Testing of DRAINMOD for Forested Watersheds with Non-Pattern Drainage

Models like DRAINMOD and its forestry version, DRAINLOB, have been specifically developed as a field scale model for evaluating hydrologic effects of crops (trees), soil, and water management practices for lands with pattern drainage (i.e. with parallel ditches) on relatively flat, high water table soils. These models conduct a water balance between the ditches to predict water table depths, drainage rates, surface runoff, ET, and soil water storage. However, a vast landscape under silvicultural management in the coastal plains along the southeast and Gulf Coast region consists of lands with non-pattern drainage systems. Reliable models are needed to determine the processes and water quality impacts of management practices on these lands also. In this study data from two naturally drained forested watersheds without the pattern drainage in Florida and South Carolina coasts were used to test the ability of DRAINMOD to predict water table depths and drainage outflow rates. A large ditch spacing and a shallow drain depth were assumed to simulate outflows from both of these flat depressional wet sites. With minimal field calibration, the model’s predictions of daily outflows from Santee watershed in South Carolina were satisfactory. The predicted outflows for Bradford watershed on Florida flatwoods with a higher level of field calibration were found to be better, as expected, for wet, dry, and normal years. The results suggest that reliable estimates of surface storage and PET inputs in DRAINMOD with a surface flow routing component may further enhance the flow predictions on these watersheds.