Modeling Non-Equilibrium Overland Flow and Transport Processes Using Hydrus-1D and Hydrus-2D.

Recent studies indicated that overland flow can be described by the Richards equation with different functional forms for the hydraulic capacity and conductivity functions. HYDRUS-1D and HYDRUS-2D, popular numerical computer codes for solving the Richards equation, were modified to solve the 1D/2D diffusion wave equations using the Picard linearization scheme. The numerical results obtained by the new model produced an excellent agreement with the analytical solution of the kinematic wave equation. Model tests demonstrated its applicability to simulate the fate and transport of many different solutes, such as non-adsorbing tracers, pesticides, bacteria, and viruses.

The diffusion wave or kinematic wave equations describe surface runoff as sheet flow with a uniform depth and velocity across the slope. In reality, overland water flow and transport processes are rarely uniform. Local soil topography, vegetation, and spatial soil heterogeneity control directions and magnitudes of water fluxes, and strongly influence runoff characteristics along the hillslope. There is increasing evidence that variations in soil surface characteristics influence the distribution of overland flow and transport of pollutants. These spatially varying surface characteristics are likely to generate non-equilibrium flow and transport processes. However, accurate physically-based modeling of non-equilibrium water flow and solute transport remains a challenge in the field of surface hydrology.

Existing subsurface physical non-equilibrium models include a hierarchical series of models of increasing complexity, from a mobile-immobile water model, dual-porosity and dual-permeability models, up to a dual-permeability model with immobile water. The same conceptualization can be applied to realistically simulate surface water flow and solute transport in order to achieve a better understanding of these natural processes. Depending upon available information and data, the presented non-equilibrium models are able to simulate various non-equilibrium features caused by depression storage zones at the soil surface or random fields of roughness.