Role of Initial Conditions and Meteorological Drought in Soil Moisture Drought Propagation: An Event-Based Causal Analysis Over South Asia

The role of meteorological droughts and initial conditions (land and atmosphere) in soil moisture drought (SMD) propagation are not yet fully understood. This work uses a drought event-based causal framework to investigate the relative importance of meteorological drought (MD) duration and intensity and initial conditions that result in surface and rootzone SMD, considering their event-level propagation time (PT) over South Asia. Initially, spatial variability of drought propagation is assessed by the Propagation Ratio (PR) computed based on MD counts that trigger SMD at various lags. PR depicts 2-3 months slower rootzone propagation than at surface. The gradual decrease in PR with increasing regional aridity indicates faster propagation over humid regions. The causal impact of initial conditions and MD parameters on propagating SMD are evaluated using normalized mutual information and a newly proposed normalized conditional mutual information. We found greater importance of triggering MD parameters followed by initial soil moisture condition on propagating SMD. This behavior is more evident for the surface layer propagation at shorter PT. There is a confounding effect of initial atmospheric conditions on drought propagation through initial soil moisture, depicting the significance of land-atmosphere interactions prior to propagation. In the rootzone propagation, initial soil moisture has a greater influence on propagation, especially at longer PT, indicating the significance of soil moisture persistence. Stronger causal links obtained through the joint influence of MD parameters on SMD suggest the importance of accounting for MD duration and intensity simultaneously, which are not considered in drought index-based propagation studies. Understanding the process of meteorological droughts transitioning to soil moisture droughts helps improve drought prediction capabilities and effectively manage agricultural water resources. This work explicitly addresses how meteorological drought "events" propagate into soil moisture drought "events" at surface and rootzone layers with variable propagation time-a critical aspect overlooked in the past. Our findings depict slower and weaker propagation to the rootzone layer compared to the surface layer and suggest that the transition rate is sensitive to regional aridity. Causal associations between initial land-atmospheric conditions and triggering meteorological drought properties with propagating soil moisture droughts are established using novel entropy-based mutual information metrics. We found higher importance of triggering drought parameters on propagating droughts both at surface and rootzone soil when their joint influence is considered rather than accounting for any individual initial condition or an individual triggering drought parameter. These causations also vary regionally and event-wise with changes in propagation times. The current work advocates investigating drought propagation at the event level instead of using standardized timeseries information. The findings would also be helpful for future studies linking land-atmosphere interactions with drought events. Propagating surface soil moisture drought has greater impact of meteorological drought (MD) than initial land and atmospheric conditions Propagating soil moisture drought (SMD) at rootzone has comparable influence of MD and initial soil moisture at longer propagation times Considering MD duration and intensity together yields higher causation between MD and propagating SMD than accounting for them exclusively