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Aim: Topography, climate and plant water stress influence the spatial patterns of burn severity (damage to soils and vegetation associated with wildfires); however, the relative importance of these predictors remains contested. We hypothesized that high‐resolution pre‐fire vegetation water stress, measured using evapotranspiration (ET), evaporative stress index (ESI) and water use efficiency (WUE), are strong predictors of burn severity. Location: California, USA. Time period: 2019–2020. Major taxa studied: Forests, chaparral, oaks and grasslands. Methods: We used random forest regression modelling with remotely sensed ECOsystem and Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) plant water stress from the spring–summer and year before the fires, predictors to represent spatial autocorrelation, and topography to predict the spatial patterns of burn severity for six fires in the Sierra Nevada and the Southern California Mountains. Results: Plant water stress was the dominant predictor for the Southern California Mountains, and spatial autocorrelation in Sierra Nevada. Elevation was a leading predictor for burn severity for both regions, with greater predictability in higher‐elevation and steeper terrain. Model results varied by vegetation type: for the Southern California Mountains, grasslands had the highest predictability [R2 =.50 (high elevation); R2 =.58 (steep)], followed by savanna and open shrublands, which are typified by chaparral‐type vegetation [R2 =.43 (savanna, high elevation); R2 =.40 (open shrubland, high elevation)]. For grasslands, annual ET was the leading predictor (R2 =.58), consistent with the idea that growing season grassland enhancement increases fuel availability. The predictability of burn severity was also sensitive to the time of day of the ECOSTRESS acquisition and varied by vegetation type. Afternoon ET was the leading predictor for high‐elevation grasslands (R2 =.50), and morning predictors were important for steep‐sloped woody savanna (R2 =.18). Main conclusions Our findings provide insights on how antecedent plant water stress predicts burn severity and can inform pre‐fire season monitoring and fuel management. [ABSTRACT FROM AUTHOR]

The ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) provides remotely-sensed estimates of evapotranspiration at 70 m spatial resolution every 1–5 days, sampling across the diurnal cycle. This study, in partnership with an operational water management organization, the Eastern Municipal Water District (EMWD) in Southern California, was conducted to evaluate estimates of evapotranspiration under ideal conditions where water is not limited. EMWD regularly uses a ground-based network of reference evapotranspiration (ETo) from the California Irrigation Management Information System (CIMIS); yet, there are gaps in spatial coverage and questions of spatial representativeness and consistency. Space-based potential evapotranspiration (PET) estimates, such as those from ECOSTRESS, provide consistent spatial coverage. We compared ECOSTRESS ETo (estimated from PET) to CIMIS ETo at five CIMIS sites in Riverside County, California from July 2018–June 2020. We found strong correlations between CIMIS ETo and ECOSTRESS ETo across all five sites (R2 = 0.89, root mean square error (RMSE) = 0.11 mm hr−1). Both CIMIS and ECOSTRESS ETo captured similar seasonal patterns through the study period as well as diurnal variability. There were site-specific differences in the relationship between ECOSTRESS AND CIMIS, in part due to spatial heterogeneity around the station site. Consequently, careful examination of landscapes surrounding CIMIS sites must be considered in future comparisons. These results indicate that ECOSTRESS successfully retrieves PET that is comparable to ground-based reference ET, highlighting the potential for providing observation-driven guidance for irrigation management across spatial scales. [ABSTRACT FROM AUTHOR]