Human contributions to evapotranspiration mitigate swings in dry to wet year transitions

California’s food and economic security depends on water availability, particularly under increasingly extreme climate scenarios. A key component of the water balance is evapotranspiration, the combination of soil and surface evaporation and plant transpiration. Evapotranspiration is influenced by natural drivers (e.g., climate, vegetation cover) and human intervention (e.g., irrigation, land management). Here, we analyze the transition between one of California’s driest years (2022) to an exceptionally wet year (2023) to assess evapotranspiration responses to climate extremes. Despite increased precipitation, total statewide evapotranspiration changed less than 10%. In 2022, human contributions accounted for 30% of statewide evapotranspiration and 80% in managed lands. In 2023, natural evapotranspiration increased, and human contributions fell by 30%, yet still comprised nearly 50% of evapotranspiration in managed areas. Our findings underscore the enduring role of human activity on California’s hydrology, even during wet years, and demonstrate a framework to separate natural and anthropogenic controls on evapotranspiration.

加州的粮食与经济安全依赖水资源供给，在气候日益极端化的背景下尤为如此。水量平衡的核心组分之一是蒸散发（evapotranspiration），即土壤蒸发、地表蒸发与植物蒸腾的总和。蒸散发受到自然驱动因子（如气候、植被覆盖）与人类活动干预（如灌溉、土地管理）的共同影响。本研究分析了加州从极端干旱年份（2022年）到异常湿润年份（2023年）的气候转变过程，以评估蒸散发对极端气候事件的响应。尽管降水量显著增加，全州范围内的总蒸散发变化幅度仍不足10%。2022年，人类活动贡献的蒸散发占全州总蒸散发的30%，在受管理土地中这一占比高达80%。2023年，自然蒸散发有所提升，人类活动贡献的蒸散发占比下降30%，但在受管理区域仍占近50%。本研究结果凸显了人类活动对加州水文系统的长期影响——即使在湿润年份亦不例外，同时也提供了一套可区分蒸散发自然与人为控制因子的分析框架。