Relationship between the Storage Coefficient and the Soil-Water Retention Curve in Subsurface Agricultural Drainage Systems: Water Table Drawdown

Water dynamics in subsurface agricultural drainage systems are described by the Boussinesq equation for an unconfined aquifer that results from the continuity equation, the Darcy law, as well as from a hydrostatic pressure distribution hypothesis. The storage coefficient that appears in the equation is conceptualized according to the unsaturated zone that results from water table drawdown during the drainage process. The difference between drained depth and drainable depth, as well as the hydrostatic pressure distribution hypothesis, allows us to infer a relationship between storage coefficient, drainable porosity, and the soil-water retention curve. This relationship is illustrated by using the van Genuchten and Fujita–Parlange soil-water retention curves. Both resulting storage coefficient expressions are validated in an experiment reported in the literature using a numeric solution of the one-dimensional Boussinesq equation. A good description of the experimental results leads to a conclusion in which the proposed relationship between storage coefficient and the soil-water retention curve can be used in the context of studies in water dynamics in subsurface agricultural drainage systems.