Driving factors of groundwater storage variability in the transboundary Bug River Basin

Monitoring variations in groundwater storage (GWS) is essential for sustainable groundwater resource management, particularly in regions where groundwater constitutes the primary source of potable water. Effective management and planning of groundwater use further require a thorough understanding of the factors controlling GWS variability, including meteorological conditions, regional hydrogeological characteristics, and anthropogenic influences.

In this study, we investigate temporal changes in GWS in the Bug River basin, located along the border of Poland, Ukraine, and Belarus. GWS estimates are derived from in-situ point measurements as well as satellite- and model-based data. Satellite-based GWS is obtained from downscaled terrestrial water storage (TWS) anomalies derived from Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) observations, in combination with data from the Global Land Data Assimilation System (GLDAS) model. We analyse long-term trends, seasonal components, and non-seasonal variability in both in-situ and satellite-derived GWS. Furthermore, we examine the relationships between GWS variations and potential driving factors, including precipitation, evapotranspiration, land surface temperature, and climate indices such as the Standardized Precipitation Index (SPI) and the Standardized Precipitation-Evapotranspiration Index (SPEI). We further analyse GWS variability in relation to groundwater table depth and lithology. Additionally, the correspondence between in-situ observations and GRACE-derived GWS is investigated.

The study demonstrates a high level of agreement between in-situ and satellite-based GWS, with correlation coefficients ranging from 0.69 to 0.95. The strength of this relationship depends on groundwater table depth, with the highest correlations observed for shallow aquifers. Seasonal variations in GWS, which are mainly controlled by precipitation and evapotranspiration, exhibit the strongest agreement between in-situ and satellite data. Overall, the study area exhibited negligible long-term GWS trends (0.0 to +1.0 mm/year) despite rising evapotranspiration over the past decade. Nevertheless, the period 2013–2023 was characterized by episodic positive and negative anomalies, which were more typical of deeper groundwater layers and more clearly captured by in-situ measurements. These findings highlight the value of integrating in-situ observations with satellite gravimetry for improving the understanding of groundwater dynamics and supporting sustainable groundwater management in transboundary river basins.