Drain Depth and Subirrigation in Layered Soils

Abstract

The Richards equation for two-dimensional saturated—unsaturated water movement was solved for subirrigation from buried drains in layered soils. Numerical solutions were obtained for soils that have subsurface layers with high hydraulic conductivities. Results showed that increasing the drain depth so that it approaches or penetrates a high conductivity bottom layer will reduce head loss due to convergence near the drain. Deeper drains significantly increased subirrigation rates for deep soils and narrow drain spacings but had a smaller effect for shallow profiles and wide spacings. The response to deeper drains also increased with hydraulic conductivity and thickness of the bottom layer. The results showed that the drain should be placed at the interface of the high conductivity layer when possible. Further increases in the depth of the drain did not result in significant increases in subirrigation rate or water table rise.