Forecasting Climate and Water Resources in the Context of Natural Variability and Climate Change

Abstract

The water resources of the Southwestern United States are under significant stress. The historical record of the Colorado River indicates that the commitment allocations (7.5 million acre-feet to both the Upper and Lower Colorado basin states, and 1.5 maf for Mexico) have overestimated the average available streamflow. Compounding the supply problem, the Bureau of Reclamation has projected an average decrease of 9% in the Colorado River streamflow between the years 2011-2060. Improving forecasts of climate and streamflow, at nearly all time scales, is imperative to most effectively manage these strained water resources. Given the challenges confronting the Southwest, three research studies are presented that could be used to assist water managers. The first study targets the lack of skill seen in seasonal forecasts of precipitation across the US issued by the Climate Prediction Center (CPC). An objective and concise methodology is shown to improve overall seasonal forecast skill as an alternative to forecasts made by the CPC. This methodology uses a combined linear and nearest neighbor model to make forecasts, with the NINO3.4 index as the only predictor. The second study shows skillful forecasts of decadal Colorado streamflow using the Atlantic Multidecadal Oscillation (AMO) and Pacific Decadal Oscillation (PDO) indices as predictors. However, even though the instrumental record showed statistically significant skillful forecasts, the reconstructed records of AMO, PDO and streamflow appear to challenge these results. Lastly, the third study investigates the effects of climate change in the 21st century on the Salt, Verde and Rio Grande river basins. Two dynamically downscaled General Circulation Models (GCMs) are first bias-corrected. Then, the output of these models is used as the climatic forcings for the Variable Infiltration Capacity (VIC) hydrologic model. Results suggest that future streamflows are projected to decrease by 22% and 37%, for the respective GCMs, averaged across the basins.