Quantitative Estimation of the Effects of Soil Moisture on Temperature Using a Soil Water and Heat Coupling Model

Soil moisture is not only an essential component of the water cycle in terrestrial ecosystems but also a major influencing factor of regional climate. In the soil hydrothermal process, soil moisture has a significant regulating effect on surface temperature; it can drive surface temperature change by influencing the soil’s physical properties and the partitioning of the available surface energy. However, limited soil temperature and moisture observations restrict the previous studies of soil hydrothermal processes, and few models focus on estimating the impact of soil moisture on soil temperature. Therefore, based on the experiments conducted in Wuchuan County in 2020, this study proposes a soil water and heat coupling model that includes radiation, evaporation, soil water transport, soil heat conduction and ground temperature coupling modules to simulate the soil temperature and moisture and subsequently estimate the effects of soil moisture. The results show that the model performs well. The Nash–Sutcliffe coefficient (NSE) and the concordance index (C) of the simulated and measured values under each treatment are higher than 0.26 and 0.7, respectively. The RMSE of the simulation results is between 0.0067–0.017 kg kg −1 (soil moisture) and 0.43–1.06 °C (soil temperature), respectively. The simulated values matched well with the actual values. The soil moisture had a noticeable regulatory effect on the soil temperature change, the soil surface temperature would increase by 0.08–0.43 °C for every 1% decrease in soil moisture, and with the increase in soil moisture, the variation of the soil temperature decreased. Due to the changes in the solar radiation, the sensitivity of the soil temperature to the decline in soil moisture was the greatest during June–July and the least in September. Moreover, the contributions of soil moisture changes to temperature increase under various initial conditions are inconsistent, the increase in sunshine hours, initial daily average temperature and decrease in leaf area index (LAI), soil density and soil heat capacity can increase the soil surface temperature. The results are expected to provide insights for exploring the impact mechanism of regional climate change and optimizing the structure of agricultural production.