7.3 Subsurface Tile & Surface Drainage System Design

Agricultural Drainage Systems

While water is essential for crops, excess water is highly detrimental. Poorly drained soils suffer from waterlogging, which deprives roots of oxygen, delays spring planting due to cold and wet conditions, increases soil compaction risks from machinery, and limits crop yields. Agricultural drainage systems are engineered to remove this excess gravitational water, creating an optimal aerobic environment in the root zone.

Benefits of Artificial Drainage

• Improved Trafficability: Drained soils dry out faster in the spring, allowing for earlier, more timely planting and field operations.
• Enhanced Root Growth: Removing the water table allows roots to penetrate deeper into the profile, expanding their access to nutrients and, ironically, improving drought tolerance later in the season.
• Warmer Soils: Water has a high specific heat capacity. Drained soils warm up much faster in the spring than saturated soils, promoting rapid seed germination and early vigorous growth.
• Reduced Salinity (in arid regions): Drainage is essential in irrigated arid lands to leach harmful salts out of the root zone.

Surface Drainage Systems

Surface drainage is designed to remove excess water before it infiltrates into the soil. It is particularly important on heavy clay soils with very low infiltration rates and in areas with very flat topography.

Key components include:

• Land Leveling/Grading: Physically reshaping the field surface to eliminate depressions where water ponds, ensuring a continuous, gentle slope toward a collection area.
• Shallow Surface Ditches: Constructed channels that collect runoff and direct it off the field safely. These are often vegetated to prevent erosion.

Subsurface Tile Drainage

Subsurface drainage involves installing perforated pipes (traditionally clay tiles, now corrugated plastic tubing) underground to artificially lower the water table. Water enters the pipes through the perforations and flows by gravity to an outlet, such as a deep ditch or a stream.

System Design: Depth and Spacing

The design of a tile system—specifically how deep the pipes are buried and how far apart the laterals are spaced—is dictated almost entirely by the soil's hydraulic conductivity (permeability).

• Spacing: The goal is to draw down the water table midway between the pipes quickly enough to prevent crop damage (typically within 24-48 hours).
  • In highly permeable sandy or loamy soils, water moves quickly laterally. Therefore, tile lines can be spaced further apart (e.g., 50 to 100 feet).
  • In low permeability clay soils, water moves very slowly. To achieve the required drawdown, tile lines must be spaced much closer together (e.g., 20 to 40 feet).
• Depth: Typical depth ranges from 2.5 to 4 feet. Deeper drains can draw from a wider area, allowing wider spacing, but they require a suitable outlet elevation and deeper, more costly trenching. In tight clay soils, tiles are often placed shallower (e.g., 2.5 feet) because deep placement is ineffective due to poor water movement in the subsoil.

The Drainage Coefficient

The drainage coefficient is a fundamental design parameter representing the depth of water (in inches) the system is designed to remove from the entire drainage area in a 24-hour period. A typical coefficient is 3/8 to 1/2 inch per day. A higher coefficient requires larger diameter main pipes to handle the increased flow rate.

Environmental Concerns: Nitrate Loss

While tile drainage vastly improves agronomic productivity, it creates a direct conduit from the soil profile to surface water bodies, bypassing the natural buffering capacity of riparian zones and wetlands.

The most significant environmental issue associated with tile drainage is the transport of nitrate-nitrogen (NO3-). Nitrate is highly soluble and moves readily with water. When excess nitrogen fertilizer is applied, or when soil organic matter mineralizes during fallow periods, accumulated nitrate is easily leached into the tile lines during precipitation events. This nutrient loading contributes heavily to downstream water quality issues, such as the hypoxic "dead zone" in the Gulf of Mexico.

Phosphorus (P) is typically less of a concern in tile drainage because it binds tightly to soil particles, though dissolved reactive phosphorus can move through macropores directly to tiles in some soils.

Mitigation Strategies

To address nitrate losses, several conservation practices are employed:

• Nutrient Management: Applying fertilizer at the right rate, time, source, and place (the 4Rs) to minimize excess nitrate in the soil.
• Cover Crops: Growing a crop over the winter scavenges residual nitrate, tying it up in plant biomass before it can leach into the tile system.
• Drainage Water Management (Controlled Drainage): Installing control structures in the tile mains. During the non-growing season, boards are added to the structure to raise the water table, keeping water and nitrate in the field. Prior to planting and harvest, the boards are removed to allow full drainage.
• Bioreactors and Wetlands: Routing tile drainage water through woodchip bioreactors or constructed wetlands where anaerobic bacteria convert nitrate gas (N2) through denitrification before the water reaches streams.