Scale-and Seasonal-Dependent Sensitivity of Hydrologic Projections in the Colorado River Basin to Different Downscaling Methods

This study created future streamflow projections using dynamically downscaled precipitation and temperature projections from the Intermediate Complexity Atmospheric Research (ICAR) model and the Variable Infiltration Capacity (VIC) model. These more physically realistic projections were compared with VIC simulations based on the widely utilized localized constructed analog (LOCA) statistical downscaling method to evaluate how downscaling choices influence water supply estimates. ICAR’s annual streamflow change at Lees Ferry, Arizona, was higher than LOCA in the ensemble mean due to equal influences from precipitation and temperature. However, annual streamflow changes at Lees Ferry were not significantly different between downscaling methods despite significant differences in precipitation and temperature as there was still a substantial spread and there were no systematic differences between downscaling methods across Earth system models. Similar streamflow projections were the result of different seasonal and spatial patterns. For example, while ICAR projected less cool-season precipitation than LOCA at high elevations (>3000 m), summer precipitation projections, which are often characterized as more uncertain, differed substantially between downscaling methods and increased ICAR’s streamflow projections relative to LOCA. Furthermore, downscaling decisions were more likely to affect streamflow projections at the local scale rather than the regional scale as 25%–32% of the smaller catchments within the upper basin showed significant differences between downscaling methods in annual streamflow changes. Finally, this study considers the uncertainty and confidence in the process representation of precipitation and temperature, emphasizing how spatial and seasonal differences in these variables__and the ways their underlying processes are represented__ directly influence streamflow projections.