VEGETATION-DEPENDENT EVAPOTRANSPIRATION RATES ACROSS A DUNE-INTERDUNAL WETLAND SYSTEM ALTER WATER TABLE ELEVATIONS AT CAPE HENRY, VIRGINIA

Evapotranspiration and precipitation, important factors in most wetlands, are the main drivers of hydrology in depressional wetlands on barrier islands. Actual ET (AET) loss is difficult to quantify accurately, in part because it can differ based on vegetation type. Most practitioners use regional weather data to get one Potential ET rate per site even if there are multiple vegetation settings. Barrier islands can have vegetation groups with notably different ET rates This study tests if more detailed Actual ET rates can improve groundwater flow models. At Cape Henry, VA a cuspate foreland contains a series of dunes and interdunal swales that create a complex mosaic with two major vegetation zones, an upland setting predominately loblolly pine and live oak and a wetland setting dominated by Bald Cypress, Swamp Tupelo, and Red Maple. To understand if the rates between the two vegetation settings can affect the water table shape Actual ET rates were derived using White’s Method - a water level fluctuation technique – that allows rates to be calculated at well sites in different vegetation groups. Results from May 2017-June 2018 indicate that the wetland setting has rates from 1.27-9.41 mm/day and the upland setting has rates from 0.84-7.56 mm/day. The average AET rates in the cypress/gum/maple swales can be 40% larger than those on the forested sand ridges but differences in the rates diminish as the growing season ends. Initial results from late 2018 and early 2019 are continuing to show similar rates and trends as previous results. A water table contour map constructed during this study shows that Cape Henry, VA has an asymmetric groundwater dome. If these differing ET rates for dunes and wetlands are applied using a relatively fine grid, the advanced MODFLOW-based version of Wetbud replicates the overall elevation and asymmetric shape of the water table across the barrier island complex.