National Aeronautics and Space Administration

Proliferation of evapotranspiration (ET) products warrants comparison of these products. The study objective was to assess uncertainty in ET output from four land surface models (LSMs), Noah, Mosaic, VIC, and SAC in NLDAS_2, two remote sensing_based products, MODIS and AVHRR, and GRACE_inferred ET from a water budget with precipitation from PRISM, monitored runoff, and total water storage change (TWSC) from GRACE satellites. The three cornered hat method, which does not require a priori knowledge of the true ET value, was used to estimate ET uncertainties. In addition, TWSC or total water storage anomaly (TWSA) from GRACE was compared with water budget estimates of TWSC from a flux_based approach or TWSA from a storage_based approach. The analyses were conducted using data from three regions (humid_arid) in the South Central United States as case studies. Uncertainties in ET are lowest in LSM ET (˜5 mm/mo), moderate in MODIS or AVHRR_based ET (10–15 mm/mo), and highest in GRACE_inferred ET (20–30 mm/month). There is a trade_off between spatial resolution and uncertainty, with lower uncertainty in the coarser_resolution LSM ET (˜14 km) relative to higher uncertainty in the finer_resolution (˜11–8 km) RS ET. Root_mean_square (RMS) of uncertainties in water budget estimates of TWSC is about half of RMS of uncertainties in GRACE_derived TWSC for each of the regions. Future ET estimation should consider a hybrid approach that integrates strengths of LSMs and satellite_based products to constrain uncertainties.