Well drainage

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Well drainage means drainage of agricultural lands by wells. Agricultural land is drained by pumped wells (vertical drainage) to improve the soils by controlling water table levels and soil salinity.

Contents 1 Definitions and explanations 2 Well spacing equation 3 References 4 See also 5 External links 6 Miscellaneous

[edit] Definitions and explanations

Subsurface (groundwater) drainage for water table and soil salinity control in agricultural land can be done by horizontal and vertical drainage systems.
Horizontal drainage systems are drainage systems using open ditches (trenches) or buried pipe drains.
Vertical drainage systems are drainage systems using pumped wells, either open dug wells or tube wells.
Both systems serve the same purposes, namely the control of the water table and soil salinity.
Both systems can facilitate the reuse of drainage water (e.g. for irrigation), but wells offer more flexibility.
Reuse is only feasible if the quality of the groundwater is acceptable and the salinity is low.

Although one well may be sufficient to solve groundwater and soil salinity problems in a few hectares, one usually needs a number of wells, because the problems may be widely spread.
The wells may be arranged in a triangular, square or rectangular pattern.
The design of the well field concerns depth, capacity, discharge, and spacing of the wells. ^[1]

1. The discharge is found from a water balance. The main water balance factors can be found in paper 4 on the Faqs page of website www.waterlog.info.
2. The depth is selected in accordance to aquifer properties (see also Aquifer test). The well filter must be placed in a permeable soil layer.
3. The spacing can be calculated with a well spacing equation using discharge, aquifer properties, well depth and optimal depth of the water table
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[edit] Well spacing equation

With a well spacing equation ^[2] one can calculate various design alternatives to arrive at the most attractive or economical solution.
The costs of the most attractive solution can be compared with the costs of a horizontal drainage system serving the same purpose to decide which system deserves preference.

A computer program (WellDrain) for well spacing calculations can be freely downloaded from the Software page on the waterlog.info website.
The numerical program takes into account fully and partially penetrating wells, layered aquifers, anisotropy (different vertical and horizontal hydraulic conductivity or permeability) and entrance resistance.

Well design proper
The well design proper is described in a ^[3]

An illustration of the parameters involved is shown hereunder.

Geometry of a well drainage system
Legend: L=well spacing, D=depth, K=hydraulic conductivity, W=well diameter

[edit] References

1. ^ Boehmer, W.K., and J.Boonstra, 1994, Tubewell Drainage Systems, Chapter 22 in: H.P.Ritzema (ed.), Drainage Principles and Applications, ILRI publ. 16, Wageningen, The Netherlands. pp. 931-964, ISBN 90 7075 3 39
2. ^ R.J.Oosterbaan, 2002, Subsurface drainage by (tube)wells, 9 pp. See item 8 on the Faqs page of waterlog.info website for free download.
3. ^ Free download of Chapter 22 (Tubewell Drainage System) from the ILRI-Alterra website

[edit] See also

• Drainage research
• Hydrology

[edit] External links

• Free download of an article on [Subsurface land drainage by tubewells]

[edit] Miscellaneous