Design Drainage Rates for Estimating Drain Spacings in North Carolina

Abstract

ABSTRACT CORN yields were simulated for a large number of drainage system designs on 12 North Carolina soils that require improved drainage for efficient crop production. Simulations were conducted for three drain depths, ten drain spacings and two surface drainage treatments for each of the 12 soils. Economic analyses were conducted to determine the drain spacing that would give maximum return to land and management for each drain depth and surface drainage treatment. Those spacings were then used in Hooghoudt's equation to determine the steady drainage rate that would result when the water table midway between the drains is at the surface. This value, normally referred to the drainage coefficient, q, in the Hooghoudt equation, was called the design drainage rate, DDR. The average DDR values obtained for eastern N.C. were 1.1 cm/day for good surface drainage and 1.3 cm/day for poor surface drainage. Use of those DDR values would result in returns greater than 90% of the maximum in 65 of 72, or 90% of the cases analyzed. Annual returns greater than 95% of the maximum were predicted in 51 of 72, or 70% of the soils considered. Hooghoudt's equation with the above DDR rates will give a good first approximation of the optimum drain spacing, but will tend to predict spacings that are somewhat too narrow for soils with low hydraulic conductivities, and somewhat too wide for soils with high conductivity values. The analyses were conducted for eastern North Carolina soil and climatological conditions and the DDR values obtained should not be assumed for other locations.