Refinement, Validation and Implementation of SWAT Model Central Texas TMDL

The development of a TMDL for the North Bosque River in central Texas was based in part on applications of the Soil Water Assessment Tool (SWAT). Public concerns regarding the initial modeling efforts for the TMDL included a lack of spatial resolution in the definition of subbasins; the exclusion of the 40 Public Law (PL)-566 flood retardation reservoirs; and exclusion of contributions associated with discharges from dairy lagoons and wastewater storage ponds not associated with routine dewatering. Addressing the concerns expressed in the public review of the TMDL, the SWAT model was reapplied with refinements. The refinements required for the new TMDL effort were, increased spatial resolution in the definition of subbasins; inclusion of the 40 Public Law (PL)-566 flood retardation reservoirs in the NBR watershed; contributions of discharges associated with dairy lagoons, wastewater storage ponds, and unauthorized discharges from WWTPs and their associated sewage collection systems; improved instream water quality kinetics in SWAT to better simulate algae growth and nutrient dynamics that have a profound effect on average daily nutrient concentration during low flow; and a new dynamic manure management component in SWAT to improve and enhance the capabilities to simulate the Natural Resources Conservation Service (NRCS) guidelines for manure management. The refined SWAT model was validated for the North Bosque River and was able to replicate results of the original TMDL while providing enhanced capabilities to look at a variety of new control practices that could not be adequately assessed with the model used in the original TMDL.