Implications of climate-driven variability and trends for the hydrologic assessment of the Reynolds Creek Experimental Watershed, Idaho

The Soil and Water Assessment Tool (SWAT) model was used to assess the implications of long-term climate trends for the hydroclimatology of the Reynolds Creek Experimental Watershed (RCEW) in the Owyhee Mountains, Idaho of the Intermountain West over a 40-year period (1967-2006). Calibration and validation of the macroscale hydrology model in this highly monitored watershed is key to address the watershed processes that are vulnerable to both natural climate variability and climate change. The model was calibrated using the streamflow data collected between 1997 and 2006 from the three nested weirs, the Reynolds Mountain East (RME), Tollgate and Outlet. For assessing the performance of the calibrated model, this study used 30years of streamflow data for the period between 1966 and 1996. This investigation suggested that the model predicted streamflow was best at RME, and inadequate at Outlet. Simulated soil moisture was also verified using the data available from five soil moisture measurement sites. The model was able to capture the seasonal patterns of changes in soil water storage considering the differences in the spatial extent of the observed and predicted soil water storage (point measurements against the spatially averaged values for the HRU) and uncertainty associated with the soil moisture measurements due to instrument effects. Water budget partitioning during a wet (1984) water year and a dry (1987) water year were also analyzed to characterize the differences in hydrologic cycles during the extreme hydrologic conditions. Our analysis showed that in the dry water year, vegetation at the higher elevation were under water stress by the end of the water year. Contrastingly, in the wet water year only the vegetation at low and mid-elevations were under water stress whereas vegetation at the higher elevations derived substantial soil moisture for ET processes even towards the end of the growing season. To understand the effect of climate change on the hydrologic cycle, the observed and simulated streamflow were analyzed for trends in Center of Timing (CT). Earlier CT timings for the simulated and observed streamflow at RME weir was obvious thus manifesting global warming signals at the watershed scale level in the Intermountain west region. Observed streamflow at the Tollgate and Outlet weirs, where streamflow is partially affected by the agricultural diversions, showed later CT timings and these results appeared to suggest that climate impact assessment studies need to carefully distinguish the system behavior that is altered by both natural and human-induced changes.