Development of a soil-plant-atmosphere continuum model (HDS-SPAC) based on hybrid dual-source approach and its verification in wheat field

HDS-SPAC, a new soil-plant-atmosphere continuum (SPAC) model, is developed for simulating water and heat transfer in SPAC. The model adopts a recently proposed hybrid dual source approach for soil evaporation and plant transpiration partitioning. For the above-ground part, a layer approach is used to partition available energy and calculate aerodynamic resistances, while a patch approach is used to derive sensible heat and latent heat fluxes from the two sources (soil and vegetation). For the below-ground part, soil water and heat dynamics are described by the mixed form of Richards equation, and the soil heat conductivity equation, respectively. These two parts are coupled through ground heat flux for energy transfer, root-zone water potential-dependent stomatal resistance, and surface soil water potential-dependent evaporation for water transfer. Evaporation is calculated from the water potential gradient at soil-atmosphere interface and aerodynamic resistance, and transpiration is determined using a Jarvis-type function linking soil water availability and atmospheric conditions. Some other processes, such as canopy interception and deep percolation, are also considered in the HDS-SPAC model. The hybrid dual-source approach allows HDS-SPAC to simulate heat and water transfer in an ecosystem with a large range of vegetation cover change temporally or spatially. The model was tested with observations at a wheat field in North China Plain over a time of three months covering both wet and dry conditions. The fractional crop covers change from 30% to over 90%. The results indicated that the HDS-SPAC model can estimate actual evaporation and transpiration partitioning and soil water content and temperature over the whole range of tested vegetation coverage.