract

Information on grazing land conservation practices and their effectiveness in controlling nonpoint source pollution and watershed health at the watershed/landscape scale is necessary for future planning and resource allocation. The objectives of this study were to test the ability of the Agricultural Policy/Environmental eXtender (APEX) model to simulate hydrologic and soil processes at the watershed scale and utilize the model to evaluate the benefit of rangeland in reducing sediment loading and the benefit of potential conservation practices within a rangeland watershed. The model was calibrated and validated for stream flow and sediment yield for two watersheds within the Cowhouse Creek Watershed in north-central Texas: Cowhouse Creek Watershed upstream of Pidcoke (CCUP) Watershed (1,178 km² [290,966 ac]) and Clear Creek (CC) Watershed (57.3 km² [14,153 ac]). Nash-Sutcliffe efficiencies for the watersheds ranged from 0.62 to 0.71 and 0.71 to 0.81 for monthly and annual stream flow, and from 0.51 to 0.84 and 0.60 to 0.84 for monthly and annual sediment yields, respectively. The percent errors ranged from −9.3% to 12.7% for stream flow and from −22.6% to 11.5% for sediment yield. In addition to APEX model calibration/validation, various modeling scenarios (e.g., cropland expansion and brush control/reduced grazing rate conservation practices) were derived and analyzed for the CCUP watershed. The cropland expansion scenario analysis (less than 0.1% continuous corn to 39% continuous corn) indicated that the current rangeland has the benefit of reducing sediment loading by 65%. Rangeland conservation practice scenario modeling identified substantial reductions in overland sediment losses for conversion of range brush to range grass on Evant soil areas, with an average reduction of 58.8% from treated areas. However, the sediment loading to the CCUP watershed outlet at Pidcoke was only reduced by 7% due to the low treated area of 6%. The reducing stocking rate scenario was less effective compared with brush removal since the current 10 ha cattle⁻¹ (24 ac cattle⁻¹) was not considered a heavy stocking rate. Reducing the current stocking rate to 15 ha cattle⁻¹ (37 ac cattle⁻¹) reduced sediment loading by 5%. This study shows that the APEX model is able to replicate measured stream flow and sediment yields for rangeland watersheds with satisfactory performance based on well-accepted statistical criteria.