Evapotranspiration of two vegetation covers in a shallow water table environment

A method is introduced to estimate evapotranspiration (ET) in shallow water table environments. The method involves measuring the diurnal fluctuations in total soil moisture above the water table to estimate (i) the net lateral and vertical subsurface flux in the aquifer and (ii) evapotranspiration from the vegetation cover. In a hillslope discharge zone, the net lateral subsurface flux was calculated from the recovery rate of soil moisture between midnight and 0400 h. Evapotranspiration was then estimated from a daily water balance in a soil column that included the water table. The method was tested on two vegetation covers, a pasture in a groundwater recharge area, and a riparian zone with woody vegetation in a groundwater discharge area. A moisture probe carrying eight sensors was used in each area to estimate the total soil moisture in a sandy soil environment. The observed water table fluctuated between land surface and a depth of 1.2 to during the study period, allowing observation and estimation of the total soil moisture in a soil column that included the water table. The results of this investigation support another hypothesis that, in humid, shallow water table environments, ET demand may be supported by adjacent ecosystems. This method provided reasonable results for the two landscapes investigated and was able to capture the variability of evapotranspiration in heterogeneous vegetation covers. It provided a relatively inexpensive alternative to characterize ET within regionally heterogeneous but microhomogenous landscapes. Though tested for coarse-textured soil, the method involving soil moisture monitoring can be easily adapted to other soil types with shallow water table. Another advantage of using this method is that ET can be successfully estimated without detailed knowledge of soil hydraulic properties, subsurface flow patterns, or vegetation characteristics.