Simple Methods for Predicting Flow to Drains

Abstract

Flow to subsurface drain tubes was investigated using numerical solutions to the Richards equation for combined unsaturated and saturated flow. Solutions to three sets of problems involving steady infiltration and transient drainage of initially curved and flat water tables were obtained for two soils and various profile depths. The numerical solutions were used to illustrate various aspects of the drainage process and to evaluate simple analytic expressions for calculating drain flow rates and water table elevations. An approximate method was developed for predicting transient drainage events by combining the Hooghoudt equation with analytic expressions for radial flow into drain tubes and for mass balance. The predictions of the approximate method were found to be in agreement with the numerical solution for drainage of an initially elliptic water table.