NSF base funding supports the NCAR Water System research program. Within that larger program, RAL conducts a Colorado Headwaters project that examines the impact of climate change on water resources in this critical region. In FY2013 researchers have continued their analysis of different scenarios of mid-21st century future hydroclimate with continuous, 8-year, very high resolution (4km) regional climate model simulations using the Weather Research and Forecasting (WRF) model. New analyses of these continuous 8-year climate downscaling runs are generating new insights into how the partitioning of the hydrologic cycle will change under a warmer climate.
The climate sensitivities obtained from 4-km WRF simulations differ from the current statistically-based guidance being provided to water managers. WRF shows wintertime increases in precipitation in the Colorado Headwaters that are consistent with a warmer and moister atmosphere, and occur when topography is adequately resolved by the regional climate model. There is now a critical need to understand the differences in the portrayal of climate change impacts on water resources obtained using different methods (e.g., dynamical WRF simulations vs. statistical downscaling), and, thus, the effectiveness of traditional climate impact assessments to support adaptation planning and decision making.
Research in the Colorado Headwaters project during FY 2013 has focused on evaluating changes in the partitioning of precipitation between evapotranspiration (ET) and runoff under a changing climate, and how climate change may affect the future availability of water resources. As one specific example of this research, the figure below illustrates the seasonal cycles of precipitation, ET, runoff, and potential evaporation (PE) for the current and future periods. The monthly changes in precipitation due to climate change are dominated by increases during winter (through May) and then decreases during summer, resulting in a net increase in precipitation of 31 mm yr-1. Offsetting the precipitation increases, monthly ET is higher in nearly all months in the future climate scenario with ET increases most pronounced during late spring and early summer. ET actually decreases slightly in August and September in the future due to the reduction in future precipitation during this period and the depletion of soil moisture (note the differences between ET and PE). The annual increase in ET is 37 mm yr-1, resulting in a net annual decrease in runoff of 6 mm yr-1.
These analyses have motivated a series of new modeling studies to investigate regional differences in the impact of climate change on seasonal cycles of snow accumulation and ablation, soil moisture, ET, and the overall partitioning of precipitation between ET and runoff. A key focus of these new studies is to evaluate the hypothesis that the hydrologic cycle in many regions will shift from a hydrologic regime where ET is primarily limited by availability of energy, to a regime where ET is limited by the availability of soil moisture as caused by earlier snow melt and increases in the amount of energy during the growing season. New runs are now underway using a modeling domain which covers the coterminous U.S. and parts of Canada and Mexico, so that we can begin to compare and contrast the change signals from the Colorado Headwaters regions to other regions such as the Pacific Northwest and the Canadian Rocky Mountains.