Steady-State Water Tables in Drained Lands Modeled Using the HYDRUS Package and Compared with Theoretical Analyses

Abstract

Steady-state water tables above parallel lines of cylindrical drains in an infinitely deep soil with uniform surface rainfall were modeled using a computer package that considers the flow in both the groundwater and unsaturated soil regions. The results were compared with analytical results obtained using a hodograph solution that considers only the groundwater region and assumes that the flux through the water table is equal to the surface input. Steady-state water tables above horizontal drains at the level of an impermeable floor were also modeled and compared with previous analytical results. Agreement of modeled and theoretical results was obtained in both drainage cases when the drainage was modeled for both sand and silty clay soils using a model for soil-water relationships that does not include an air-entry value in their soil-water characteristics. Water tables were lower than the analytical values when simulations were undertaken with sand and silty clay soils with characteristics described in a model that included air-entry values. Midway between drains they were higher than those obtained analytically for a completely tension-saturated soil above the water table, but in the vicinity of the drain lines they were slightly lower. The study shows that steady-state drainage theory accurately describes water table profiles in many soils while indicating the need for less intense drainage for soils that display tension-saturated capillary fringes. It gives confidence in the assumption of one-dimensional vertical flow in the unsaturated soil above the water table in the general consideration of groundwater flows.