Unsaturated Flow in Ground-Water Hydraulics

Water tables as zero-pressure surfaces do not separate saturated hydraulic conductivity from impermeability in systems of flow of water through soil. Hydraulic conductivity relationships on the negative pressure side of the water table are a property of the soil and must be experimentally determined. The various methods for obtaining these relations are examined, and curves relating hydraulic conductivity to negative soil-water pressure are shown for 28 soils. These curves, which are generally sigmoid, can be simplified to step functions for including flow at negative soil-water pressure in the analysis of steady flow systems. Such systems are then solved with a certain negative pressure (called the critical pressure head), rather than with zero pressure, at boundaries where the system is in contact with atmospheric air in the soil. Critical pressure heads calculated for the 28 soils show that this factor may vary from -15 cm of water or more for sands to -100 cm water or less for structureless loams and clays. The procedure is demonstrated for systems with mainly horizontal flow, downward flow, and for flow through restricting layers.