Soil Conservation
Soil Drainage
Soil drainage is probably the most important soil characteristic influencing choice of management options, and failure to consider drainage when planning production programs is a common reason for poor crop performance. Soil drainage is a natural process by which water moves across, through and out of the soil as a result of the force of gravity. Drainage can improve crop yields, reduce year-to-year yield variability, and provide trafficable conditions for field operations at critical times of planting or harvest. Drainage enables crops to establish deeper root systems in fields without impeding or compacted layers, allowing them greater access to nutrients and soil water. Drainage can increase nitrification (the conversion of ammonia to nitrate) in most soils, providing more nitrate for plant uptake.
Clay Soils
Clay soils often pose an additional challenge with respect to drainage because they remain in the plastic state for extended periods after wet conditions. Once the upper inch of the soil surface dries out, it becomes a barrier that greatly reduces further evaporation losses. This is often referred to as self-mulching. This barrier keeps the underlying soil in a plastic state, preventing it from being worked or trafficked without causing excessive smearing and compaction damage.
Types of Field Drainage
Two forms of artificial drainage can be distinguished: surface and subsurface drainage. Surface drainage removes standing water from the soil surface before it enters the soil profile. Subsurface drainage is designed to remove excess water from the soil profile.
Surface Drainage
Surface drainage systems typically consist of an outlet channel (existing natural stream or constructed channel emptying to a natural stream), lateral ditches, and field ditches. Such systems are used primarily in flat areas having poor natural drainage to remove water that collects on the land surface when the rainfall rate exceeds the infiltration capacity. A primary goal in the design and construction of surface drainage systems is to remove the water from the surface as quickly as possible while avoiding soil erosion that can occur when the water moves too rapidly.
Subsurface Drainage
Surface drainage may not be sufficient or practical in poorly drained soils where the water table is naturally near the ground surface. In these cases, subsurface drainage is needed to remove the excess water and lower the water table. There are 4 main types of subsurface drainage systems:
PVC Slotted Pipes. Perforated corrugated pipes (polyvinyl chloride/PVC, high-density polyethylene/PE) are buried in the ground to remove the excess water and lower the water table. They are still often referred to as drain “tile,” although that word dates back to when clay pipes were used.
Mole Drains. Mole drains are used on heavier, low permeability, clayey soils where downward movement of water through the soil profile is slow. Heavy soils of low hydraulic conductivity often require very closely spaced drainage systems for satisfactory water control.
Interceptor Drains. These drains are installed at the base of slopes at the change of gradient, usually where a steeper slope meets the flats to intercept the downhill flow of subsurface water. Often the soil type on the slope is more permeable than those of the flats and this forces the water to come to the surface, usually at the change of slope.
Ground Water Pumps. Where it is impossible or uneconomical to install outlets for drainage, groundwater pumps can be used to remove excess surface water or groundwater. The act of pumping causes a drawdown of ground water leading out from the pump's location—its effect will be much less at depth.
Constructed Wetlands
A wetland (sometimes nicknamed “Nature’s Kidney” is a shallow pool that filters sediment, nitrate and other nutrients while also offering flood mitigation and habitat benefits. Constructed wetlands can be specifically designed for agriculture landscapes to intercept drainage tiles, treating the water, before releasing it back into a tile line, ditch or stream. Additional benefits with wetlands include:
Concerns with Land Drainage
TDrainage alters the hydrology of a field. In undrained systems, the soil acts as a sponge to store water, providing it is not already saturated, and releases it slowly to adjacent streams or water bodies. With surface and subsurface drainage downstream flooding may result transferring the water to adjacent streams or water bodies. Surface and subsurface drainage systems can increase the flashiness of the system (the rapid rise and fall in stream flow), causing greater high flows (peak flows) and smaller low flows (baseflow).
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