Final Report: Projecting Future Streamflow in the Colorado River Basin

This project aims to better characterize how the changing climate of the Southwest is affecting cool and warm season precipitation in the Colorado River basin, and the corresponding response of streamflow in select individual sub-basins. The principal research objective is to assess whether the level of complexity of downscaling, applied to the official global climate change projection models used by the Intergovernmental Panel on Climate Change (IPCC), substantially affects resultant future streamflow projections used for operational planning purposes. The current methodological standard used for future streamflow projection by the US Bureau of Reclamation (BOR) applies the Bias Correction and Spatial Disaggregation (BCSD) technique to IPCC global climate change projection models to generate surface temperature and precipitation, which is then used as input to a pre-calibrated hydrologic model in a given river basin or sub-basin. Within this work select IPCC global climate models (GCMs) are dynamically downscaled with regional climate models (RCMs), to generate alternative temperature and precipitation inputs to hydrologic models. The differences in resultant modeled streamflow then inform as to the sensitivity of the downscaling approach in generating projected streamflow in the Colorado River basin, and therefore robustness of the current BCSD-based BOR streamflow projections. The other research objective is to assess how regional climate models can improve upon the representation of natural climate variability in the western United States during the cool and warm season, by dynamically downscaling select IPCC global climate change projection models and a historical 20th century atmospheric reanalysis. The investigation of historical climate variability in the context of a dynamically downscaled 20th century reanalysis is a new aspect to the project not included in the original proposal, and informed by the stakeholder interaction during the course of the project. There was a strong desire from the participating water resource providers to have more reliable historical climate information for the early twentieth century. This particular period is of interest for several reasons related to water resource planning: 1) it is when most of present-day water resource delivery infrastructure in the southwestern United States was constructed (i.e. major dams and reservoirs), 2) it corresponds to the period when the Colorado River Compact was enacted, which governs Colorado River water allocation among the states 2) where significant long-term wet and dry periods occurred, that are used as a basis for present-day operational planning purposes.