Recognition of Hydrogeological Droughts by Spi and Sgi Indices in the Southern Bug River Basin

It is obvious that hydrogeological drought is caused by meteorological drought and is caused by a decrease in groundwater recharge and an increase in its flow. However, it remains unclear which factor is more influential: rising air temperatures, decreased precipitation, declining river stages, or other related variables. The article highlights a method for identifying hydrogeological drought based on the analysis of SGI indices and confirmation of meteorological drought. Based on retrospective data, the duration of meteorological droughts was assessed to determine whether it was sufficient to trigger significant and prolonged recession in groundwater levels. Groundwater level depths affected by meteorological drought were also analyzed. Spatial and temporal patterns of drought manifestation were identified, along with the degree of inertia in groundwater level response to climatic changes and meteorological phenomena. The analysis was conducted using data from hydrogeological monitoring wells in the Southern Bug River Basin, by calculating and comparing the Standardized Precipitation Index (SPI) and the Standardized Groundwater Index (SGI). A series of SPI and SGI maps for the year 2016 was developed using the spline interpolation method in the ArcMap environment. Additionally, the long-term dynamics of SGI over the 1980–2020 period were analyzed using data from nine monitoring wells. The results showed spatial correlation between the indices, but with a temporal lag: the most intense meteorological drought was recorded in July–September, while the greatest groundwater level decline was observed in August–October. The analysis allowed the identification of the most drought-prone regions in the central part of the basin and substantiated the feasibility of combining SPI and SGI indices for comprehensive water resource monitoring. The proposed approach can serve as a foundation for the development of a regional early warning system for groundwater droughts and adaptive groundwater management under climate change conditions.