Watgis: a Gis–based Lumped Parameter Water Quality Model

A Geographic Information System (GIS)–based, lumped parameter water quality model was developed to estimate the spatial and temporal nitrogen–loading patterns for lower coastal plain watersheds in eastern North Carolina. The model uses 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 in–stream dynamics. Travel times from any point in the drainage network to the watershed outlet are obtained from simulations using a combined physically based field hydrology and drainage canal routing model (DRAINMOD–DUFLOW). Nitrogen load from contributing areas in the watershed delivered to the main watershed outlet is obtained as the product of field export with the corresponding delivery ratio. The total watershed load at the outlet is the combined loading of the individual fields. Nitrogen exports from source areas are measured. The lumped water quality model is integrated within a GIS framework with menu interface, display options, and statistical procedures. Within this framework, the model can be used as a screening tool to analyze the effects of different land and water management practices on downstream water quality. A description of the model is presented along with the results from the evaluation of the model to characterize the seasonal and annual export of nitrogen from a drained forested watershed near Plymouth, North Carolina. Results of the study showed that the lumped parameter model can reasonably predict the loads at the outlet of the watershed. Predicted loads for 1997 were highly correlated with the observed loads (correlation coefficients of 0.99, 0.90, and 0.96 for nitrate–nitrogen, TKN, and total nitrogen respectively). Sensitivity and uncertainty analyses indicated that predicted outlet loads were sensitive to field flow predictions and export concentrations. Overall, the results indicate that the lumped parameter model can be an effective tool for describing the monthly nitrogen loads from a poorly drained coastal plain watershed.