Coupled model of surface runoffand surface-subsurface water movement

Surface runoff has been recognized as an important component in agricultural water management. Extensive studies have been developed to measure surface runoff, and numerical methods have been applied to simulate surface water dynamics. For simulations of surface runoff in agricultural systems, three processes should be considered: (1) the movement of water along the soil surface, i.e., surface runoff; (2) the accumulation of water in depressions; and (3) the water fluxes across the soil/atmosphere interface (i.e. infiltration, evaporation and exfiltration through seepage faces). The objective of this study is to develop a physical-based surface runoff model that includes all the three processes. Numerical implementation of the new runoff model was developed and incorporated within 2DSOIL, a simulation package for soil water, energy and solute movement. The new model describes the subsurface flow near the soil surface following a unified boundary condition, expressed with the Heaviside step function. This expression enables continuous and automatic changes of boundary conditions between infiltration and runoff. The surface water flow is simulated using Saint-Venant equations. The ponded water height on the soil surface and the infiltration rate are adjusted based on the runoff flux and topography. Numerical tests based on an experimental dataset are used to evaluate the accuracy of this model, and numerical examples of surface water flow along a variety of topography are used to demonstrate model performance. The simulations match the experimental results, and the surface water mass balance errors of the numerical examples are less than 1%. A practical example of using the surface runoff model to estimate the runoff efficiency in a ridge-furrow water harvesting is carried out. In conclusion, the newly developed surface runoff model can successfully simulate surface water dynamics. This model can further support the design and evaluation of agricultural water management strategies and field water budgets.