Assessing climate change impact on watershed hydrological processes and stream temperature by considering CO₂...

Carbon dioxide (CO₂) emissions are a critical indicator influencing climate change and have significant impacts on the health of riverine ecosystems. The effects of CO₂ emissions on streamflow and stream temperature have not been explicitly considered in process-based models, which limits the models' capability to simulate streamflow and stream temperature under varying CO₂ concentration scenarios. This study modified an equilibrium temperature model and a CO₂ effect model to overcome this limitation, which were subsequently coupled with the Soil and Water Assessment Tool (SWAT) model. The coupled model was tested and applied in the Chaohe River basin in China from 2021 to 2080, and daily streamflow and stream temperature were simulated under the RCP8.5 and RCP4.5 scenarios based on the ACCESS and HadGEM climate models. The study showed that the coupled model performs well in simulating streamflow and stream temperature, with the PBIAS of less than ±10 %, and both the NSE and R² exceeding 0.85. Under both the ACCESS and HadGEM climate models, the simulations of streamflow and stream temperature exhibit a consistent pattern: increased CO₂ concentration leads to higher air temperatures, which in turn elevates stream temperatures and changes streamflow mainly through evapotranspiration process. However, the lower CO₂ concentrations or where snowmelt is significant in regions, streamflow and stream temperature exhibit greater variability. When CO₂ levels are high to induce stomatal closure in plants, decreased evapotranspiration can lead to increased streamflow. In addition, headwater tributaries, primarily fed by rainfall, snowmelt, and groundwater, are located in high-altitude areas influenced by natural factors, while the main stem, mainly supplied by tributary inflows and precipitation, is situated in low-altitude areas affected by both natural and anthropogenic factors. This difference in water sources and influencing factors leads to distinct patterns in streamflow and stream temperature. Therefore, it is essential to develop algorithms that explicitly account for the impacts of CO₂ concentration on hydrological processes and stream temperature dynamics, to accurately simulate the effects of climate change on streamflow and stream temperature, enabling the prediction of future climate change impacts on the thermal regime of river basins. The coupled model developed in this study provides a valuable tool for simulating the effects of CO₂ on streamflow and stream temperature, offering insights into the complex interactions between climate change and hydrological processes.