Hydrological Response of High Elevation Watersheds in the Conterminous United States to Climate Change under the Representative Concentration Pathway Scenarios

There is growing evidence that some high-elevation watersheds have experienced and are projected to experience future increases in temperature that are more rapid than the global land average. Climate changes in headwater watersheds have important implications for human welfare downstream, but course spatial resolution of future climate scenarios generated by general circulation models (GCMs) poses challenges for hydrological projections in small headwater watersheds where we have accurate measurements and knowledge on processes. We applied a hydrological and biogeochemical model PnET-BGC with statistically downscaled future climate change scenarios by 4 GCMs under the RCP4.5 and RCP8.5 scenarios to 11 intensively-studied high-elevation watersheds in the US to evaluate the impact of future temperatures and precipitation on the hydrological process of headwater watersheds across the country.

Climate change scenarios suggest future annual precipitation is projected to increase more in eastern than western sites. Projections of annual stream discharge show large variation in flow patterns at eastern sites, and more consistent increases at western sites. Our monthly time-step hydrological model projects large decreases in the magnitude of future snow water equivalent at most sites, but the timing of future snowpack accumulation and ablation changed significantly only in the Rocky Mountains. Snowpack changes have important implications to seasonal patterns of stream discharge and plant growth. We used a Budyko curve approach to explore future partitioning of precipitation into evapotranspiration and runoff. Future hydrological status of most studied headwater watersheds show a tendency towards drought, but they remain energy-limited systems. The changes are generally well predicted by the Budyko curve. However, some sites show changes in partitioning of precipitation to ET and discharge. These sites are largely temperate forests, which is different from previous expectations of low elasticity values and low resilience in alpine and subalpine ecosystems.