Surface flux boundary simplifications for flow through clay under landscaped conditions

The quality of the solution of moisture flow through expansive soils for the purpose of depth of moisture influence determination for residential foundation design depends on properly described flux boundary conditions including appropriate environmental factors and inclusion of the microclimate created by human activity. In this study, both climatic and human imposed conditions typical to Phoenix, Arizona, were considered in 1-D unsaturated flow modeling. Many years of recorded precipitation data were obtained, and common irrigation practice from surveys of municipalities, together with evapotranspiration data from the regions, were used to determine the surface flux conditions for modeling. Rigorously described surface flux boundary conditions were used in the analyses, and simplifications to these conditions were systematically made to determine the impact of simplified boundary conditions on the final solution. It was found that major simplifications, through averaging of flux conditions and increased time-steps for application, result in only negligible difference in computed matric suction compared to more detailed simulations of flux when the capacity of the soil to absorb applied surface water is not exceeded, such as for the desert landscape conditions of this study. Otherwise, as observed for the turf irrigation case of this study, averaging surface flux can result in significant over-estimate of the extent and degree of wetting in the profile.