Evaluating Micrometeorological Estimates of Groundwater Discharge from Great Basin Desert Playas

Groundwater availability studies in the arid southwestern United States traditionally have assumed that groundwater discharge by evapotranspiration (ET_g ) from desert playas is a significant component of the groundwater budget. However, desert playa ET_g rates are poorly constrained by Bowen ratio energy budget (BREB) and eddy-covariance (EC) micrometeorological measurement approaches. Best attempts by previous studies to constrain ET_g from desert playas have resulted in ET_g rates that are within the measurement error of micrometeorological approaches. This study uses numerical models to further constrain desert playa ET_g rates that are within the measurement error of BREB and EC approaches, and to evaluate the effect of hydraulic properties and salinity-based groundwater density contrasts on desert playa ET_g rates. Numerical models simulated ET_g rates from desert playas in Death Valley, California and Dixie Valley, Nevada. Results indicate that actual ET_g rates from desert playas are significantly below the uncertainty thresholds of BREB- and EC-based micrometeorological measurements. Discharge from desert playas likely contributes less than 2% of total groundwater discharge from Dixie and Death Valleys, which suggests discharge from desert playas also is negligible in other basins. Simulation results also show that ET_g from desert playas primarily is limited by differences in hydraulic properties between alluvial fan and playa sediments and, to a lesser extent, by salinity-based groundwater density contrasts.