Utility of L-band and X-band vegetation optical depth to examine vegetation response to soil moisture droughts in South Asia

Abstract

Vegetation Water Content (VWC) holds essential information about plant photosynthetic activity, phenological changes, ability to withstand extreme events, and soil-plant-atmosphere interactions. However, there is no consensus on the extent to which Soil Moisture (SM) and atmospheric dryness (Vapor Pressure Deficit, VPD) affect VWC during droughts. In this work, we determine the causal relationships with VWC in response to VPD and SM when plants are subjected to SM droughts during the growing season. We use satellite-derived Vegetation Optical Depth (VOD), obtained from two frequencies (L- and X- bands), to represent VWC in different parts of the plant (stem, branches, and leaves). The holistic response of VOD considering VPD and SM interactions is determined using bivariate and multivariate Granger causality tests. The analysis is carried out over South Asia.The results show a statistically significant coupling between VPD and SM at semi-weekly scales over 53.20% of the study area. In these locations, we observe a dominant influence of VPD compared to SM on VODL and VODX while subjected to soil moisture droughts during the growing season. In the areas where VPD and SM are decoupled, we find both VPD and SM to influence VODL and VODX. VODL and VODX largely exhibit contrasting causal responses. We observe a surge in VODL due to water stress while subjected to soil moisture droughts under all land cover conditions. In the case of VODX, we noticed both an increase and a decrease in leaf water content while subjected to SM droughts. The former scenario is noticed predominantly under moderate to severe drought intensity conditions during which plants attempt to maintain a stable water column (through increased stomatal regulation) for their functioning as a response to SM droughts. The latter scenario is noticed in most pixels, including forests and croplands. The observed leaf water content loss in forests is due to their mild drought intensities. We also observe a stronger causal influence of VPD and SM in low LAI areas than in high LAI areas, indicating a pressing plant water stress in sparse vegetation conditions during SM droughts.