Spatio-Temporal Analysis of Drought: From Identification to Propagation Pathways

Drought develops gradually as a consequence of sustained rainfall shortages and extends through the land surface system, leading to reductions in soil moisture and impacts on agricultural production. In regions such as India, where the climate is strongly influenced by the monsoon, clarifying the linkage between meteorological drought conditions and subsequent agricultural drought is essential for improving drought assessment and early warning capabilities. This study develops a spatio-temporal framework to examine drought propagation by combining statistical drought indices with network-based analysis. Meteorological drought is quantified using the Standardized Precipitation Index (SPI) at multiple accumulation time scales to represent short- and long-term rainfall anomalies. Agricultural drought is represented using the Standardized Soil moisture Index (SSI), which is calculated from soil moisture anomalies. Drought events are identified using run theory, from which their onset, duration, and severity are determined. The relative timing between meteorological and agricultural drought is evaluated by examining lagged correlations between SPI and SSI, which allows the response delay of agricultural drought to be estimated for different regions. The observed lag patterns differ across space, reflecting variations in soil properties, local climate conditions, and interactions between the land surface and the atmosphere. 
                  To assess spatial coherence, separate single-layer spatial networks are constructed for SPI and SSI, where grid cells represent nodes and statistically significant correlations define network connections. Network measures such as degree and betweenness centrality are applied to determine the regions that exert the greatest influence on drought connectivity. The analysis shows that meteorological drought tends to form more widespread and spatially coherent connectivity patterns, whereas agricultural drought exhibits stronger spatial contrasts linked to land-surface processes. These differences indicate that drought does not propagate uniformly but follows region-specific pathways shaped by local response times within the hydrological system. The proposed framework improves understanding of drought evolution from rainfall deficits to soil moisture stress and provides useful insights for drought monitoring, early warning, and climate-resilient agricultural planning.

Keywords: Standardized Precipitation Index, Standardized Soil moisture Index, Drought propagation, Network analysis