Saturated and Unsaturated Flow to Parallel Drains

Richards equation for transient two-dimensional flow during drainage was solved using numerical methods and the solutions compared to solutions of the more approximate Boussinesq equation. The effect of field variability of the soil properties on solutions to the Richards equation was also examined. For the cases studied, the hydraulic head distributions predicted by the Richards equation were in general agreement with the D-F assumptions except for times near t = 0. Convergence near drainage ditches is more critical immediately after drainage begins than for latter times. It results in a surface of seepage for ditches and water standing over the drain for drain tubes. When convergence near the drain is accounted for, solutions to the Boussinesq equation can be used to adequately predict drawdown for design purposes. Unless enough measurements are made to define field-effective values of the unsaturated soil-water properties, there is little to be gained by solving the Richards equation rather than using simpler approximate methods for drawdown prediction.