Subsurface Tile Drainage Management

Controlled Subsurface Drainage Benefits Crops, Water Quality

Without underground (subsurface) drainage, the incredible bounty of some of our most productive agricultural land would be dramatically compromised. Tile drainage can transform poorly drained fields, facilitating timely planting in wet springs, increasing rooting depth, and dramatically increasing crop yields.

The potential downside is that tile drainage can also increase nitrate losses through the root zone and out into surface waters. Tile drainage from Midwest crop fields — where an estimated 50 million acres rely on subsurface drainage — has been associated with the annual hypoxic area in the Gulf of Mexico where excessive algal bloom depletes oxygen in the water. Additionally, uncontrolled drainage can rob crops of water and nutrients and reduce the efficiency of applied fertilizer.

The good news is that managing tile drainage systems can help strike a balance, enabling the practice to enhance crop productivity while reducing the loss of nitrate and protecting water quality.

The ability to raise and lower the water table enables the grower to respond to crop needs and reduce the amount of nitrate and phosphate that escape from fields.

Farmers are learning to better manage subsurface drainage systems by using structures that allow them to control when and how much water leaves the field.

In the U.S., farmers have been installing underground (subsurface) drainage pipes beneath poorly drained agricultural land since the late 1800s. Originally made of clay tile, most of today’s drainage systems utilize perforated plastic pipes installed three to five feet below the soil surface. Tile drainage is employed in many parts of the U.S. and Canada but it is particularly prevalent in the U.S. Midwest where an estimated one-third of agricultural land is tiled.

The goal of a drainage system depends on the characteristics of the area in which it’s installed. For most farmers, though, drainage allows field access for planting or cultivation equipment earlier in the growing season and during wet periods. It also improves soil aeration and, as a result, increases crop production where excessive moisture at certain times of the year is a production-limiting factor.

The interest in tile drainage has increased in many areas as computer yield monitors on modern combines indicate greater yields in tiled areas and as land values and commodity prices have increased.

As the drainage water leaves the field, however, it is funneled into streams, lakes, rivers and oceans. Nitrate and phosphorus from fertilizer may be carried in that flow, turning those important nutrients into pollutants and reducing the nutrients in the soil accessible to the crops for which they were intended.

Actively Managing the Water Table

Changes in how tile drainage systems are installed and managed can now maximize the upside of drainage and minimize its downside. Adjustable, in-line control structures attached to drainage pipes in strategic locations in a field allow the farmer to periodically adjust the height at which the water table triggers drainage. The ability to raise and lower drainage height enables the grower to respond to crop needs and reduce the amount of nitrate and phosphorus that escape from the field.

The water table level may be set high during winter or other periods when crops are not growing, decreasing the loss of water and nutrients from the field. In spring, the water level may be lowered enough for the grower to plant his or her crop. And during the growing season, the drainage manager is able to respond to crop and weather conditions, adjusting the water level accordingly. For example, the control structure is:

• Lowered in spring and fall so the drain flows freely, aerating soil prior to field operations such as planting and harvesting.
• Raised after harvest to limit outflow and reduce nitrate outflow to surface water.
• Partially raised after spring field work to capture and store water for crop use.

Controlled tile drainage has been shown to be very effective in reducing nitrate losses. In Natural Resources Conservation Service (NRCS) funded demonstrations in several states, controlled drainage decreased nitrate losses by approximately 45 percent and the amount of water drained from fields by about 40 percent.
In areas where water can be a limiting factor in agricultural production, controlled drainage has shown significant yield advantages by holding water in the soil for use by plants during dry periods.

Controlling Limits Nitrate Losses

“The key is that when you manage drainage, you tend to drain less water from the field and less water leaving the field means less nitrate as well,” says Dan Jaynes, a soil scientist with the Agricultural Research Service (ARS), the chief scientific research agency of the U.S. Department of Agriculture. “Traditionally, we put the pipe into the ground and never touched it. Now, we’re learning to manage drainage to benefit both the crop and surface water quality.

“In Iowa, most of the ground needs to be drained to be worked so we would open up the system and let the tile do its job. The stop logs are completely removed in the spring and at harvest to allow for maximum drainage. With drainage water management, the farmer has the option to partially raise the water table in the summer to capture some water that would have otherwise drained and make it available to the crop — potentially increasing yield. Then, during the offseason, all the stop logs can be inserted to raise the water table just below the soil surface to prevent as much drainage and loss of nutrients as possible.”

In one Iowa study Jaynes cites, nitrate losses were reduced by 30 percent by management of the drainage system. At the Agroecosystems Management Research Unit in Ames, Iowa, Jaynes and other scientists study how to reduce the flow of nitrate from farm fields into surface waters.  Projects cover a wide range of issues in addition to tile drainage management, including the effect of cover crops on nitrogen load and optimal nitrogen rates of corn management zones delineated from soil and terrain attributes.
 

Meeting the Challenge

Meeting global food demands in a sustainable manner and protecting water quality poses significant challenges to the agricultural community. A growing awareness of best management practices as well as continued scientific research promises both a renewed environmental commitment as well as a more productive agricultural industry.

Tile drainage management is an example of how agriculture can synchronize productivity and economic demands while responding to environmental concerns.