Coupling of SWAT and CE-QUAL-W2 Models

NTMWD sought to model nutrient loading from the Lavon Lake watershed and its impacts on water quality in Lavon Lake by coupling a SWAT model of the watershed with a CE-QUAL-W2 model of the lake. During its long history of supporting permitting activities for the Wilson Creek WWTP, Plummer had already developed a well-calibrated CE-QUAL-W2 model of Lavon Lake (“Lake Model”) that can simulate a wide range of hydrological conditions. The Lake Model is dynamic and gives realistic predictions of algal responses to nutrient loads from the WWTP. By itself, however, the Lake Model cannot predict nutrient loads from the watershed. Prior to coupling, watershed loads were estimated through a combination of a water balance analysis and best professional judgment. This limited the model’s ability to predict the impacts on lake water quality of land use changes due to urbanization.

To simulate the impacts of land use changes, a Watershed Model was developed using Texas A&M’s SWAT model software. Plummer incorporated site-specific information and calibrated it to NTMWD water quality data collected in Lavon tributaries. This was an intensive effort, because nutrient loads are sensitive to many parameters associated with soil-water balance, nutrient kinetics, surface erosion, etc. Plummer employed a combination of manual and automated calibration methods to match predicted nutrient loads with observed values.

To allow nitrogen and phosphorus loads predicted by the Watershed Model to be used as inputs to the Lake Model, Plummer developed computer programs to convert SWAT outputs into CE-QUAL-W2 input files. This effectively linked (“coupled”) the two models together and allowed simulation of point and non-point source impacts. The coupled model can provide estimates of the relative nutrient contributions from WWTP discharge, watershed runoff, and diversions from other water sources. 

Plummer also constructed and simulated future nutrient loading scenarios. These scenarios considered future point source loads and non-point source loads from projected land use changes due to population growth. The coupled model provided estimates of algal impacts from the future scenarios.