A method for estimating subdaily evapotranspiration of shallow groundwater using diurnal water table fluctuations

Diurnal water table fluctuations are a common feature of well hydrographs recorded in wetlands, riparian areas, and similar environments where shallow groundwater supports phreatophytic vegetation. Historically, this periodic signal has been used to estimate daily groundwater consumption by this vegetation and has shown that this is typically a very large component of the water budget. As interest in ecohydrology grows and the need for finer resolution estimates of groundwater consumption increases, new, cost‐effective measurement methodologies must be developed. Here, a method is proposed that uses the observed rate of water table decline during the day to estimate phreatophytic groundwater consumption while using the nighttime records to account for other groundwater fluxes to/from the vicinity of the well at all times. Variably saturated groundwater flow modeling was used to create a synthetic data set to test the methodology, and results showed that accurate, sub‐daily (15 min) estimates of phreatophytic groundwater consumption were obtained. The method was also applied to a preexisting data set to demonstrate the usefulness of the technique.