Comparison of Drainmod Based Watershed Scale Models

Watershed scale hydrology and water quality models (DRAINMOD-DUFLOW, DRAINMODW, DRAINMOD-GIS and WATGIS) that describe the nitrogen loadings at the outlet of poorly drained watersheds were examined with respect to their accuracy and uncertainty in model predictions. Latin Hypercube Sampling (LHS) was applied to determine the impact of uncertainty in estimating field exports and decay coefficients on the uncertainty of the simulated nitrogen loads at the outlet of a 2950 ha coastal plain watershed in eastern North Carolina. Mean daily flow predictions were all within 1% of the observed flows. Except for the WATGIS model, mean daily nitrate-nitrogen load predictions were within 2% of the observed load. Statistical test indicated no difference between the predictions of the different models. Uncertainty analysis indicated that uncertainty in quantifying the field exports has greater impact on the uncertainty of outlet loads than does the uncertainty associated with decay coefficient. The uncertainty of predicted outputs from the DRAINMOD-GIS and WATGIS models are similar.