The Impact of Climate Change on Summer Low Flow of a Headwater Basin in Montana
Climate change is an important driver of hydrological extremes in cold regions. The summer low flows are among the hydrological extremes which have been impacted by climate change and will be prone to future changes in climate. In southwestern Montana, various stakeholders compete for limited water resource during the summer season. In this headwater region, the summer low flows mainly originate from groundwater storages which are often replenished during the previous seasons. The future changes in air temperature, precipitation, and snowmelt timing not only will affect groundwater recharge but also will have important implications for summer low flows. Here, our main objective was to simulate the future streamflow for a headwater region in southwestern Montana and analyze summer low flow variability in the distant future. Specifically, our focus was to assess summer low flow variability by mid-century and late-century compared to the early-century flows. To achieve our objectives, we ran Tsinghua Hydrological Model based on the Representative Elementary Watershed (THREW) using two future climate scenarios: RCP 4.5 (i.e. climate change mitigation with lower greenhouse gas emissions) and RCP 8.5 (i.e. business-as-usual). The study area was first divided into multiple Representative Elementary Watersheds which were further discretized into several hydrologic zones such as snow, vegetated, bare soil, saturated soil, unsaturated soil, sub-stream network and the main channel. THREW was first calibrated and validated at Big Hole watershed and then applied to the whole Jefferson basin in southwestern Montana. It was found that the summer low flow will decrease under RCP 8.5 climate scenario by mid-century and late-century. The decrease in summer low flow was higher for RCP 8.5 climate scenario and during late-century compared to for RCP 4.5 climate scenario and during mid-century, respectively. In conclusion, our results showed that under business-as-usual climate scenario, the decrease in summer low flows would lead to significant water stress by late-century in the headwater region of southwestern Montana.