Hydrological Droughts in the Carpathian Basin: From Historical Evidence to Future Risk under CMIP6 Climate Scenarios

Abstract

This study extends a multi-index (SPI, STI, SPEI, SDI) assessment of hydrological drought across the Carpathian Basin (1961–2024, 36 stations) with new future projections for the Danube River at the Bezdan station under CMIP6 climate scenarios. Historical analysis shows that precipitation (SPI) has remained largely stationary across the basin, while temperature (STI) has warmed significantly at every station, driving a predominantly water-balance (SPEI) and, in small tributary catchments, hydrological (SDI) drying signal; annual SDI at Bezdan itself shows no trend (Kendall's tau = 0.02). Building on this framework, monthly SDI projections (2025–2050) were generated from three CMIP6 general circulation models (CNRM-ESM2-1, EC-Earth3-CC, IPSL-CM6A-LR), bias-corrected against station records, under SSP1-1.9 and SSP5-8.5. Results show moderate-drought frequency roughly doubling under SSP5-8.5 (9–13%) relative to SSP1-1.9 (3–7%), with a severe episode robustly projected across all three models in mid-2026. Low inter-model correlation underscores persistent structural uncertainty, arguing for multi-model, temperature-aware drought monitoring.