Criteria for calculating drain spacing and depth

Agricultural and biological engineers have had a leading role in developing drainage design methods that span the range from simple drainage equations to complex computer simulation models. While current efforts are focused on development of complex models to quantify crop and drainage water quality response to design and management alternatives, there is still a place for simple drainage equations. Application of drainage design equations is limited by lack of design criteria for most locations in the U.S. This article reports results of a simulation study to determine design criteria for the steady-state Hooghoudt equation and the transient van Schilfgaarde equation. Simulations were conducted to determine drain spacings that maximize profit for three drain depths on four soils at five locations in eastern U.S. Drainage design rates (DDR) for the Hooghoudt equation and the time required for 30 cm water table drawdown (TDD30) were determined for each of the "optimum" drain spacings. Results were analyzed to develop equations for predicting design criteria (DDR and TDD30) in terms of growing season rainfall, drain depth, profile transmissivity, and drainable porosity for the humid region of the U.S. These criteria may be used, with their respective equations, to estimate drain depth and spacing for any location in eastern U.S.