Influences of rainfall unsaturated infiltration on the change of heat transfer in soils

In the high temperature season, the surface temperature of the soil is very high, and the internal moisture content of soil is low. A sudden rainfall has a great influence on the dynamic change of soil temperature, and as well as the water-heat exchange and migration. Temperature change of soil surface and water movement can be interacted with each other during this time. It is also very important to reveal the influence mechanism of water movement caused by rainfall unsaturated infiltration, on temperature change of soil and the law of heat transfer. However, there are only a few research achievements on the hydrothermal interaction of soil in the process of rainfall unsaturated infiltration. In this study, a mathematical model of soil heat transfer in the process of rainfall infiltration was established, and the corresponding finite element numerical calculation program was compiled, in order to reveal the soil moisture movement in unsaturated rainfall infiltration process of high temperature, and the influence mechanism of soil heat transfer. Taking the typical silt soil layers in Nanjing as research objects, a numerical analysis was carried out for the unsaturated rainfall infiltration exerting to the function of soil heat transfer. The results showed that without considering rainfall infiltration, the soil temperature change and heat transfer were mainly caused by the heat exchange between the surface soil and the environment, where the depth of heat transfer was about 0.2 m. Under the action of rainfall intensity of 45 mm/h, the soil matric suction and volume moisture content in the silt layer changed gradually with the duration of rainfall, due to the unsaturated infiltration process of rainfall. When the soil moisture front moves down and reached different depths, the volume moisture content increased rapidly, and the soil volume moisture content after the moist front gradually approached the saturated volume moisture content and the soil infiltration capacity gradually decreased until reaching the saturation infiltration rate. In rainfall unsaturated infiltration, the heat exchange occurred between the low-temperature rainfall in soil pores and soil particles, which can change the distribution of original temperature field in soil. Moreover, the influence of rainfall infiltration process on soil heat transfer gradually weakened, with the continuous increase in the depth of rainfall infiltration. Compared with the data of field measurement and the simulation results, the relative error of measured and calculated soil volume moisture content remained within ±3.99%, and the Root Mean Square Error (RMSE) was 0.01 cm3/cm3. The relative error of soil temperature between measured and numerical values remained within ±2.72%, and the RMSE was 0.55℃, indicating that the simulation results and the measured data were consistent with better. It infers that there was a strong adaptability in the model for the rainfall unsaturated soil infiltration process, and heat migration in soil, and the model was verified by numerical program. The causes of errors were related to the initial moisture content of the soil, the initial temperature, and the accuracy of rainfall infiltration during the test. The findings can provide an important reference for water conservancy engineering in modern agriculture, including the soil and water conservation, distribution and utilization of soil hydrology and water resources, control of urban water resources and ecological environment protection. It is of great significance to further study the coupled migration mechanism of soil water and heat transfer caused by rainfall infiltration. © 2020, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.