Predicting drainage rates under varying water table conditions

Finite element solutions for the two dimensional Richard's equation are used to study the relationship between drainage rates and water table elevation for parallel drains. The use of traditional drainage equations based on midpoint water table elevation lead to errors in estimating drainage rates for high water table conditions. An algorithm for predicting drainage rates in terms of average water table elevation was developed. The algorithm, which combines classical solution by Kirkham for ponded conditions, Hooghoudt for drawdown conditions and an empirical equation for the transition, was tested for conventional and controlled drainage conditions. The new algorithm can be used to predict drainage rates for water table conditions ranging from drain level to a fully saturated profile with ponded surface.