An Evaluation of Future Climate Change Impacts on Key Elements of the Water–Carbon Cycle Using a Physics-Based Ecohydrological Model in Sanchuan River Basin, Loess Plateau

The cycle of carbon and water in ecosystems is likely to be significantly impacted by future climate change, especially in semiarid regions. While a considerable number of investigations have scrutinized the repercussions of impending climatic transformations on either the carbon or water cycles, there is a scarcity of studies delving into the effects of future climate change on the coupled water–carbon process and its interrelationships. Based on this, the Sanchuan River Basin, an ecologically fragile region of the Loess Plateau, was chosen as the research area. General circulation model-projected climate scenarios (SSP1-2.6, SSP2-4.5, SSP5-8.5) and an ecohydrological model were integrated to predict (2021–2100) changes in actual evapotranspiration (ET), surface runoff (Rs), net primary productivity (NPP), and soil organic carbon (SOC). The results indicated that under the impacts of future climatic warming and humidification, ET, Rs, and NPP will increase by 0.17–6.88%, 1.08–42.04%, and 2.18–10.14%, respectively, while SOC will decrease by 3.38–10.39% in the basin. A path analysis showed that precipitation and temperature had significant effects on ET and NPP, Rs was more sensitive to precipitation, and temperature had a significant impact on SOC. Furthermore, all climate scenarios had an average ET-NPP correlation coefficient greater than 0.6, showing that the basin’s water–carbon cycle was tightly coupled. However, under SSP5-8.5, the correlation coefficient of Rs-NPP decreased from −0.35 in the near-future period to −0.44 in the far-future period, which may indicate that the positive effect of increased precipitation on Rs-NPP would barely offset the negative effect of large future temperature increases. As a foundation for achieving sustainable water resource management and ecosystem preservation policies, this study can be utilized to build adaptation methods to manage climate change.