CILMATE CHANGE: COMPLICATING WATER RESOURCES MANAGEMENT (Invited Presentation)

Climate change adds uncertainty for water managers because it lessens a hydrologist‘s ability to use historical data to project future climate conditons, which is a key factor for decision making. Hydrologists incorporate changing climate conditions into projections of water supply, hopeing the results will be useful to water resources managers. One approach for assessing water resources is to use hydrologic models to better understand cause and effect relationships in the hydrologic cycle. In the present work, we use an integrated hdyrologic model to better understand why historical streamflow records indicate decreasing summertime streamflow while previous studies indicate predominantly increasing trends in precipitation across the Western United States. These non-intuitive trends make it difficult for managers because increased annual precipitation is typically thought to increase annual groundwater recharge. To gain insight on this seemingly paradoxical finding, we rely on a calibrated, integrated hydrologic model to simulate climate impacts on surface-water/groundwater interactions using 12 general circulation model projections of temperature and precipitation from 2010 to 2100. Hydrologic simulations show groundwater flow to streams peaks following the decrease in stream depth caused by snowmelt recession, and the shift in snowmelt causes a corresponding shift in groundwater discharge to streams. Our results show that groundwater discharge to streams is depleted during the summer due to earlier drainage of shallow aquifers adjacent to streams even if projected annual precipitation and groundwater recharge increases. These projected changes in surface-water/groundwater interactions result in more than a 30-percent decrease in the projected ensemble summertime flow, over a range in future climate conditions and model input parameters. These findings provide important information to water resources managers to better plan for the effects of future climate change, which may not follow general preconceptions of the basic interplay between precipitation and the resulting amount of water flowing in streams.