Simulation of coupled transport of soil moisture and heat in a typical karst rocky desertification area, Yunnan Province, Southwest China

Understanding the transport processes of soil moisture and heat is critical for vegetation restoration in karst rocky desertification areas where serious soil erosion and extensive exposure of carbonate rocks occur. Numerical simulation can provide an important approach to explore the transport processes of soil moisture and heat, but few studies employing this technique have been carried out in karst rocky desertification areas of southwest China. In this study, a model of coupled soil moisture and heat transport was established using HYDRUS-1D based on the high-resolution data of soil moisture, soil temperature, and meteorological parameters obtained throughout a year in a typical karst rocky desertification area in Yunnan province, southwest China. The modeling results reflect the rainfall-infiltration-evaporation processes in rocky desertification areas well. The frequently rainfall events in small intensity in the study site often induced great variations of soil moisture in the near-surface soil layer (< 1-cm depth). However, soil moisture in deep soil layer (> 10-cm depth) kept stable during light rainfall events, implying that the deep soil was only influenced by heavy rainfall events. The variations of soil temperature showed a high sinusoidal fitting trend. At the annual scale, variations of soil temperature were distinct apparent evident below the depth of 40 cm, but no evident daily variations were observed. The simulated fluxes of soil water showed that the vapor fluxes were lower than the liquid water fluxes by 3-6 orders of magnitude, suggesting the control of soil thermal gradients. Our results also indicate that the vapor flux has great significance for plant water utilization in the drought periods. The simulation errors are small for soil temperature but slightly more significant for the soil moisture in deep soil layer. This primary failure may result from the occurrence of preferential flows at the rock-soil interface, which needed to be further investigated in the future.