Wet fields are difficult to work in, can affect crop yield and can delay harvest.

“Soils that stay waterlogged or saturated/wet for several days after a rain can hinder appropriate soil aeration needed for crop root development and can result in crop stress,” Kapil Arora, Iowa State University Extension and Outreach Field Specialist, said.

“Soils that stay wet do not provide support to farm equipment as it passes over such areas in a farm field,” he said. “Equipment will typically get stuck in such areas, unable to move, resulting in loss of valuable time.”
Equally important, “such areas can also lead to significantly reduced crop yields, causing the crop production system to lack profitability or suffer economic loss,” he added.

A well-planned subsurface drainage system can assist farmers with improving the productivity of their fields, alleviate crop stress and allow for properly timed field operations.

What is subsurface drainage?
Subsurface farmland drainage, also called tiling, is a method that relies on a system of underground drainage tiles (pipes) installed at depths of between 2 and 4 feet beneath the surface. The system is designed to remove excess water from farm fields. Today, corrugated plastic tubing is the preferred material and ranges from 3 inches to 10 inches in diameter, with 4 inches and 6 inches the most commonly used size. The plastic hose can be perforated, meaning it has small slits so that it can take in water, or it is nonperforated to simply carry water from one point to another.
Drainage pipe is often called drainage tile because original short lengths of clay tile were used. Small spaces between traditional clay tiles allowed water to trickle off the surface and drain away.

Moving extra water off field surfaces is important for crop health. Healthy plants require a balanced mixture of air and water beneath the soil surface. Excess water fills air pockets and suffocates plant roots. Hence, the ultimate goal of a drainage system is to lower the soil’s water table to allow room for air to encourage healthy root growth. Generally, the system should be designed to lower the water table within 24 hours after a rain event.

Subsurface drainage systems are designed in one of four layouts: parallel, double main, herringbone and random. Design depends on field topography, soil type and the crop that will be grown in the field. Agronomists and environmental engineers have established recommendations for installation depth and spacing. Experienced drainage contractors are familiar with these standards and use them to design a system to fit specific scenarios.

A herringbone design is typically used for long, relatively narrow wet areas. Parallel tile lines are placed at an angle on both sides of a main tile line.

A parallel system, used on flat fields with uniform soil types, is set up with a main “header” and parallel lateral lines feeding into the one mainline.

A double-main design, used where a depression exists in the field, essentially places two parallel systems side-by-side.

A random design combines multiple tile configurations and is installed where the land is rolling and/or where there are isolated wet spots.
Regardless of the design layout, trenches are needed for each of the drainage tiles. A specially designed plow attachment for a tractor or a dedicated machine is required to efficiently trench. Trenching begins at the outlet end of the system and continues upgrade. The width of the trench should be at least equal to the outside diameter of the drain and can be up to a half foot wider than the drain. Backfill is required once the drainage tile has been placed.

Getting started with subsurface drainage
Planning and installing a subsurface drainage system is a complex process that requires approvals from multiple government agencies. “To begin with, an evaluation of local, state, and federal regulations is required to see how it impacts the system design and installation,” Arora said. Regulations will vary based on location, but likely you will need approval from the U.S. Department of Agriculture (USDA).

The approval process starts with your local Farm Service Agency office by providing them with a detailed plan of a proposed drainage system. The Natural Resource Conservation Service (NRCS) will be notified of your plans and will decide if any “wetlands” are included in the proposed drainage area. “Wetlands” areas are protected and not eligible for subsurface drainage systems.

“Additional things that must be considered are soils information, where the system outlet will be located, crop yields, current and future cropping strategies, environmental impacts of the associated drainage discharge, and the drainage laws in the state,” he added.

Pipe size, depth of installation and slope or grade when installed depends on multiple factors that range from the volume of water that will move through the system and the speed at which it will flow the tile, soil density and the crop planned for the field. For accurate specifications, work with an agronomist, engineer or your local extension agent for guidance on properly sizing the system.

What does it cost?
Like most farm improvement projects, the cost varies based on multiple factors. “Costs are highly variable due to size of the project, size and drainage pipe length installed, connections made, excavation needed, project precision and quality, fuel costs, and outlet restrictions,” he said. “Current estimates suggest a subsurface drainage project will cost between $1,000 and $1,200 per acre.” These costs can be significantly higher or lower on a case-by-case basis.

The expense of installing subsurface drainage does not necessarily need to be accumulated all in one year. A system design should take into account the entire system, but can be planned for phased installments to occur over one or more years.

Maintenance is important
Though the drainage tile is out-of-sight does not mean it is out-of-mind. Subsurface drainage systems do not require extensive upkeep, but do need regular maintenance for optimum performance.

Watch water levels after a rainfall. If water remains in the fields rather than draining it’s likely that a drain may be partially or fully blocked. Keep outlet ditches clear from sediment, which can block or slow the flow of excess water. Also be sure to check inlets and remove debris.

Rodents and tree roots can also cause problems for subsurface drainage systems. Flaps can be installed over inlets to prevent rodents and other small animals from entering the system. Tree roots from as far as 50 to 100 feet can reach underground tile and should be removed to prevent damage to the system.

Increase productivity
The installation of subsurface drainage systems may be necessary for some farm fields to optimize crop production and increase yield. A good design that takes soil type, typography and existing wetlands into account will result in a system that efficiently redirects excess water from farmed fields to drainage ditches for years.