Evaluation and inter-comparison of multi-source evapotranspiration products over the continental United States: Implications for the next phase of NLDAS development

Authors: ZHANG, B. - Lanzhou University; XIA, Y. - National Oceanic & Atmospheric Administration (NOAA); LONG, B. - Lanzhou University; HOBBINS, M. - University Of Colorado; HAIN, C. - Goddard Space Flight Center; LI, Y. - China Agriculture University; Anderson, Martha

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/13/2019
Publication Date: 1/15/2020
Citation: Zhang, B., Xia, Y., Long, B., Hobbins, M., Hain, C., Li, Y., Anderson, M.C. 2020. Evaluation and inter-comparison of multi-source evapotranspiration products over the continental United States: Implications for the next phase of NLDAS development. Agricultural and Forest Meteorology. https://doi.org/10.1016/j.agrformet.2019.107810.
DOI: https://doi.org/10.1016/j.agrformet.2019.107810

Interpretive Summary: The National Oceanic and Atmospheric Administration (NOAA) maintains several land-surface modeling systems to support weather and hydrologic forecasting systems. The National Aeronautics and Space Administration (NASA) has developed a platform to intercompare these models to track their relative performance, as well as improvements in performance as individual models are upgraded. This paper compares performance of several of these models, particularly in their ability to model evapotranspiration (ET) – the net exchange of water vapor between the land surface and the atmosphere. The models are compared against satellite remote sensing estimates of ET and water balance estimates at the watershed scale. The findings from this study will help to guide further refinement of these land-surface models, with the goal of ultimately improving our national forecasting capacity.

Technical Abstract: Terrestrial evapotranspiration (ET) is a major component of the surface hydrological cycle and controls land-atmosphere feedbacks by modulating the surface energy budget. Accurate ET quantification at global or grid scales is crucial for understanding variations of carbon and water cycling in a changing environment. Although various grid-based ET products have been developed from multiple approaches, these products vary in concept and physics, leading to varying performances. We examine uncertainties associated with limitations in method physics to assist in product selection and improvement. We evaluate multi-source ET products, including estimates derived from a variety of land surface models (LSMs) forced by the operational North American Land Data Assimilation System (NLDAS) phase 2 (NLDAS-2) and experimental NLDAS phase 3 (NLDAS-3) drivers, and satellite retrievals against water budget-derived ET and tower observations. Overall, all products are able to capture the spatial variability of mean annual water balance-based ET (ETwb) and monthly seasonal cycles of tower ET measurements although there is a large range within the population of estimates. NOAH28, FLUXNET, SSEBop, LandFlux, and GLEAM perform the best, demonstrated by their higher correlation and smaller bias and RMSE values. Simple relative uncertainty analysis shows that the NLDAS-3 ensemble mean has a slightly smaller uncertainty than that of the NLDAS-2 ensemble. Our study indicates that NLDAS-3/VIC412 (NLDAS version/LSM version) is improving and NLDAS-3/CLSM is deteriorating relative to NLDAS-2/VIC403 and NLDAS-2/Mosaic. NLDAS-3/NOAH36 and NLDAS-3/NOAHMP36 are comparable to NLDAS-2/NOAH28 although biases between models and ETwb are opposite in sign. These findings will help further improvement of these models and support future NLDAS development.