Warming and Forest Thinning Effects on the Hydrologic Cycle in the Beaver Creek

Observational evidence is mounting on the reduction of winter precipitation and an earlier snowmelt in the southwestern United States. It is unclear, however, how these changes, along with forest thinning, will impact water supplies due to complexities in the precipitation-streamflow transformation. In this study, I use the Triangulated Irregular Network-based Real-time Integrated Basin Simulator (tRIBS) to provide insight into the independent and combined effects of climate change and forest cover reduction on the hydrologic response in the Beaver Creek (~1100 km2) of central Arizona. Prior to these experiments, confidence in the hydrologic model is established using snow observations at two stations, two nested streamflow gauges, and estimates of spatially-distributed snow water equivalent over a long-term period (water years 2003-2018). Model forcings were prepared using station observations and radar rainfall estimates in combination with downscaling and bias correction techniques that account for the orographic controls on air temperature and precipitation. Model confidence building showed that tRIBS is able to capture well the variation in snow cover and streamflow during wet and dry years in the 16 year simulation period. The results from this study show that the climate change experiments increased average annual streamflow by 1.5% at +1°C of warming. However, a 28% decrease in streamflow occurs by +6°C of warming as evapotranspiration (ET) increases by 10%. Forest thinning shifted the warming threshold where ET increases reduce streamflow yield until +4°C of warming as compared to no forest thinning when this threshold occurs at +2°C. An average increase in streamflow of 12% occurs after forest thinning across all climate scenarios. While the snow covered area is unaffected by thinning, the volume of snowmelt increases and is linked to the higher water yield. These findings indicate that water managers can expect decreases in streamflow due to climate change but may be able to offset these impacts up to a warming threshold by thinning forested areas within the Beaver Creek.