Within a field that is used for agricultural production, a clay unit is found at a depth 8 ft below the land surface. The surface soil has a hydraulic conductivity of 0.6 ft/d. If parallel drains are placed 2 ft above the clay unit, and the maximum prolonged infiltration rate is expected to be 2 inches per day, and if the water level is to be maintained below 18 inches from the ground surface. Required:
What is the maximum allowable lateral drain spacing?