Watgis : a Gis - Based L Umped P Arameter W Ater Q Uality

AWIXACT. A Geographic Information System (GIS)-based, lumpedparameter water quality model was developed to estimate the spat ia l and temporal n i trogenloadingpat terns for lower coastal p lain watersheds in eastern North Carol ina. The model uses a spatially distributed delivery ratio (OR) parameter to accountfor nitrogen retention or loss along a drainage network. Delivery rat ios are calculatedfrom t ime of travel and an exponential decay modelfor in-stream dynamics. Travel t imes from any point in the drainage network to the watershed out le t are obtainedfmm simulat ions using a combined physical ly based f ie ld hydrology and drainage canal rout ing model (DRAINMOBDUFLOW). Ni trogen loadfrom contr ibut ing areas in the watershed del ivered to the main watershed out le t is obtained as the product offield export wi th the corresponding del ivery rat io . The to tal watershed load at the out le t i s the combined loading of the indiv idual f ie lds . Ni trogen exports f rom source areas are measured. The lumped water qual i ty model is in tegrated wi thin a GIS framework wi th menu inter$ace, d isplay opt ions, and s tat is t ical procedures. Within this framework, the model can be used as a screening tool to analyze the ef fects of diffeerent land and water management practices on downstream water quality. A description of the model is presented along wi th the resul t s fmm the evaluat ion of the model to characterize the seasonal and annual export of ni trogen from a drained forested watershed near Plymouth, North Carol ina. Resul ts of the s tudy showed that the lumped parameter model can masonably predict the loads at the outlet of the watershed. Predicted loads for 1997 were highly correlated with the observed loads (correlat ion coeficients of 0.99, 0.90, and 0.96for ni trate-ni trogen, TKN, and total ni trogen respect ively) . Sensi t iv i ty and uncertainty analyses indicated that predicted outlet loads were sensitive to field flow predictions and export concentrations. Overall , the nzsults indicate that the lumped parameter model can be an eflective tool for descr ibing the monthly n i trogen loads from a poorly drained coastal p lain watershed.