How Will the Hydrograph Change in Predicted Future Climates? A Detailed Comparative Analysis of Macro-Scale Hydrological and Climate Model Output From the California Sierra Nevada

Future climate scenarios project spatially heterogeneous changes in both temperature and precipitation over the next 100 years. Potentially one of the greatest climate change impacts to freshwater systems is changes in watershed hydrology, namely stream discharge. Because of anticipated hydrologic alteration, water managers in much of the arid to semi-arid western US are concerned about earlier spring peak flow events caused by a shift from snow to rain events. While climate impacts to average flows, seasonal timing, flood peaks, and droughts have been analyzed, we are not aware of a comprehensive analysis of projected climate-related changes to the entire stream hydrograph.

To fill this gap in understanding, we compiled a dataset of projected stream discharge for eight streams along California’s west slope of the Sierra Nevada. Historic observed climate data and six bias-corrected statistically downscaled future climate projections were run through the Variable Infiltration Capacity (VIC) macroscale hydrologic model to produce daily stream discharges in the past (1958-2008) and future (2008-2058) for eight rivers. Effects of flow regulation were removed to create an unimpaired model observed dataset. With these datasets we used the Indicators of Hydrologic Alteration (IHA) analysis tool to measure 67 parameters of the hydrographs for the first and last 30 years of the past and future flow record.

Results within each downscaling method were consistent, even when the results for individual parameters differed from the observed. Model results matched observed data within 10% across all IHA parameters, with only a few exceptions. In general, less annual discharge is projected in the next 50 years, with a strong shift in the timing of the largest peak (earlier) and duration of the peak flow event (shorter). The future rise and fall rates of both the large and small peak events are projected to be faster than in the historic record. The median flows of May through July are projected to have the largest decrease over the entire year. These results suggest that ecological processes dependent on the timing, magnitude and duration of peak events (both large and small), as well as those dependent on elevated summer low flows are projected to see the greatest change. This detailed information about how these various hydrologic indicators are projected to change can be useful for water managers in developing climate adaptation strategies.