Ecohydrological Controls on the Altitude Effect of Stable Isotopes in Surface Water Across Slope Aspects and Seasons

ABSTRACT The isotope altitude effect is a key indicator of mountain hydrological processes, yet its ecohydrological regulation in complex terrain remains poorly understood. This study investigates the spatial and seasonal variations in δ 2 H, δ 18 O and d ‐excess in surface water across three slope aspects of Cangshan Mountain, Southwest China, based on 116 surface water samples collected during the 2022 rainy season and 2023 dry season. Integrating isotope data with remotely sensed ecological indicators, we examine how ecological factors modulate the δ 18 O–elevation relationship. Results show surface water isotopes are more enriched in the dry season, with δ 18 O following a North > East > West spatial pattern and d ‐excess opposite. The local surface water line (δ 2 H = 4.09δ 18 O‐40.98, R 2 = 0.82) indicates substantial evaporative enrichment. The δ 18 O lapse rate exhibits strong slope‐dependence: steepest on the North (−6.56‰ to −3.67‰/km), weaker on the East (−0.35‰ to −1.47‰/km) and reversed on the West (+0.45‰ to +1.25‰/km). Structural equation modelling indicates a physical‐driver–ecological‐modulation framework mechanism: altitude affects δ 18 O indirectly through land surface temperature (LST), while normalized difference vegetation index (NDVI), modified normalized difference water index (MNDWI) and evapotranspiration (ET) jointly determine lapse rate amplification, attenuation, or reversal across slope aspects and seasons. These results demonstrate that isotopic altitudinal gradients in mountain streams are highly sensitive to slope aspects and ecohydrological heterogeneity, with implications for water‐source identification and isotope‐informed hydrological assessment in complex mountain terrain.