Field Scale Modeling to Estimate Phosphorus and Sediment Load Reductions Using a Newly Developed Graphical User Interface for Soil and Water Assessment Tool

Abstract

Streams throughout the North Canadian River watershed in northwest Oklahoma, USA have elevated levels of nutrients and sediment. Soil and Water Assessment Tool (SWAT) was used to identify areas that likely contributed disproportionate amounts of Phosphorus (P) and sediment to Lake Overholser, the receiving reservoir at the watershed outlet. These sites were then targeted by the Oklahoma Conservation Commission (OCC) to implement conservation practices, such as conservation tillage and pasture planting as part of a US Environmental Protection Agency Section 319(h) project. Conservation practices were implemented on 238 fields. The objective of this project was to evaluate conservation practice effectiveness on these fields using the Texas Best Management Evaluation Tool (TBET), a simplified Graphic User Interface (GUI) for SWAT developed for field-scale application. TBET was applied on each field to predict the effects of conservation practice implementation on P and sediment loads. These predictions were used to evaluate the implementation cost (per kg of pollutant) associated with these reductions. Overall the implemented practices were predicted to reduce P loads to Lake Overholser by nine percent. The ‘riparian exclusion’ and ‘riparian exclusion with buffer’ practices provided the greatest reduction in P load while ‘conservation tillage’ and ‘converting wheat to bermuda grass’ produced the largest reduction in sediment load. The most cost efficient practices were ‘converting wheat to bermuda grass’ or ‘native range’ and ‘riparian exclusion’. This project demonstrates the importance of conservation practice selection and evaluation prior to implementation in order to optimize cost share funds. In addition, this information may lead to the implementation of more cost effective practices and an improvement in the overall effectiveness of water quality programs.