ROLE OF ACTUAL EVAPOTRANSPIRATION ON GROUNDWATER DOME ASYMMETRY AND HISTORIC POND LEVELS AT CAPE HENRY, VIRGINIA

The broad beach ridge plain at Cape Henry, Virginia consists of numerous Holocene dune-capped arcuate ridges with intervening swales. Tall precipitation dunes border the northern and eastern edge of the plain. Many swales contain freshwater ponded wetlands with Bald Cypress, Swamp Tuepelo, and Red Maple, while uplands on the dunes are Loblolly Pine and Live Oak habitats. Most of the wetlands lie within First Landing State Park. Throughout the 18-month study period shallow wells across the Park revealed an asymmetric groundwater dome with the crest shifted north of the geographic center of the barrier complex. Earlier studies concluded the asymmetry arose from a presumption that ponded forested wetlands have higher evapotranspiration rates than forested uplands. To test this presumption Actual Evapotranspiration (AET) rates were calculated using White’s hydrograph method based on diurnal water level fluctuations. Rates calculated for each month in the Bald Cypress/Swamp Tuelpelo setting ranged from 1.27-9.41 mm/day; rates in the Loblolly Pine/ Live Oak setting, 0.84-7.56 mm/day. These rates were applied to a 2-D MODFLOW-based version of Wetbud. Using measured AET rates this model suggests that higher wetland AET losses might generate a slight northward shift in the groundwater dome but not as large of a shift as observed in field data. In addition, an Effective Monthly Recharge analysis allowed construction of a synthetic monthly hydrograph from 1897 to 2018 to estimate historic pond levels. These calculations suggest that during the last century, pond-drying seasonal droughts were common but fewer than 10 one-to-two-year droughts occurred.