Development and application of a WASP model on a large Texas reservoir to assess eutrophication control

Abstract

Ernst, M.R. and J. Owens. 2009. Development and application of aWASP model on a large Texas reservoir to assess eutrophication control. The Tarrant Regional Water District developed and calibrated an 11-yr WASP-Eutro model for Cedar Creek Reservoir, a 13,350 ha (33,000 ac) drinking water supply and recreation reservoir located near Fort Worth, Texas, that is experiencing eutrophication. Cedar Creek was partitioned into 22 segments with up to three vertical segments in the main pool. Loading to the reservoir was divided into four categories: (1) watershed, (2) nine wastewater treatment plants (5.6 mgd), (3) atmospheric loading, and (4) internal NH4 and OPO4 sediment flux. Watershed loading was derived from a SWAT model of the 1007 mi2 watershed. Wastewater treatment plant loading was based on 1 yr of weekly plant nutrient and flow data. Atmospheric loading was estimated from periodic rain sampling adjacent to the reservoir. Internal flux was estimated from hypolimnetic accumulation during stratification. Calibration was judged by conformity of median observed and predicted water quality data and longitudinal profiles in the reservoir. Sensitivity analysis of these four loadings revealed that Chla and total phosphorus were most influenced by the watershed nonpoint source load. Internal flux was the next most influential load; form, timing and location of loading are more important than actual magnitude of this load. Estimates of the nonpoint source reduction necessary for a significant reduction in Chla were about 30%, while estimates of reductions in OPO4 flux to significantly reduce Chla were 75–100%. A combination of watershed and internal nutrient control are needed to control eutrophication in Cedar Creek Reservoir.