There are several environmental benefits that result from a properly designed and installed agricultural drainage system. Here we explore 5.
1. Improved Water Storage Space
Fields with drainage tile offer more temporary storage space for water than non-drained fields. Subsurface drainage increased the soil’s storage capacity because it continually removes excess, or “loose” water. Also called Gravitational Water, this “loose” water is not available for use by plant roots, and instead fills the soil normally occupied by air and leads to drowning of crops.
Plant roots use Capillary Water that’s held to soil particles by surface tension. Soils with tile drainage were found to have a greater storage capacity than naturally well-draining soils that did not have tile drainage.
When a field has drainage in place, it normally takes the water table about 3–4 days to fall to drainage depth. However, undrained fields may take several weeks for evaporation alone to lower the water table to a similar depth – and if there is any rain it will take longer.
Even more, some soils drained with tiles may actually have a higher capacity to store water because tile drainage improves soil structure. Better soil structure means that the soil is more porous, and is therefore better able to store water.
2. Infiltration vs Surface Runoff
Tile drained soils can have a higher capacity to store added moisture. Because of this, more water is able to infiltrate into the soil, and reduce surface runoff volumes. Extensive review articles have confirmed that tile drainage reduces surface runoff – in some cases by as much as 65%. Reduced surface runoff can decrease the losses of soil, chemicals, and nutrients from a field.
3. Peak Flows Reduced
Good subsurface drainage significantly reduces the volume of peak flow. Peak flow refers to the greatest amount of flow resulting from the total of surface runoff and subsurface drainage. Because increased storage capacity allows more water to infiltrate, the soil acts as a buffer for rainfall and spreads the runoff over a longer period of time.
4. Improved Groundwater Levels
A study that collected data for 16 years from a central New York site found that only 7.5% of the average annual precipitation came out in the tile discharge. This same study also found that most of the average annual tile flow (84%) occurred during the months from November to April during periods of abundant moisture – where water would be lost in runoff anyway.
5. Reduction in Pesticide Runoff
Tile drainage may actually reduce the amount of pesticides that arrive to the surface water by reducing runoff volumes. In fact, one study determined that, of the total amount of the herbicide lost to runoff, 51–62% was removed from the field through surface runoff compared to only 16–24% through tile drainage.