Assessing the Relationship between the Satellite-Derived Evaporative Stress Index and Physical Drivers of Evapotranspiration during the Growing Season

The Evaporative Stress Index (ESI), defined as the ratio of actual evapotranspiration to reference evapotranspiration, conveys valuable information about the evolution of moisture-related stress in vegetation over intraseasonal-to-seasonal time scales. Numerous studies have shown that it is very useful for monitoring drought conditions and can also provide early warning of drought development, especially during rapid-onset flash drought events. This study seeks to improve our understanding of the physical processes that drive changes in the ESI over intraseasonal time scales. To do this, correlations were computed between the ESI and major drivers of variations in evapotranspiration, namely soil moisture, precipitation, air temperature, solar radiation, wind speed, and humidity. The analysis revealed that across the central U.S. where land surface – atmosphere interactions are generally largest, that the ESI is strongly correlated to soil moisture during the growing season, and that this relationship is strongest during the summer when soil moisture is often a limiting factor on actual ET. It is interesting to note that the ESI was strongly correlated to the dew point depression – an indicator of near surface humidity – throughout the growing season, with the correlations being even stronger than those due to soil moisture during the spring. Compared to soil moisture and humidity, precipitation and air temperature actually exhibited much weaker correlations, thereby indicating the indirect nature of their contribution to variability in the ESI. These results indicate that ESI variability across the central U.S. is largely controlled by moisture availability, both in the root zone and over the vegetation leaf surface. Results will also be discussed for the eastern and western parts of the U.S.