Developing optimum subsurface drainage design procedures

The DRAINMOD simulation model was used to investigate combinations of drain depth and spacing for major soils of southern Minnesota to maximize profitability and minimize drained volume and surface runoff. Six soil types (Canisteo, Harps, Nicollet, Normania, Okoboji, and Webster) and three locations (Lamberton, Waseca, and Willmar) were included in the original study and one soil-location combination was reported on, herein (Webster–Waseca). Four drain depths (90, 105, 120, and 135 cm) were used and drain spacings were selected to simulate drainage coefficients from 3.2 to 19 mm/day. Soil input files for DRAINMOD were created using a combination of measured data from field sampling (soil texture and bulk density), data estimated with the Rosetta soil parameter model/van Genuchten equation (water retention curves), and publicly available data (soil permeability). Long-term simulations were conducted for each location using 90 years of historical climate data (1915–2005). Outputs from DRAINMOD were used to create “performance” indices that enable the concurrent consideration of profitability, drainage volumes, and surface runoff, when choosing drain depth and spacing. A spreadsheet design tool was developed to compute and display these indices, based on a few simple user inputs to define cost and crop price. Internal rate of return was used as the basis for profitability considerations. Results obtained indicated that in general, profitability may be increased with increased drain depth and spacing, due to system costs associated with narrower drain spacings. It was found that when considering both profitability and drainage volume, deeper, more widely spaced drainage systems were generally preferable, because of increased profitability and decreased drainage volumes.