Estimating Groundwater Evapotranspiration Using the Water Table Fluctuation Method

Abstract

research posterEstimating evapotranspiration for an ecosystem’s water budget can be particularly challenging. In an unconfined aquifer, plants with roots that interact with the water table are able to extract groundwater and transpire it to the atmosphere. The Water-Table Fluctuation (WTF) Method was developed and applied frequently for groundwater recharge and others have used this method to calculate evapotranspiration (ET). Assuming the changes in water table elevation are plant mediated transpiration, measured changes in the water level in combination with the specific yield can be used to understand plant interactions with shallow groundwater and estimate ET resulting from phreatophytes. Previously, shallow wells screened across the water table were installed on Isherwood Farms in Portage County, WI, USA. The area is a mixed agricultural, coniferous, deciduous, and shrub forested area with groundwater in close proximity to the land surface. Wells were instrumented with Solinst 3001 LTF15 level loggers and water levels were monitored on a 30-minute interval. Measurements were adjusted to account for barometric pressure. The WTF method was then applied to the data in order to quantify evapotranspiration variability within a growing season as well as between growing seasons and vegetation types. Water level information was compared to weather data to validate ET estimates, allowing for a better understanding of ET dynamics in shallow groundwater environments