Adapting a drainage model to simulate water table levels in coastal plain soils

Seasonal saturation in soils is expensive and time consuming to document, but the information is needed for land use assessments. Hydrologic models can be used to assess saturation occurrence quickly if the models are calibrated for individual sites. This study determined whether a drainage model (DRAINMOD) could predict water table levels in soils with and without a perimeter ditch. Water table levels were monitored for up to 3 yr at two toposequences that contained a total of 21 soil plots Q m by 3 m). Soils included Typic Paleudults, Aquic Paleudults, and Umbric Paleaquults. Each plot was instrumented with a recording well to monitor daily water table levels. DRAINMOD was calibrated for each soil plot using measurements of in situ saturated hydraulic conductivity, soil water characteristic, depth to impermeable layer, depth of rooting, and rainfall. A plot's water table fluctuation was simulated by a system of virtual drains whose distance and depth were adjusted to produce simulated water table fluctuations in line with those actually measured. Further calibration adjusted drainable porosity in the upper 20 cm of the soil, depressional storage, evapotranspiration rate, and depth to impermeable layer. Adjustments were made by iteration to minimize the absolute average deviation between simulated and measured water table levels. Calibration had to be done by plot. Average absolute deviations were generally <20 cm for periods ranging from 1 to 3 yr. The results showed that DRAINMOD could be adapted to simulate water table levels in landscapes that do not contain a network of parallel drains.