Evaluation of a DRAINMOD based watershed scale model

A DRAINMOD based watershed scale model with a flow routing component was evaluated using 11 months of measured outflow data from a 3200 ha forested watershed in the lower coastal plain of North Carolina. Given the complexity of the soils, vegetation, and drainage network, model predictions of outflow rates were excellent except during large hurricane and tropical storm events. The average absolute difference between the predicted and observed daily drainage rates was 0.45 mm/day. Total cumulative outflow was underpredicted by only 5%. The results of simulations to evaluate the effects of variability in rainfall and reference evapotranspiration (REF-ET) indicated that areal distribution of rainfall and estimates of REF-ET characteristic to the dominant vegetation at the site are important in this scale of watershed modeling. Simulations also showed that the effects of clearcutting less than 10% of the area on Cape Fear sandy loam (mineral) soil resulted in a 5% increase in annual drainage outflow and only 2.5% when the same area was clearcut in Belhaven muck (organic) soil. This study showed that the model can be used to assess impacts of distribution of rainfall and ET, land use changes and drainage practices on the hydrology of watersheds composed of poorly/artificially drained soils.