Ecosystem Water Use Efficiency Responses to Drought in the Nile River Basin: Spatial Patterns, Environmental Controls, and Resilience

Ecosystem water use efficiency (WUE) is a key indicator of terrestrial carbon–water coupling, yet the mechanisms governing drought sensitivity across heterogeneous climatic and vegetation regimes remain poorly understood. This study examines the spatial variability, environmental controls, drought memory, and post-drought resilience of WUE across the Nile River Basin (NRB) from 2000 to 2024, using satellite-derived gross primary productivity (GPP) and evapotranspiration (ET), together with multi-timescale Standardized Precipitation Evapotranspiration Index (SPEI) data. WUE exhibited a pronounced north–south gradient, with higher, more stable values in humid upstream regions and lower or declining values in arid downstream areas. Air temperature, solar radiation, wind speed, and vapor pressure deficit were the dominant environmental controls, whereas precipitation and soil moisture exerted weaker, largely indirect influences. The relative contributions of GPP and ET to drought-related WUE variability varied with drought intensity, vegetation type, and climatic conditions. Drought exhibited substantial memory effects, with cumulative responses affecting 68% of the basin and an average response timescale of approximately 4 months. Lagged responses averaged 5.52 months and were generally longer in humid and sub-humid regions, consistent with greater soil and subsurface water storage. Approximately 64% of the basin was classified as resilient, although recovery dynamics varied among ecosystems, with forests exhibiting stronger drought buffering but slower recovery. Overall, WUE dynamics across the NRB reflect interactions among climate, vegetation, drought memory, and hydrological controls, highlighting the importance of distinguishing between atmospheric drought and ecosystem water stress under increasing hydroclimatic variability.