Field comparison of methods for estimating groundwater discharge by evaporation and evapotranspiration in an arid-zone playa

Evaporative losses typically play a substantial role in the water balances of arid regions. However, they are often poorly understood due to low flux rates and difficulty in direct measurement. We compared six field methods to quantify groundwater discharge due to evaporative and evapotranspirative fluxes from Stirling Swamp, a playa in central Australia; Bowen ratio-energy balance (BREB), maximum entropy production (MEP), chloride and stable isotope profiling, change in groundwater level, and 14C profiles within the aquifer. The latter method has not been previously used to determine groundwater discharge. Evaporative groundwater discharge estimates varied between 0 and 300 mm/y, partly due to variability in spatial and temporal scales captured by the individual methods. Within playa systems where evapotranspiration within the soil is negligible but the depth to groundwater is small, land surface energy balances were found to have the advantage of integrating over hundreds of metres, and when upscaled to annual estimates they agreed well with expected evaporative flux values. Soil profile methods yielded a wide range of results depending on the values of several constants that must be assumed, and the assumption of steady state was found to be a disadvantage. Groundwater methods also had the advantage of integrating over some distance within the aquifer; however, advective transport in the subsurface may have led to under-estimation of evaporative flux with these methods.