Remote sensing’s role in improving transboundary groundwater monitoring and sustainable management: The Bug Basin, Polish-Ukrainian-Belarusian Borderland

Despite the progress in Polish-Ukrainian cooperation on transboundary waters through the establishment of joint regulatory bodies and legislative agreements, the problem of integrated groundwater monitoring still remains unresolved. This study presents the application of remote sensing to address data gaps concerning transboundary groundwater resources. The main advantage of remote sensing measurements is that the data they provide are temporally consistent and include spatial information, unlike point-based in-situ observations. Moreover, due to the use of multiple sensors, remote sensing enables comprehensive studies over large areas simultaneously, which is typically challenging in inaccessible regions.

Currently, only the GRACE/GRACE-FO mission (Gravity Recovery and Climate Experiment and GRACE Follow-On) provides direct measurements of terrestrial water storage (TWS) changes, which are largely governed by groundwater storage capacity. Our study presents the quantification of groundwater resources in terms of fluctuations in the shallow unconfined water table by integrating GRACE/GRACE-FO gravity data, precipitation observations, evapotranspiration, river runoff, and groundwater depth. Using machine learning algorithms, data from multiple sources were assimilated, achieving accurate groundwater quantification at a spatial resolution of 0.1°. Previous assessments of transboundary groundwater resources in the Bug River basin were based on a sparse and uneven observational network with a density of 0.003 points/km², as well as old (often from the 1980s) hydrogeological maps at a scale of 1:50,000.

The results of our novel approach indicate that groundwater resources in the study area are depleting, primarily due to increased evapotranspiration, despite a stable precipitation level of around 700 mm/year. According to GRACE/GRACE-FO observations, between 2012 and 2023, TWS in the Bug River basin decreased at a rate of 8.8±5.2 mm/year. Our comprehensive study serves as a source for the reassessment of available groundwater resources, providing information on the sustainable allocation of transboundary groundwater resources between Poland, Ukraine, and Belarus.

The study was conducted as part of the project GRANDE-U “Groundwater Resilience Assessment through iNtegrated Data Exploration for Ukraine” (NSF Awards No. 2409395 and 2409396).