Future shift of the relative roles of precipitation and temperature in controlling annual runoff in the conterminous United States

Abstract. Precipitation and temperature are the two key climatic variables that control the hydrological cycle and water availability for humans. This study examines the potential shift of the relative roles of precipitation and temperature in controlling annual runoff in the conterminous United States (CONUS), using a water-centric ecohydrological model driven with historical records and climate scenarios constructed from 20 CMIP5 (Coupled Model Intercomparison Project Phase 5) climate models. The results suggest that precipitation has been the primary control of runoff variability and trend during the latest decades. However, the influence of temperature is projected to increase in a continued warming future in the 21st century. Despite considerable uncertainty and regional diversity, the multi-model ensemble reveals a high degree of consistency in the general increasing trend of both precipitation and temperature in the future, imposing positive and negative effects on annual runoff, respectively. The magnitude of temperature effect tends to exceed that of precipitation, and thus leads to an overall decrease of 8 ~ 30 mm yr⁻¹ (3 % ~ 11 %) runoff by 2100. Overall, temperature and precipitation changes are expected to contribute to runoff change by 58 % ~ 65 % and 31 % ~ 39 % separately, indicating that the role of rising temperature may outweigh that of precipitation in the later part of the 21st century. Across the CONUS, runoff decrease and increase in 34 % ~ 52 % and 11 % ~ 12 % of the land area are expected to be dominated by long-term changes in temperature and precipitation, respectively. We found that the vast croplands and grasslands across the central and forests in the northwestern regions might be particularly vulnerable to water supply decline caused by the changing climate.