Seasonal variation and key factors influencing evapotranspiration partitioning in alpine ecosystems of the Qinghai Lake Basin

• Evapotranspiration partitioning by SPAC model, isotope method and uWUE methods showed consistent results. • Alpine ecosystems exhibit seasonal T/ET variations, peaking in summer and dropping in winter. • Canopy conductance integrates LAI and environmental factors, impacting T/ET dynamics. • Soil freezing/thawing impacts T/ET, with levels fluctuating during these periods. This study partitioned evapotranspiration (ET) into vegetation transpiration (T) and soil evaporation (E) across three alpine ecosystems in the Qinghai Lake Basin, China, from 2019 to 2021 using a soil–plant-atmosphere continuum (SPAC) model. The model’s effectiveness was validated by its consistency with observed energy fluxes, ground surface temperature, and the transpiration fraction of evapotranspiration (T/ET) derived from isotope methods, the underlying water use efficiency (uWUE) approach, and remote sensing data. Results show that T/ET ratios vary minimally from year to year, peaking in July or August and reaching their lowest in January. The subalpine shrub ecosystem exhibited the highest average T/ET, followed by temperate grasslands and alpine grasslands. Leaf area index, linked to canopy stomatal conductance, was the main driver of T/ET variability, along with environmental factors like radiation, soil moisture, and temperature. During the growing season, vegetation transpiration dominated ET, with T/ET rising in May and June due to thawing seasonally frozen soils and active plant growth. This research quantifies T/ET across various ecosystems in the Qinghai Lake Basin, highlighting the importance of canopy stomatal conductance in seasonal variations and its implications for ecohydrological processes and water resource management in alpine regions. The findings provide both theoretical support and practical guidance for optimizing water use efficiency, achieving sustainable ecosystem management, and ensuring water ecological security in alpine areas.