Relative impacts of vapour pressure deficit and root-zone soil moisture on vegetation during flash droughts

Abstract

The impact of long-term drought conditions on agriculture and the ecosystem is well-known. However, short-term rapid decline in soil moisture leads to irreversible damage to vegetation, as observed over various parts of the world. Soil moisture varies spatially and vertically in the soil due to heterogeneities in climatic conditions, land cover, and soil properties. It has prominent retention and persistence effects, especially in the deeper layers, which supply water to plants. Flash Drought (FD) is associated with a rapid decline in root-zone soil moisture (RSM) for a short duration and causes significant damage to the crops and vegetation in an area. Prolonged hot and dry conditions or a severe heatwave often trigger FD. Consecutively, rapid changes in the land surface temperature and excessive evaporative stress during FD could impact vegetation at an ecosystem scale. Although the role of Vapor Pressure Deficit (VPD) is examined in the evolution of FD, its impact and antecedent soil moisture conditions in the rootzone on the vegetation still need to be better understood. The vegetation’s health depends on the available water content, which the roots supply during photosynthesis. Vegetation Water Content (VWC) is a potential descriptor that connects the soil-plant-atmosphere continuum in water movement from one medium to another. Vegetation Optical Depth (VOD) obtained from passive microwave sensors is a strong indicator of VWC. Through this work, we investigate the relative impacts of VPD and antecedent rootzone soil moisture conditions on VWC during FD in South Asia. We use European Centre for Medium-Range Weather Forecasts’ ERA5-Land, Global Land Evaporation Amsterdam Model (GLEAM), and Vegetation Optical Depth Climate Archive (VODCA) products for VPD, RSM and VWC, respectively, for the period 2002-2018. We use conditional mutual information to determine the strength of the relationship between these variables during the FD. Findings from this work will help us comment on FD’s impact on vegetation at ecosystem scales. This work will pave the way to examine the vegetation resilience to FD in the future.