Agricultural Drainage

Bulletin 871-98

Beneficial and Adverse Water Quality Impacts of Drainage

Historically, drainage systems were not primarily installed as water quality management tools, but they do have an impact on soil and chemical transport. Where it has been necessary to drain agricultural land for production, research has shown that drainage systems can have a positive impact on some nonpoint source pollution problems in comparison to agricultural land without drainage. For example, under certain conditions artificial drainage acts to lower soil erosion by increasing the movement of water through the soil profile and thus reducing runoff. However, subsurface drainage expedites the transport of nitrate-nitrogen (nitrate-N) from the soil zone to surface waters.

Based upon the published literature, research results on the water quality impacts of drainage can be summarized by the following statements that compare agricultural land with subsurface drainage to that without subsurface drainage:

• The percentage of rain that falls on a site with subsurface drainage and leaves the site through the subsurface drainage system can range up to 63%.

• The reduction in the total runoff that leaves the site as overland flow ranges from 29 to 65%.

• The reduction in the peak runoff rate ranges from 15 to 30%.

• Total discharge (total of runoff and subsurface drainage) is similar to flows on land without subsurface drainage, if flows are considered over a sufficient period of time before, during, and after the rainfall/runoff event.

• The reduction in sediment loss by water erosion from a site ranges between 16 to 65%. This reduction relates to the reduction in total runoff and peak runoff rate.

• The reduction in loss of phosphorus ranges up to 45%, and is related to the reductions in total runoff, peak runoff rate, and soil loss.

• In terms of total nutrient loss, by reducing runoff volume and peak runoff rate, the reduction in soil-bound nutrients is 30 to 50%.

• In terms of total nitrogen (N) losses (sum of all N species), there is a reduction.

• However, nitrate-N, a soluble N ion, has great potential to move wherever water moves. Numerous studies throughout the Midwest and southeast U.S., and Canada document that the presence of a subsurface drainage system enhances the movement of nitrate-N to surface waters. Proper management of drainage waters along with selected in-field BMPs helps reduce this potential loss.

These results indicate that subsurface drainage is a management tool that reduces the potential for erosion and phosphorus enrichment of surface waters from agricultural activities. However, nitrate-N loadings exported from drainage conduits to surface waters continue to be a major water quality concern. The following pages provide information on agricultural drainage research conducted in individual states of the North Central Region. Climate, geography, landform, and potential natural vegetation differ between regions so that research results are unique, yet they are relevant to similar circumstances.

First, the basic concepts of water table management are reviewed so that research in individual states can be better understood.

Under certain conditions, artificial drainage acts to lower soil erosion
rates and amounts by increasing the movement of water through the soil
profile and reducing peak and total runoff.