A water balance based, spatiotemporal evaluation of terrestrial evapotranspiration products across the contiguous United States

Authors: CARTER, E. - Cornell University; HAIN, C. - Goddard Space Flight Center; Anderson, Martha; STEINSCHEIDER, S. - Cornell University

Submitted to: Journal of Hydrometeorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/16/2018
Publication Date: 4/24/2018
Citation: Carter, E., Hain, C., Anderson, M.C., Steinscheider, S. 2018. A water balance based, spatiotemporal evaluation of terrestrial evapotranspiration products across the contiguous United States. Journal of Hydrometeorology. https://doi.org/10.1175/JHM-D-17-0186.1.

Interpretive Summary: Evapotranspiration (ET) – the water lost from the land-surface to the atmosphere through the combined processes of evaporation and plant transpiration – is an important component of the Earth’s hydrological cycle. There are principally two types of physical modeling approaches for estimating: models based on water balance, and models based on energy balance that consider the thermodynamic cooling effect that ET has on the evaporating surface. The two approaches can be considered independent, operating under very different constraints and using different kinds of driving inputs (precipitation vs. radiation load). This paper compares ET estimates developed across the continental United States using a water balance approach and three models based on energy balance to determine regions of persistent agreement and bias to better understand the limitations of each approach. The study found reasonable inter-annual agreement between ET estimates, particularly in the central United States. Higher biases were identified in the more humid and topographically variable parts of the United States, potentially due to errors in assumed rainfall in the water balance or treatment of slope and advection in the energy balance models. In general, this comparison between independent estimates of ET provides a proxy method for model evaluation over large areas.

Technical Abstract: Accurate gridded estimates of evapotranspiration (ET) are essential to the analysis of terrestrial water budgets. In this study, ET estimates from three gridded energy-balance based products (ETEB) with independent model formations and data forcings are evaluated for their ability to capture long term climatology and inter-annual variability in ET derived from a terrestrial water budget (ETWB) for 671 gaged catchments across the continental United States (CONUS). All three ETEB products have low spatial bias and accurately capture inter-annual variability of ETWB in the central US, where ETEB and ancillary estimates of change in total surface water storage ('TWS) from the GRACE satellite project appear to close terrestrial water budgets. In humid regions, ETEB products exhibit higher long-term bias, and the covariability of ETEB and ETWB decreases significantly. Several factors related to either failure of ETWB, such as errors in 'TWS and precipitation, or failure of ETEB, such as treatment of snowfall and horizontal heat advection, explain some of these discrepancies. These results mirror and build on conclusions from other studies: on inter-annual timescales, 'TWS and error in precipitation estimates are non-negligible uncertainties in ET estimates based on a terrestrial water budget, and this confounds their comparison to energy balance ET models. However, there is also evidence that in at least some regions, climate and landscape features may also influence the accuracy and long-term bias of ET estimates from energy balance models, and these potential errors should be considered when using these gridded products in hydrologic applications.