Ohio State University Extension Bulletin

Agricultural Drainage

Bulletin 871-98

Drainage, Agriculture and the Environment

The purpose of this bulletin is to review the status and importance of drainage for agricultural production in the North Central Region, identify the positive and negative effects of agricultural drainage, and then summarize new drainage methods which work to increase productivity, enhance the positive effects of agricultural drainage, and mitigate the adverse effects of agricultural drainage on the environment.

The North Central Region of the United States is one of the most productive agricultural regions of the world and this productivity relies heavily on the practice of drainage. Agricultural drainage is the removal of excess water from the soil surface and/or soil profile of cropland, by either gravity or artificial means. Once land is drained both positive and adverse water quality impacts can be documented. Compared to uncleared land under natural conditions, improved drainage and agricultural production usually increase peak runoff rates, sediment losses, and pollutant loads on surface-water resources. However, where land is used for agricultural production, improved drainage has been found to reduce runoff, peak outflow rates, and sediment losses, compared to undrained agricultural land.

Historically, the main reason for drainage on agricultural land has been to enhance crop production. Drainage removes excess water from the soil and helps to create a well-aerated root environment that enhances plant uptake of nutrients. Drainage on wet agricultural soils allows timely field operations, and helps plant growth to begin early, continue vigorously, and achieve improved levels of productivity. In states that depend heavily on irrigation from surface water supplies, subsurface drainage is often used to prevent harmful buildup of salt in the soil. Drainage benefits crop production by minimizing risks, improving efficiency, and increasing net income.

While agricultural drainage is necessary for economical and efficient crop production in much of the North Central Region, there have been environmental costs. Better than half the original wetlands in the United States have been lost to drainage practices. Much of this wetland loss is related to agricultural production in areas that were swampy and too wet to farm. Loss of wetlands is associated with declines in wildlife habitat and adverse effects on water quality. The loss of wetlands and associated benefits to healthy ecosystem function have become an important environmental issue.

As the public considers the loss of wetland ecosystems associated with agricultural drainage, the continuing need for economical food production must also be considered. Currently, research in the North Central Region is focusing on methods that mitigate the adverse impacts of agricultural drainage. These techniques have the goal of sustaining agricultural productivity and profitability on existing cropland while addressing environmental problems.

These innovative drainage methods are used on land that is already under agricultural production and do not require further drainage of remaining wetlands. Instead, they work to help make agriculture more environmentally sound and sustainable. Modern water management for agricultural production focuses on the management and enhancement of existing drainage systems to benefit water quality and the profitability of agriculture.

(a) Soybean plants left standing in flooded conditions will die in 72
hours; (b) This open ditch is an example of an agricultural surface
drain; (c) Subsurface drainage pipes are installed to drain excess
water from an agricultural field.

Agricultural drainage improvements can be made on either the soil surface, the subsurface, or a combination of both. Surface drainage is designed to remove standing water from the soil surface. It affects the water table by reducing the volume of water entering the soil profile. This type of drainage includes land leveling and smoothing; the construction of surface water inlets to subsurface drains; and the construction of shallow ditches and grass waterways, which empty into open ditches and streams.

Subsurface drainage is designed to remove excess water from the soil profile. The water table level is controlled through a series of drainage pipes (tile or tubing) that are installed below the soil surface, usually just below the root zone. Subsurface drainage pipes are typically installed at a depth of 30 to 40 inches, and at a spacing of 20 to 80 feet. The subsurface drainage network generally outlets to an open ditch or stream. For the same amount of treated acreage, subsurface drainage improvements are generally more expensive than surface drainage improvements.

An important distinction must be made between improving drainage of land presently in agricultural production and converting additional wetlands. Present agricultural trends are toward intensive use of existing cropland, with much of the emphasis on new management technologies. Maintaining and improving existing drainage and associated yields on wet agricultural soils presently in production minimizes the economic need for landowners to convert wetlands. This encourages a new emphasis on protecting existing wetlands and establishing new wetland areas, while maintaining our highly productive agricultural lands.

The top illustration (a) represents agricultural land without drainage
improvement. The water table is near the soil surface and water ponds
in surface depressions. The middle illustration (b) represents
agricultural land with surface drainage improvement that allows for the
safe removal of surface water. The land in the bottom illustration (c)
is drained by a system of subsurface plastic tubing or clay tile. The
water table is lowered which allows timely field operations and helps
increase crop yields.

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