VARIATION IN ESTIMATES OF ACTUAL EVAPOTRANSPIRATION RATES LEADS TO UNDERSTANDING OF HYDROLOGIC RESPONSE TO WETLAND EXPANSION AT HYBLA VALLEY, VIRGINIA

Monitoring wells in wetlands located at Huntley Meadows Park, Fairfax County, Virginia provide water table data used to determine estimates of actual evapotranspiration (AET) rates with a version of White’s method. Huntley Meadows Park is a 630 ha park with more than 365 ha of wetlands situated in Hybla Valley, an old meander scar of the Potomac River. A recent restoration effort at the park provided the opportunity to study the hydrologic effects of restoring and expanding the existing wetland. Using data from the Reagan National Airport Weather Station, a water budget constructed with Wetbud wetland water budget modeling software was calibrated using monitoring wells at the park. During many months evapotranspiration (ET) removed the majority of the groundwater from Huntley Meadows Park; ET accounted for 85% or more of the water lost during summer months. Development of a MATLAB script allowed calculation of several hundred AET short-term rates from detailed hydrographs of 16 shallow wells. Here we compare those estimated AET rates derived from diurnal fluctuations of the water table with potential evapotranspiration (PET) values estimated with Thornthwaite’s equation, or the FAO Penman-Monteith method. Initial results indicate PET rates are reasonable estimates of AET for most settings, but variations in vegetation can produce significantly different AET rates across a given site during the growing season. While these differences in AET rates could be in part attributed to estimates of specific yield, we postulate that the higher rates of AET observed in forested settings relative to shrub-scrub and emergent vegetative communities are artifacts of the plants removing groundwater from the system. Improvements in estimates of evapotranspiration rates would produce more accurate wetland water budgets and could help planners anticipate changes to water budgets as a result of wetland expansion and conversion of vegetative communities.