Coupled soil heat and moisture transfer -- a simplified soil energy and hydrologic model

Abstract

While Soil temperature and moisture information are urgently needed in meteorology, agriculture, and forestry, field measurements are not easy to be carried out due to geographical conditions, soil types, and other restrictions. Modeling of the physical processes in soil provides an alternative method to get access to the information. Soil heat and moisture are interrelated, and the relationship of the two-state variables makes a considerable difference in the modeling of soil physical processes. In order to study the importance of soil moisture and improve the energy simulation of soil heat transfer, a one-dimensional, finite difference, coupled soil heat and moisture transfer model is established. The difference between the uncoupled and coupled soil heat and moisture models shows different behavior according to the boundary conditions imposed, the resulting differences are up to $1\ ^oC$ in soil temperature profile and $0.04$ in soil moisture profile. The coupling model is tested with both idealized boundary conditions and realistic boundary conditions, by utilizing energy fluxes outputs from the land surface model. The effect of soil moisture on soil thermal properties and the temperature effect on water content distributions are addressed in the coupling model. Radiation and atmospheric forcings adapted to the coupled model gives agreeable results of the diurnal temperature change of the whole soil profile. This gap between the simulation results and the ERA5 data can be narrowed by increasing the model's initial soil moisture content according to the ERA5 data, which shows the adjustability of the coupled model. The coupled model displays a better defined diurnal cycle than ERA5, with more sensitivity to the variations in incoming shortwave and longwave, because of the higher spatial resolution.