A computer simulation study of pocosin hydrology

Abstract

The hydrology of pocosins is dependent on plant, soil, site, and climatological factors. A simulation study was conducted to determine the effects of natural factors, such as depressional storage, and changes in drainage and land use on pocosin hydrology. The water management model DRAINMOD was used to simulate the hydrology of drained and undrained pocosins. Hourly rainfall data from a 33-year period of climatological record were used in DRAINMOD to predict evapotranspiration (ET), subsurface drainage, runoff, and water table depth on a day-by-day basis. Results were summarized to determine annual and long-term average effects. Pocosins with a large amount of surface depressional storage have water ponded on the surface during most of the year, high ET, and low surface runoff. As the depth of depressional storage decreases, average annual ET decreases, and runoff increases. Average annual runoff predicted for a natural pocosin on a Portsmouth soil near Wilmington, North Carolina increase from 277 mm to 384 mm as the depth of depressional storage was decreased from 300 mm to 5 mm. However, year-to-year variation in annual runoff was much greater than the effects of all other factors considered. Drainage ditches at spacings greater than 400 m had no effect on average annual runoff for constant surface depressional storage and conditions analyzed in this study. Decreasing the ditch spacing from 400 to 100 m increased average total annual outflow by only 7%. However, more than half of the predicted outflow for the 100 m spacing occurred as subsurface flow compared to less than 5% of the total for the 400 m spacing. Conversion from natural pocosin vegetation to a managed pine forest with a deeper rooting zone decreased predicted annual outflow by about 9%. Conversion to agricultural uses increased predicted average outflow by 7% compared to natural conditions.