Drains as a Boundary Condition in Finite Elements

Four different methods of representing drain tubes in finite elements for field drainage problems were studied. A finite‐element solution to the Richards equation was used to compare the performance of each method for three conditions: flow to parallel drains from a ponded surface, transient drainage, and flow to and past an interceptor drain. Differences in predictions of hydraulic heads and drain flow rates were found among the four methods. A method using logarithmically varying adjustment factors for hydraulic conductivity and a modified grid system in the vicinity of the drain was found to be relatively easy to implement and to produce accurate estimates of drain flow rates and hydraulic heads for the drainage problems simulated.