HYDRAULIC PROPERTIES OF UNSATURATED SOILS

Introduction Many agrophysical applications require knowledge of the hydraulic properties of unsaturated soils. These properties reflect the ability of a soil to retain or transmit water and its dissolved constituents. For example, they affect the partitioning of rainfall and irrigation water into infiltration and runoff at the soil surface, the rate and amount of redistribution of water in a soil profile, available water in the soil root zone, and recharge to or capillary rise from the groundwater table, among many other processes in the unsaturated or vadose zone between the soil surface and the groundwater table. The hydraulic properties are also critical components of mathematical models for studying or predicting site-specific water flow and solute transport processes in the subsurface. This includes using models as tools for designing, testing, or implementing soil, water, and crop management practices that optimize water use efficiency and minimize soil and water pollution by agricultural and other contaminants. Models are equally needed for designing or remediating industrial waste disposal sites and landfills, or assessing the for long-term stewardship of nuclear waste repositories. Predictive models for flow in variably saturated soils are generally based on the Richards equation, which combines the Darcy–Buckingham equation for the fluid flux with a mass conservation equation to give (Richards, 1931):