ACTUAL EVAPOTRANSPIRATION (AET) RATES FOR DIFFERENT VEGETATION GROUPS MAY CONTROL GROUNDWATER DOME ASYMMETRY, FIRST LANDING STATE PARK, VIRGINIA

Evapotranspiration (ET) constitutes a major loss to wetland water budgets but local ET values prove difficult to quantify accurately, in part because rates can vary between different vegetation groups in the same landscape. Previous studies suggest long-term differences in groundwater recharge (precipitation minus ET) between wetlands and dune ridges on barrier islands can control water table elevations and shapes. First Landing State Park, southeastern Virginia, sits on a 4km-wide cuspate foreland – a 10+m-thick sand body formed by a sequence of progradation events – surfaced by alternating dunes, beach ridges, and fresh-water peat-filled swales. There the asymmetry of the groundwater dome has been attributed to the uneven distribution of recharge caused by different ET patterns across the mosaic of dune and wetland vegetative communities. To test those results and to quantify Actual ET (AET) differences more accurately, three shallow groundwater monitoring wells with transducers were installed in Bald Cypress/Swamp Tupelo/Red Maple wetlands and three in Pine/Oak sand ridge uplands. A modified version of White’s method, which uses nighttime groundwater recovery after daytime ET-induced drawdowns, was used to calculate the AET rates. Preliminary results from early growing-season months for 2017 suggest that on a given day, the average AET rates in the cypress/gum/maple swales are approximately twice those of the forested sand ridges. If this trend persists throughout the growing season it would confirm that the park’s asymmetrical groundwater dome is due to greater ET rates in the southern portions of the park which contain more ponds and swales than sand ridges. Monthly crop coefficients developed for each vegetative group will reflect the average daily ratios of calculated Actual (local White method) and Potential (regional Penman method) ET values. These coefficients can be used to more accurately estimate ET in wetland water budgets and in groundwater flow models.