Declining Transboundary Inflows Drive Rising Water Stress in Azerbaijan: A 24-Year WSI Decomposition Corroborated by GRACE Storage Trends

Azerbaijan faces escalating water stress driven by climate variability, socio-economic pressures, and pronounced transboundary dependence. This study presents a transparent, reproducible framework that decomposes the Water Stress Index (WSI = TAWW/TRWR × 100) into its hydrological (internal runoff, transboundary inflows, groundwater) and sectoral (agricultural, domestic, industrial) components, and integrates it with hydro-climatic drivers (P, ETₐ) and GRACE/GRACE-FO total water storage (TWS). Applied to Azerbaijan for 2000–2023, the framework yields mean TRWR of 21.8 km3 year−1, mean TAWW of 11.7 km3 year−1, and mean WSI of 56% (range 35–88%); the high-stress threshold (40%) was exceeded in 20 of 24 years, peaking at 88% in 2020. TRWR declined at −524 Mm3 year−1 (p < 0.001), driven almost entirely by falling transboundary inflows, while TAWW rose at +89 Mm3 year−1 and WSI at +1.8 percentage points year−1. GRACE mascons show a basin-scale TWS decline of −1.1 to −1.5 cm year−1 over 2002–2025 (p < 0.001). A key new result is a national water-balance closure test: a climate-only budget (ΔS = P − ETₐ − Q) is essentially flat (+0.10 cm year−1) and negatively correlated with GRACE (r = −0.61), whereas a full budget that includes the documented anthropogenic groundwater over-abstraction (which grew 4.3-fold between 2000 and 2020) matches GRACE with r = +0.96, RMSE = 2.5 cm, and trend agreement within 0.07 cm year−1. This provides an independent, budget-based attribution of the observed storage decline to unsustainable groundwater extraction rather than climatic variability. Sensitivity analysis reveals a supply-side dominance: a 20% reduction in Q, raises WSI by ≈10 percentage points, versus ≈ 7 for an equivalent demand reduction.