Estimating groundwater evapotranspiration rates using diurnal water-table fluctuations in a semi-arid riparian zone

In semi-arid climates, phreatophytes draw on shallow aquifers, and groundwater evapotranspiration (ETG) is a principal component of groundwater budgets. Diurnal water table fluctuations, which often are a product of ETG, were monitored in the riparian zone of Red Canyon Creek, Wyoming, USA. These fluctuations were higher in a riparian wetland (2-36 mm) than a grass-covered meadow (1-6 mm). The onset and cessation of water-table fluctuations correspond to daily temperatures relative to freezing. Spatial differences were due to vegetation type and specific yield, while temporal changes were due to vegetation dormancy. Ratios of ETG to potential evapotranspiration (PET), K-c,K-GW, were similar to ratios of actual evapotranspiration (ET) to PET, K-c, in semi-arid rangelands. Before vegetation senescence, K-c,K-GW increased between precipitation events, suggesting phreatophytes pull more water from the saturated zone as soil moisture decreases. In contrast, K-c decreases with soil moisture following precipitation events as ET becomes increasingly water-limited. Error in ETG is primarily from estimates of specific yield (S-y), which is difficult to quantify in heterogeneous sediments. ETG values may be more reliable because the range of acceptable S-y is smaller than K-c and S-y does not change with vegetation type or soil moisture.