Evaluation of ECOSTRESS Collection 2 Evapotranspiration Products: strengths and uncertainties for evapotranspiration modeling

The ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) 29 collects thermal observations from the International Space Station to support 30 evapotranspiration (ET) research at fine spatial resolutions (70 m x 70 m). Initial ET estimates 31 from ECOSTRESS Collection 1 have been used in a wide range of scientific studies and 32 applications, though subsequent analyses identified areas for improvement. This study provides 33 an overview of updates to ECOSTRESS Collection 2 ET and presents an accuracy assessment of 34 ET and auxiliary variables against in situ data from AmeriFlux. Key updates in Collection 2 35 include: four independent model estimates of ET and improved auxiliary forcing data. We find the multi-model ensemble ET estimate achieves a root mean square error (RMSE) of 109 Wm-2 36 37 for instantaneous observations and 1.5 mm/day for daily retrievals. When considering 38 uncertainty in energy balance closure approaches for site-level data, the RMSE improves to 48 Wm-2 39 for instantaneous ET. We observe variable performance based on time of day of 40 ECOSTRESS image acquisition, climate and vegetation type. Evaluation of auxiliary data 41 highlight limitations in down-scaled net radiation and relative humidity, contributing to a 42 diurnal hysteresis in ET estimates. We provide accuracy metrics and model sensitivity to 43 auxiliary data to facilitate user confidence, data adoption, interpretation, and applications. 44 ECOSTRESS is the only instrument capable of providing ET at different times of day at high 45 spatial scales; thus, this work is an important step toward enhancing the capabilities of satellite46 driven ET models in resolving diurnal ET variations and guiding directions for future 47 improvements.