This study examines the relative roles of climatic variables in altering annual runoff in the conterminous United States (CONUS) in the 21st century, using a monthly ecohydrological model (the Water Supply Stress Index model, WaSSI) driven with historical records and future scenarios constructed from 20 Coupled Model Intercomparison Project Phase 5 (CMIP5) climate models. The results suggest that precipitation has been the primary control of runoff variation during the latest decades, but the role of temperature will outweigh that of precipitation in most regions if future climate change follows the projections of climate models instead of the historical tendencies. Besides these two key factors, increasing air humidity is projected to partially offset the additional evaporative demand caused by warming and consequently enhance runoff. Overall, the projections from 20 climate models suggest a high degree of consistency on the increasing trends in temperature, precipitation, and humidity, which will be the major climatic driving factors accounting for 43-50, 20-24, and 16-23g% of the runoff change, respectively. Spatially, while temperature rise is recognized as the largest contributor that suppresses runoff in most areas, precipitation is expected to be the dominant factor driving runoff to increase across the Pacific coast and the southwest. The combined effects of increasing humidity and precipitation may also surpass the detrimental effects of warming and result in a hydrologically wetter future in the east. However, severe runoff depletion is more likely to occur in the central CONUS as temperature effect prevails.
Bibliographical noteFunding Information:
Acknowledgements. This work was supported by the National Science Foundation EaSM program (AGS-1049200) awarded to North Carolina State University, and the Eastern Forest Environmental Threat Assessment Center (EFETAC), USDA Forest Service, Raleigh, NC. The MACAv2-LIVNEH dataset was produced under Northwest Climate Science Center (NW CSC) US Geological Survey grant number G12AC20495. Partial support was provided by the Natural Science Foundation of Jiangsu Province, China (BK20151525), and the Pine Integrated Network: Education, Mitigation, and Adaptation project (PINEMAP), which is a Coordinated Agricultural Project funded by the USDA National Institute of Food and Agriculture (award #2011-68002-30185). The authors would like to give special thanks to Dennis Lettenmaier, Paul CD Milly, William Farmer, Brian Finlayson, and two anonymous reviewers for their valuable comments and suggestions.
1Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, NC, USA 2Eastern Forest Environmental Threat Assessment Center, USDA Forest Service, Raleigh, NC, USA 3Coweeta Hydrologic Laboratory, USDA Forest Service, Otto, NC, USA 4Key Laboratory of Meteorological Disaster of Ministry of Education, Nanjing University of Information Science & Technology, Nanjing, Jiangsu, China 5State Climate Office of North Carolina, North Carolina State University, Raleigh, NC, USA