Refining global water cycle components under a changing climate

Accurate quantification of global water-cycle components remains a major challenge in Earth system science. In this study, we refine historical and future estimates of global river flow by applying an emergent constraint (EC) approach, which integrates outputs from 26 Earth System Models (ESMs) with observed streamflow from 50 large river basins (covering 26.9% of global land area). For the historical period (1980–2014), we estimate global river flow at  39.1±5.4×10³ km³ yr^-1 and a river flow-to-precipitation ratio of 0.35±0.03 , both lower than previous assessments. Land evapotranspiration is estimated at 73.4±6.2×10³ km³ yr^-1. Under future climate change, the EC-constrained projection indicates a global river flow increase of ,7.8±5.5 mm yr^-1 K^-1, which is 9.3% lower than the ESM ensemble mean and reduces inter-model uncertainty by 66%. Our results highlight a systematic overestimation of river flow increases in current ESMs, underscoring the importance of incorporating observational constraints and human impacts to improve the reliability of hydrological projections. This study provides a benchmark for global water-cycle partitioning and supports more accurate water resource planning under climate change.

This work is supported by the National Natural Science Foundation of China (Grant No. 42330506 and 42361144709), the Talent Program of the Ministry of Science and Technology of China, and the PIFI outstanding international team project by the Chinese Academy of Sciences.