Tile drainage is a subsurface water management method used primarily in agriculture to remove excess water from the soil profile. This practice involves installing a network of underground pipes that collect and transport water away from the root zone of crops. The system is designed to lower the water table, transforming poorly drained, waterlogged fields into productive farmland. By managing the saturation level of the soil, tile drainage allows for better growing conditions and improved field accessibility. This technique has been a common feature of agricultural landscapes for over a century, evolving from original designs using fired clay to modern materials and precision installation methods.
How Subsurface Drainage Systems Function
The mechanism of a tile drainage system relies on gravity to move water through the soil and into a network of pipes. The system begins with a series of smaller, perforated pipes, called lateral lines, which are laid at a specific depth and slope beneath the field surface. These laterals are the primary collectors of excess soil water.
Water in saturated soil moves downward through the spaces between soil particles, drawn by gravity toward the buried pipes. Once the water reaches the laterals, it seeps into the pipe through small slits or perforations. A filter material, often called a drain envelope, surrounds the pipe to prevent fine soil particles from entering and clogging the system.
The water collected by the laterals flows into larger-diameter pipes known as main collector lines. These main lines consolidate the flow from the entire drainage network. The main collector line transports the water to a designated discharge point, typically a drainage ditch, stream, or other surface water body. The system is engineered with a continuous downward gradient to ensure uninterrupted, gravity-fed flow out of the field.
Primary Benefits for Agricultural Land
The removal of surplus water from the root zone offers several substantial advantages for farming operations. A primary outcome is the immediate improvement in soil aeration. When soil becomes saturated, pore spaces fill with water, displacing the oxygen that plant roots require for healthy growth.
Removing excess water prevents waterlogging and ensures a proper balance of air and moisture in the soil, which enhances root respiration and development. This healthier environment allows crop roots to grow deeper, giving them greater access to nutrients and moisture reserves during drier periods. Improved soil structure also reduces the risk of soil compaction that occurs when heavy machinery drives over wet ground.
The ability to rapidly dry out the field significantly lengthens the effective growing season for crops. Farmers can access fields earlier in the spring for planting because the ground is firm enough to support equipment. Fields also remain accessible later into the fall for harvesting. This extension of the planting and harvesting window allows for more timely operations, which is directly linked to higher crop yields.
Installation and Long-Term Maintenance
Modern tile drainage systems predominantly utilize corrugated plastic tubing made from high-density polyethylene (HDPE). This material is flexible, durable, and highly resistant to corrosion, offering a long service life, often exceeding 50 years. Installation involves specialized machinery, such as drainage plows or trenchers, that simultaneously dig a trench and lay the pipe at the correct depth and grade.
Achieving the proper slope is essential for the system’s function, ensuring gravity continuously moves the water toward the outlet without pooling. Laterals are installed at depths ranging from 2.5 to 4 feet below the surface, with the specific depth determined by soil type and local drainage requirements. The main collector lines must be large enough to handle the combined flow from all the laterals to prevent bottlenecks.
Long-term maintenance focuses on ensuring the system’s outlets remain clear and functional. Outlets, where the main collector line discharges, should be inspected regularly for blockages caused by debris, sediment, or rodent activity. Periodic flushing of the lines may be necessary to remove sediment buildup that accumulates over time. Farmers must also monitor for sinkholes or “tile blowouts,” which indicate a breakage or failure in the underground pipe that needs immediate repair.
Environmental and Water Quality Considerations
While tile drainage provides numerous agricultural benefits, it creates direct pathways for water to move from the soil to surface water bodies, leading to environmental consequences. The most significant concern is the transport of soluble nutrients, particularly nitrate-nitrogen. Nitrate is highly mobile in water and not bound to soil particles, allowing it to easily move with the drained water.
The resulting high concentrations of nitrate-nitrogen in the discharge water contribute to the eutrophication of lakes and rivers. This process involves the excessive growth of algae, which can lead to oxygen depletion and the creation of hypoxic zones. Conversely, tile drainage can have a positive effect by reducing surface runoff, which slows the movement of water across the soil surface. This reduction in surface flow can decrease the transport of sediment and phosphorus, which are often bound to soil particles, into waterways.
To address nutrient loss, several mitigation strategies can be implemented alongside the drainage system.
Mitigation Strategies
Controlled drainage involves installing structures at the outlet to manage the water table height, effectively reducing water and nutrient loss during non-growing seasons.
Edge-of-field practices, such as denitrification bioreactors, are installed to filter the tile water before it is discharged. These structures use organic material, such as wood chips, to create an environment where microbes convert nitrate into harmless nitrogen gas, improving the quality of the water leaving the field.