Water Movement and Heat Transfer Simulations in a Soil under Ryegrass

Soil temperature is an important factor that affects most processes and reactions occurring in the unsaturated zone. In this paper, a mathematical model based on the solution of the heat transfer equation in the soil with the finite element method is developed. The heat transfer model has been incorporated as part of the integrated model WANISIM (water and nitrogen simulation), which describes the soil water movement and the mass transport and nitrogen transformations in the soil. Three different types of boundary condition at soil surface are considered: soil temperature varies cosinusoidally; heat balance; and Erosion–Productivity Impact Calculator (EPIC) model approach. The soil temperature of EPIC model approach is examined in comparison of the conduction–convection heat transfer model. Data from a soil covered by grass for a time period of 10 months are used for the application and checking of the model. A significant agreement was observed between the simulation results and the measurements of water content and soil temperature. The heat transfer model with heat balance at soil surface boundary condition gives more accurate results (average error |Em|⩽0·96 °C and root mean square error RRMSE ranged from 11·79 to 19·05), in comparison with soil surface temperature boundary condition (|Em|⩽1·39 °C and RRMSE ranged from 18·82 to 27·76). Different functions of thermal conductivity evaluation result in non-significant differences of soil temperature evaluation.