Technical, Planning and Design Pumped outlets for subsurface drainage

Generally speaking, surface drainage is not as effective as subsurface drainage for satisfying the drainage needs of many soils. Enhancing surface drainage with subsurface, or tile drainage, has the potential to improve crop productivity and farm efficiency where wet soils persist. In addition, improved tile drainage has been found to reduce (in some cases dramatically) runoff and peak outflow rates, as compared with surface drainage alone. In terms of nutrient and sediment losses, tile drainage provides a mixed bag of impacts. Compared to surface drainage alone, tile drainage decreases the loss of some constituents such as sediment, phosphorus, and organic nitrogen, while increasing the loss of mobile constituents, such as nitrate-nitrogen and some salts. Planning an effective tile drainage system requires consideration of a number of factors. The Minnesota Drainage Guide, local drainage experience and expertise, appropriate agency personnel, soil survey information, site topography surveys, field evaluation, wetland restrictions, and outlet limitations should all be considered in the planning process. Pumped outlets enable improved drainage at reasonable cost, in areas that lack a natural gravity outlet. A pump drainage system consists of a collection system (tile or surface drainage system), a pumping plant, and a free outlet. A typical pumping plant is shown in Figure 2. For pumped drainage to be feasible, the economic benefits of improved drainage must be weighed against the startup and operating costs associated with the pumping plant. Potential economic benefits must be assessed locally, and will depend on the adequacy of the current drainage system, soil texture, and crop to be grown. Operating costs depend on design of the pumping plant (in particular, size of the pump and lift) and rainfall characteristics over the drainage season.