HYDROLOGIC MODELINGS/GIS AS AN AID IN LOCATING MONITORING SITES

Abstract

With the increasing importance and awareness of non-point source pollution, critical siting of water quality monitoring stations becomes important. Within large watersheds this siting becomes difficult because of the time and expense to travel the entire watershed and evaluate the area. Previous work has shown that hydrologic models can assist in evaluating water quality in large watersheds. In this study, the hydrologic model SWAT (Soil and Water Assessment Tool) was used to simulate flows, sediment and nutrient loadings on a 9000 km2 watershed in central Texas for the period 1970-1984. The model is a continuous, daily time step model that predicts surface runoff, percolation, lateral subsurface flow, groundwater flow, transmission losses and flood routing. The Geographic Resources Analysis Support System (GRASS) and available Natural Resources Conservation Service databases provided input into SWAT. Subwatersheds demonstrating areas of highest per acre loadings were identified from model output. Modeled output of streamflow, nitrogen, phosphorus, and sediment loadings were analyzed. For 1972-1974, the correlation coefficient between observed and simulated streamflow was 0.83, while the Nash-Sutcliffe coefficient was 0.57 indicating the model is a better predictor than using the mean. Average annual loads from the entire basin were 3.9 kg ha–1 NO3 and 0.03 kg ha–1 soluble P. Subwatersheds in the lower part of the watershed had loads which were higher than the watershed average and as a result monitoring stations have been installed. The results demonstrate that a hydrologic model and available spatial databases can be used to aid in locating sites.