EVAPOTRANSPIRATION CROP COEFFICIENTS CALCULATED FOR WETLAND VEGETATION IN HYBLA VALLEY, VIRGINIA ENHANCE WETBUD WATER BUDGET MODELS

Crop coefficients for large stands of emergent and forested wetland vegetative communities have the potential to improve estimates of evapotranspiration (ET) used in water budget calculations. Building on recent studies relating to the restoration and expansion of wetlands at Huntley Meadows Park, Fairfax County, Virginia, this work simulates the long-term hydrologic effects of converting non-wetland vegetative communities to wetlands. The 630 ha park, located in the Coastal Plain of Virginia within Hybla Valley, an old meander scar of the Potomac River, contains more than 365 ha of wetlands. Twenty monitoring stations positioned in different vegetative communities collected hydrologic data. These data permitted calculation of (1) actual evapotranspiration (AET) rates within several vegetative communities across the park from August 2014 through March 2016, (2) potential evapotranspiration (PET) from March 2015 through March 2016 in the extensive emergent wetland at the center of the park, and (3) surface water flux at the wetland. AET was estimated from diurnal fluctuations in the water table using a script developed in MATLAB, while PET rates were estimated using the FAO Penman-Monteith Method. Potential evapotranspiration was also calculated using weather data from the nearby Reagan National Airport weather station. ET crop coefficients (K_c) were developed for emergent wetlands, forested/shrub wetlands, and forested non-wetland areas, with reference ET coming from both the temporary Huntley Meadows weather station and the Reagan National Airport weather station. Average monthly K_c at individual monitoring points developed using the Huntley Meadows weather data ranged from 0.48 to 7.14, while those developed using the Reagan National Airport data ranged from 0.45 to 6.36. Monthly K_c were applied to analytical models and numerical groundwater flow models using Wetbud to evaluate their effects on wetland water budgets.