How well does the Evaporative Stress Index from ECOSTRESS capture site-based stresses?

Evaporative Stress Index (ESI) is a commonly used indicator for assessing ecosystem-level water stress, particularly for monitoring agricultural and forest droughts. This study assesses performance of two remote sensing ESI products: ECO4ESIPTJPL and ECO4ESIALEXI from the ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS), against in-situ data from 59 AmeriFlux sites across the continental United States. Results indicate that ECOSTRESS ESI, overall, underestimates site-based estimates, suggesting a tendency to indicate higher evaporative stress. When aggregated across all sites and time steps, ECO4ESIPTJPL underestimates ESI 55.1% of the time, while ECO4ESIALEXI underestimates 62.42%. However, under high evaporative stress conditions, both the products, particularly ECO4ESIPTJPL, overestimate ESI, meaning they indicate the stress is lower than it actually is. Further analysis reveals that ECOSTRESS-derived evapotranspiration (ET), rather than potential ET, is the primary source of bias in the ECOSTRESS ESI products. These findings emphasize the need for refining ECOSTRESS ESI products, particularly improving ECOSTRESS ET, to enhance accuracy of assessing evaporative stress across diverse landscapes and climatic conditions.