An Analytical Approach for Quantifying the Role of Vapor Pressure Deficit in Soil Moisture Depletion During Flash Droughts
Vishal Singh, Tushar Apurv
Abstract
Abstract Several studies have attributed the rapid drying during flash droughts to vapor pressure deficit (VPD) by using VPD anomaly based metrics. In this study, we use an analytical approach to investigate the role of VPD in the depletion of soil moisture through evapotranspiration (ET) and evaluate if the anomalies reflect its actual contribution toward drying. We model the energy and water balance of the land and lower atmosphere using a simplified slab model and use the energy and water balance equations of the model to decompose the daily changes in soil moisture during flash droughts into those caused by external forcings such as precipitation, wind and radiation on a day and the system response driven by the initial conditions of ground temperature, air temperature, soil moisture and specific humidity on the day. We apply this approach to investigate the role of VPD in ET‐driven soil moisture depletion in 10 major watersheds of India. Contrary to previous studies, we find negligible contribution of VPD toward soil moisture depletion in humid regions and high contribution in arid regions. In arid regions, even low anomalies of VPD can contribute significantly to soil moisture depletion due to the large magnitude of VPD, whereas in humid regions even large VPD anomalies do not lead to significant impact on soil moisture depletion. Our results show that the influence of VPD is underestimated in arid regions and overestimated in humid regions in previous studies which have used VPD anomaly based metrics for quantifying its effect on flash droughts.