CYPRESS-MAPLE SWAMPS AND PINE-OAK DUNES DEVELOP DIFFERENT ACTUAL EVAPOTRANSPIRATION RATES, FIRST LANDING STATE PARK, VIRGINIA BEACH, VIRGINIA

Evapotranspiration (ET) constitutes a major loss to many wetland water budgets, but is often a difficult value to calculate. Potential ET (PET) can be calculated for regions using local weather data, but Actual ET (AET) loss is more difficult to quantify accurately, in part because it can differ based on vegetation type. On barrier islands with large areas of dune ridges and interdune depressional wetlands, vegetation communities between the two settings may have significantly different ET rates. Previous studies show that long-term differences in groundwater recharge (precipitation minus ET) between the dune ridges and swales can cause changes in the 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. Earlier studies indicate the groundwater dome is not symmetric and centered in the middle of the island, but has a crest farther north than expected. To determine if variations in AET influence the shape in the water table mound, seven monitoring wells with pressure transducers were installed throughout the park. Three wells were installed in each of the two major dominant vegetation groups, a Bald Cypress/Swamp Tupelo/Red Maple setting and a Pine/Oak setting, and one well was installed at the crest of the groundwater dome. AET was calculated using a modified version of the White’s Method, a calculation that uses nighttime groundwater recovery and multiday drawdown. Preliminary results from the early/mid growing-season months for 2017 suggest that on a given day, the average AET rates in the cypress/gum/maple swales (up to 12.7 mm/day) are approximately twice those of the forested sand ridges (up to 7.9 mm/day), but as the growing season ends differences in the rates diminish. Monthly crop coefficients developed for each vegetative group 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.