WATGIS: A GIS–BASED LUMPED PARAMETER WATER QUALITY MODEL

Abstract

A Geographic Information System (GIS)based, lumped parameter water quality model was developed to estimatethe spatial and temporal nitrogenloading patterns for lower coastal plain watersheds in eastern North Carolina. The modeluses a spatially distributed delivery ratio (DR) parameter to account for nitrogen retention or loss along a drainage network.Delivery ratios are calculated from time of travel and an exponential decay model for instream dynamics. Travel times fromany point in the drainage network to the watershed outlet are obtained from simulations using a combined physically basedfield hydrology and drainage canal routing model (DRAINMODDUFLOW). Nitrogen load from contributing areas in thewatershed delivered to the main watershed outlet is obtained as the product of field export with the corresponding deliveryratio. The total watershed load at the outlet is the combined loading of the individual fields. Nitrogen exports from sourceareas are measured. The lumped water quality model is integrated within a GIS framework with menu interface, displayoptions, and statistical procedures. Within this framework, the model can be used as a screening tool to analyze the effectsof different land and water management practices on downstream water quality. A description of the model is presented alongwith the results from the evaluation of the model to characterize the seasonal and annual export of nitrogen from a drainedforested watershed near Plymouth, North Carolina. Results of the study showed that the lumped parameter model canreasonably predict the loads at the outlet of the watershed. Predicted loads for 1997 were highly correlated with the observedloads (correlation coefficients of 0.99, 0.90, and 0.96 for nitratenitrogen, TKN, and total nitrogen respectively). Sensitivityand uncertainty analyses indicated that predicted outlet loads were sensitive to field flow predictions and exportconcentrations. Overall, the results indicate that the lumped parameter model can be an effective tool for describing themonthly nitrogen loads from a poorly drained coastal plain watershed.