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Title: Adapting HYDRUS-1D to simulate overland flow and reactive transport during sheet flow deviations

item LIANG, JING - University Of California
item Bradford, Scott
item SIMUNEK, JIRI - University Of California
item HARTMANN, ANNE - Ugt Environmental Measurement Devices Gmbh

Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/4/2017
Publication Date: 6/8/2017
Citation: Liang, J., Bradford, S.A., Simunek, J., Hartmann, A. 2017. Adapting HYDRUS-1D to simulate overland flow and reactive transport during sheet flow deviations. Vadose Zone Journal. 16(6):1-18. doi: 10.2136/vzj2016.11.0113.

Interpretive Summary: : Runoff of water and contaminants from agricultural fields contributes to contamination of surface water supplies in many portions of the United States. A popular numerical model (HYDRUS 1D) was modified to simulate overland flow and contaminant transport from agricultural runoff. The developed model has important advantages over other codes, including: a graphical user interface for pre- and post-processing, an inverse parameter optimization routine, and ability to simulate non-uniform water flow and reactive contaminant transport over heterogeneous soil surfaces. The developed model improves our understanding and ability to describe overland flow and transport processes from agricultural fields and hillslopes. This modeling tool will be of interest to scientists, engineers, consultants, and government regulators that are concerned with assessing the impact of agriculture on the quality of surface water supplies and minimizing contamination.

Technical Abstract: The HYDRUS-1D code is a popular numerical model for solving the Richards equation for variably-saturated water flow and solute transport in porous media. This code was adapted to solve rather than the Richards equation for subsurface flow the diffusion wave equation for overland flow at the soil surface. 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 transport and fate of many different solutes, such as non-adsorbing tracers, nutrients, pesticides, and microbes. However, 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. 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. HYDRUS-1D includes a hierarchical series of models of increasing complexity to account for both physical equilibrium and non-equilibrium, e.g., dual-porosity and dual-permeability models, up to a dual-permeability model with immobile water. The same conceptualization as used for the subsurface was implemented to simulate non-equilibrium overland flow and transport at the soil surface. The developed model improves our ability to describe non-equilibrium overland flow and transport processes, and improves our understanding of factors that cause this behavior.