Watertable control

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Watertable control is the practice of controlling the water table in agricultural land by subsurface drainage with proper criteria to improve the crop production.

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[edit] Description/Definitions

Crop yield (Y) and depth of water table (X in dm)
Crop yield (Y) and depth of water table (X in dm)

Subsurface land drainage [1] aims at controlling the water table of the groundwater in originally waterlogged land at a depth acceptable for the purpose for which the land is used. The depth of the water table with drainage is greater than without.

In agricultural land drainage, the purpose of water table control is to establish a depth of the water table that does no longer interfere negatively with the necessary farm operations and crop yields (Figure, from SegReg model, see Segmented regression).
The development of agricultural drainage criteria [2] is required to give the designer and manager of the drainage system a target to achieve in terms of maintenance of an optimum depth of the water table.

Optimization of the depth of the water table is related the benefits and costs of the drainage system. The shallower the permissible depth of the water table, the lower the cost of the drainage system to be installed to achieve this depth. However, the lowering of the originally too shallow depth by land drainage entails side effects. These have also to be taken into account, including the costs of mitigation of negative side effects.

An overview of the effects (both positive and negative) of water table control can be found in Chapter 17: "Agricultural Drainage Criteria" of ILRI publication 16: "Drainage Principles and Applications" that can be viewed and downloaded from the ILRI-Alterra website.

Historically, agricultural land drainage started with the digging of relatively shallow open ditches that that received both runoff from the land surface and outflow of groundwater. Hence the ditches had a surface as well as a subsurface drainage function.
By the end of the 19th century and early in the 20th century it was felt that the ditches were a hindrance for the farm operations and the ditches were replaced by buried lines of clay pipes (tiles), each tile about 30 cm long. Hence the term "tile drainage".
Later, land drainage became a powerful industry. At the same time agriculture was steering towards maximum productivity.
As a result, many modern drainage projects were over-designed [3] while the negative environmental side effects were ignored. In circles with environmental concern, the profession of land drainage got a poor reputation, sometimes justly so, sometimes unjustified, notably when land drainage was confused with the more encompassing activity of "wetland reclamation".

Nowadays, in some countries (not in all), the hardliner trend is reversed. An article on "Agricultural land drainage: a wider application through caution and restraint" can be viewed and downloaded from www.waterlog.info.

Subsurface drainage of groundwater can also be accomplished by pumped wells (vertical drainage, in contrast to horizontal drainage). Drainage wells have been used extensively in Pakistan [Salinity control and reclamation program (SCARP) in the Indus valley. Although the experiences were not overly successful, the feasibility of this technique in areas with deep and permeable aquifers is not to be discarded. The well spacings in these ares can be so wide (more than 1000m) that the installation of "vertical" drainage systems could be relatively cheap compared to "horizontal" subsurface drainage (drainage by pipes, ditches, trenches, at a spacing of 100m or less).

See "Energy balance of groundwater flow" for drain spacing equations and "spacing of drainage wells".

[edit] References

  1. ^ Nosenko, P.P. and I.S. Zonn 1976. Land Drainage in the World. ICID Bulletin 25, 1, pp.65-70.
  2. ^ Free download of an article on drainage criteria
  3. ^ R.J.Oosterbaan, 1990. Agricultural Land Drainage: a wider application through caution and restraint. In: ILRI Annual Report 1991, pp. 21-36, International Institute for Land Reclamation and Improvement, Wageningen, The Netherlands.

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