reasons for and benefits of agricultural drainage

Agricultural drainage serves two purposes. The first is for the removal of excess surface and subsurface water.  The second is to remove excess soluble salts with the (excess) water from the drained soil profile. 

drainage
source:  bae.ncsu.edu

There are two options for drainage systems. The first is a surface drainage system. A surface drain removes excess water from the surface of the land.

The second is a subsurface drainage system. A subsurface drainage system is used to control the water table in the soil. They can be open drains, which are open ditches with an exposed water table, or pipe drains which are buried pipes.

water table drainage
source: ohioline.osu.edu

Both types of drainage are employed to direct excess surface water to a collector drain in order to prevent ponding.

ponding
source: purdue.edu

The three main components of drainage systems are as followed:

  1. A field drainage system
    This system is used to control the water table and prevent ponding. This component is the most important part of the drainage system. It is comprised of a network which gathers excess water from the land. This is accomplished with the assistance of field drains. Supplemental measures to direct water to the drains may be taken.
  2. A main drainage system
    This part of the drainage system brings the water away from the farm to the outlet point. The water comes from the field drainage system, surface runoff and groundwater flower using a main drain known as a canalized stream. This means that there was an existing stream that was altered to improve the flow.
  3. An outlet
    This is where the drainage water is led out of the area and discharged into another body of water (lake, river, sea). An outlet will either be gravity powered or require a pumping station.  If it is gravity powered,  the water levels rise and fall.  An outlet will require a pumping station if the water levels in a drainage system are lower than the levels of the receiving body of water.
outlet
source: bae.ncsu.edu

There are several benefits of agricultural drainage.  The biggest and most important being the improvement of aeration within soils which results in better yields. This is due to the fact that:

  1. The crops can root more deeply
  2. The choice of types of crops that can be planted is expanded
  3. There will be fewer weeds
  4. Efficiency in fertilizer use will be improved
  5. Denitrification will be reduced
  6. Grass swards will be better

There are also additional benefits not related to aeration. They include:

  1. Easier access to the land
  2. Greater bearing capacity in the land
  3. The soil has better tilth and workability
  4. Tillage operations can take place over a longer period of time
  5. A better environment for micro-fauna (ex. Earthworms) is created which improves permeability
  6. Crops can be grown earlier due to increases in soil temperature

Drainage also makes it possible to inhibit soil salinity.  Soil salinity levels indicate the amount of salt presence within a soil. If there is too much salt, plant growth will be retarded.  To remove the excess salts from the soil, leaching is encouraged.  Leaching is the process of nutrients or salts being removed from the soil with water.  Leaching can occur naturally or as a result of irrigation and drainage systems. Sometimes leaching is necessary to repair land that has been negatively impacted by agricultural activities. It is also used to protect the root zone from being salinized by the capillary rise of saline water. Such safeguards allow for a wider variety of crops to be grown.

sources:

https://www.bae.ncsu.edu/topic/drainageadvisory/background.php

https://www.researchgate.net/publication/272507858_Drainage_of_Irrigated_Lands

http://www1.frm.utn.edu.ar/laboratorio_hidraulica/Biblioteca_Virtual/Subsurface%20drainage%20practices/Part_1.pdf