Intensifying compound drought-heatwave events weaken ecosystem water-use efficiency in the Nile River Basin.

Climate warming has intensified compound drought-heatwave (CDHW) events worldwide, posing growing threats to ecosystem carbon-water dynamics. However, the impacts of CDHWs on ecosystem water-use efficiency (WUE), particularly event sequencing, drought memory, and climatic heterogeneity, remain poorly understood at large river-basin scales. This study investigated the spatiotemporal evolution of CDHW events and their impacts on ecosystem functioning across the Nile River Basin (NRB) during 2000-2025 using SPEI, heatwave detection, harmonized MODIS-derived GPP and ET, phase-space trajectory analysis, and machine-learning approaches. CDHW frequency, duration, severity, and spatial extent generally increased across the basin, with heatwave-leading events showing a 29.5% increase in spatial extent compared with 12.9% for drought-leading events. A chronological Pettitt test identified 2014 as a significant change point in CDHW frequency. Although basin-wide GPP and ET increased, WUE declined across 58.6% of valid WUE grid cells, indicating substantial spatial heterogeneity in ecosystem carbon-water coupling. CDHWs were generally associated with stronger reductions in GPP and ET than individual drought or heatwave conditions, while WUE commonly increased at the event scale because ET declined proportionally more than GPP. WUE responses varied across SPEI accumulation timescales, with longer antecedent timescales prevalent across much of the basin. Precipitation contributed most in arid and semi-arid regions, whereas temperature, radiation, and VPD contributed relatively more in wetter environments. These findings provide a comprehensive basin-scale assessment of ecosystem WUE responses to CDHWs in the NRB, highlighting spatial heterogeneity, event-scale impacts, and antecedent hydroclimatic associations, and inform ecosystem management under a changing climate.