Artesian and Anisotropic Effects on Drain Spacing

Subsurface drainage systems installed in soil overlying artesian aquifers should be spaced to handle both upward artesian water flow and normal downward seepage flow from irrigation and rainfall. Proper drain spacing depends on several parameters, while a narrower than normal spacing is required for drains subject to artesian conditions. The hydraulic conductivity of the soil above an artesian aquifer determines the ease with which water flows to the drains and the magnitude of the upward artesian water flux for a given piezometric head and soil layer thickness. Since water movement to subsurface drains depends on both the horizontal and vertical components of hydraulic conductivity, anisotropy of the soil affects the drain spacing. For most soil formations, the hydraulic conductivity in the horizontal direction exceeds that in the vertical direction. Neglecting anisotropy may lead to under‐design of the drainage system for a formation above an impermeable layer and can lead to over‐design of a drain system for a formation subject to artesian conditions with no downward flow. For a soil subject to upward flow as well as downward flow, anisotropy effects depend on the magnitude of the different parameters influencing the problem.