Assessment of hydrological drought vulnerability using SPI and SWI indices in Tiruttani block Tamilnadu

Drought is a natural phenomenon that manifests in multiple forms, each characterized by distinct indicators and impacts on both environmental systems and human society. Recent meteorological observations have highlighted a significant decline in rainfall, which has directly contributed to reduced groundwater storage in aquifers. This study presents an integrated approach to assess hydrological drought vulnerability by combining 30 years of rainfall data (1995-2024) from three rain-gauge stations and Climate Hazards Group InfraRed Precipitation with Station gridded precipitation, along with groundwater level time series and PET data for SPEI computation. The research outlines the methodology for deriving SPI, SPEI, and SWI, integrating them into a single hydrological drought severity index (HDSI), generating spatial and temporal drought maps in GIS, and developing a hydrological drought vulnerability map through weighted overlay analysis incorporating soil, aquifer depth, land use, slope, and HDSI for the Tiruttani block in the Thiruvallur district of Tamil Nadu, India. DrinC and ArcGIS Pro 3.1 software's were utilized for the analyses. Rainfall drought years were identified using the 3-month, 6-month and 12-month standardized precipitation index (SPI) and SPEI, while groundwater drought years were determined using the standardized water level index (SWI). SPI results showed 14 years Near Normal droughts, 7 years moderately wet, and 7 years drought (1 severe). Also distinct drought clustering across two major temporal phases 1995-2007 and 2008-2024 corresponding to shifts in rainfall distribution and hydroclimatic behavior over the semi-arid region. SWI results indicated moderate drought was observed in the years 1995, 2000, 2001, 2007 and extreme droughts in the years 2005, 2006, 2009, 2010, 2012, 2013, and 2014. The integrated SPI, SPEI, and SWI values were ranked and used in a weighted overlay analysis with soil, aquifer depth, land use, slope, and HDSI to evaluate the spatial variability of hydrological drought vulnerability across the study area. The findings revealed that 72% of the area falls under severe to extreme hydrological drought conditions, 12% under moderate drought, 3.5% under mild drought, while only 0.05% remains relatively drought-free. The study concludes that the region is facing escalating hydrological stress driven by inadequate rainfall conservation and groundwater mismanagement. Improved groundwater recharge practices and effective rainwater conservation strategies are essential to prevent further intensification of drought conditions.