European Climate Signals in Water Budget Dynamics

Abstract

Monitoring the water budget helps us understand and manage the available water in a specific area, as well as track its use. Knowing the water budget (i.e., the difference between water inflow and outflow) allows us to predict water shortages or surpluses. In these times of climate change, the risk of extreme events such as droughts and floods is rising. The study considered the area of the European continent, classified according to the Koppen-Geiger climate zones, as it features a highly diverse climate. This research aims to evaluate the water budget across Europe and perform further analysis using indices: the Relative Retention Index (RRI), Storage Change Index (SCI), Water Stress Proxy (WSP), and Modified Runoff Ratio with EV correction (MRR). The research shows that area 4 (Scandinavia, with the coldest climate) exhibits the best water retention and balance. Monthly MERRA-2 data for precipitation, evapotranspiration and surface runoff were analysed for the period [2002-2025] across six European climatic regions. The results revealed a strong and coherent seasonal water budget cycle across all regions, while the magnitude of seasonal and interannual variability differed substantially among them. Correlations between regional water budget series and the European mean ranged from 0.834 to 0.912, indicating strong temporal coherence but also regional differences in the magnitude of water budget variability. Long-term trends in the annual water budget were weak and spatially heterogeneous, with values ranging from −3 × 10⁻⁹ to 5 × 10⁻⁹ yr⁻¹, and no statistically significant monotonic trend was detected across the six regions. The four derived indices complemented the aggregate water budget by characterising different aspects of water partitioning and hydroclimatic variability, including relative runoff response, storage-related changes and seasonal water stress. Their combined use revealed regional contrasts that were not apparent from the aggregate water-budget signal alone, particularly the stronger warm-season water-stress signal in southern regions and the distinct seasonal runoff response in the northern region.