Abstract

Mountains act as “water towers,” sustaining critical hydrological functions for ecosystems and societies, particularly in arid and semi-arid regions where they provide a disproportionate share of water resources and support unique ecosystems. While their role in storing and releasing surface water is well recognized, groundwater contributions in mountain landscapes remain underexplored. In Mediterranean mountains, where water scarcity and climate-driven changes in precipitation and snowpack dynamics threaten groundwater recharge, this gap is particularly concerning. Here, we identified and mapped potential groundwater-dependent vegetation (pGDV) across contrasting hydrogeological environments in Sierra Nevada (SE Spain) using Sentinel-2 NDVI (2019–2023) ecohydrological indicators: (i) dry-season NDVI, (ii) dry–wet seasonal NDVI difference, and (iii) interannual NDVI variability. We combined these indicators into 64 classes, later simplified into three levels of groundwater dependence (Unlikely, Moderate, and Likely GDV). To explore hydrogeological controls, we assessed spatial patterns across three aquifer zones (alpine, carbonate, and weathered hard-rock) and examined vegetation-climate relationships through correlations with the Standardized Precipitation Evapotranspiration Index (SPEI). Our results reveal the hidden importance of weathered hard-rock aquifers within metamorphic substrates, which, despite limited storage capacity, sustain the largest share of Likely GDV (31.8%), mainly riparian forests, deciduous woodlands, and high-mountain meadows. In comparison, carbonate and alpine aquifers accounted for 14.9% and 1.3% of Likely GDV, respectively. These findings highlight how shallow groundwater in weathered hard-rock aquifers sustains vegetation greenness and functioning during dry periods. Management strategies should prioritize recharge areas and traditional water management systems to preserve groundwater-dependent vegetation under increasing climate stress.