Projecting Climate Change Impacts on Seasonal Water Supply Forecasting Error

This study explores changes in water supply forecasting error associated with climate change, with a focus on snowmelt-dominated basins in the western U.S. For study basins where climate change would appear to adversely affect error, the study also explores potential mitigation strategies in forecasting procedures.

Understanding projected climate impacts on seasonal water supply forecasting is relevant to understanding our future capabilities in managing western U.S. reservoir systems as climate changes. Every year, seasonal water supply forecasts inform the scheduling of seasonal to annual plans of operation for such reservoir systems. Forecasts describe the expected seasonal runoff volume and uncertainty during the snowmelt period when much of the annual surface water supply is captured in reservoir systems (e.g., April-July reservoir inflow volumes, with forecasts issued initially in January and updated monthly heading into spring). In snowmelt-dominated basins, snowpack offers significant predictive information in these forecasts. While it has been well-studied that climate warming reduces snowpack and associated monthly runoff patterns, it has not been well-studied how such hydrologic impacts would also affect water supply forecasting error.

The study focuses on eight basins distributed in the western U.S. (CA, OR, ID, MT, WY and CO). Methods involve process-based hydrologic simulation using NWS SacSMA/Snow17 models to develop synthetic hydroclimates from historical to future (i.e., generating runoff projections consistent with 1950-2099 temperature and precipitation projections obtained from the "Statistically Downscaled WCRP CMIP3 Climate Projections" archive). NWCC statistical procedures are then applied to generate water supply forecast projections. In doing so, water supply forecast models are developed within the basin hydroclimate projections, and are subjected to periodic model updating consistent with real-world practice. Results are analyzed for projected change in snow predictor influence in these forecast models and change in forecast error characteristics. The presentation will summarize project findings and highlight their ramifications for climate change vulnerability assessments on reservoir operations and water management.