Incorporating Near-Surface Processes in Modeling Moisture Movement in Soils

The movement of moisture into and through near-surface soils is of major importance in numerous areas of geotechnical and geoenvironmental engineering, impacting soil strength, contaminant transport, efficiency of waste containment systems, and slope stability. Numerical models are often used to simulate moisture movement and behavior of soil systems due to the cost, complexity, and time required to undertake field studies. However, these simulations often ignore or grossly simplify near-surface processes such as precipitation, snowmelt, run-off, and evapotranspiration. It is not uncommon to see complex systems simulated using a one-dimensional model in which the surface moisture flux for the site is represented by a steady-state infiltration rate that is based on the annual average precipitation. In this paper we examine the impact of incorporating non-steady state models for near surface processes such as evapotranspiration, precipitation, and surface runoff on simulations of soil moisture movement in capillary barrier systems. Simulations with and without transient near surface processes have been conducted for a variety of sites with different climates. These sites include a semi-arid climate, a site where the bulk of the precipitation occurs during the winter months, and a wet, humid climate where rainfall exceeds 2 meters per year. The results demonstrate that simplification or omission of near-surface processes can lead to erroneous results.