There exist mathematical models to determine the optimum depth and spacing of field drains from measurements of hydraulic conductivity, or indirect estimates of this. The results are reasonably useful as long as the hydraulic conductivity exceeds perhaps. 1–2 m/day. For heavier, less permeable soils, it is necessary either to use very cheap “pipes” (mole drains) at the calculated spacing or normal pipes at wider spacing after increasing the hydraulic conductivity by subsoiling.