On groundwater fluctuations, evapotranspiration, and understory removal in riparian corridors

This study utilizes 7 years of continuously monitored groundwater-level data from four sites along the Río Grande riparian corridor in central New Mexico to calculate evapotranspiration from groundwater and assess impacts of understory vegetation removal during a restoration project. Diurnal groundwater fluctuation measurements were used to compare the well-known White method for estimating evapotranspiration from groundwater (ET_g) to colocated measurements of total riparian evapotranspiration (ET) measured using the eddy covariance method. On average, the two methods were linearly correlated and had similar variability, but groundwater hydrograph estimates of ET _g tended to be larger than tower ET estimates. Average ET_g estimates for two wells at one site ranged from 91.45% to 164.77% of measured tower ET estimates, but were also shown to range from 57.35% to 254.34% at another site. Comparisons between the methods improved with deeper water tables, reduced groundwater and river connectivity, and where soil profiles were dominated by coarse-sized particles. Using a range of texture-based estimates of specific yield (S_y) with water table position improves the field application of the White method. River-induced fluctuations in groundwater increased the variability of ET_g measurements. Removal of understory vegetation at one site resulted in a small but significant reduction in diel groundwater fluctuation amplitude of 19-21%. Caution is required when understory vegetation removal is used as a means to decrease overall riparian ET. Diel groundwater fluctuation amplitudes can be useful in gauging the hydrological effects of vegetation removal. Riparian groundwater hydrographs are critical to investigating the hydrologic connectivity between river and shallow groundwater, the temporal patterns of vegetative consumption, and monitoring changes to the vegetation community.