Well drainage


Well drainage

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

Introduction

Subsurface (groundwater) drainage for water table and soil salinity 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.

Map of a well field for subsurface drainage with radial flow across concentrical cylinders representing the equipotentials

Both systems serve the same purposes, namely water table control and soil salinity control .
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.

Design

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.[2]
  2. The depth is selected in accordance to aquifer properties. 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.

The determination of the optimum depth of the water table is the realm of drainage research .

Flow to wells

Geometry of a fully penetrating well drainage system in a uniform, isotropic aquifer
Geometry of a partially penetrating well drainage system in an anisotropic layered aquifer

The basic, steady state, equation for flow to fully penetrating wells (i.e. wells reaching the impermeable base) in a regularly spaced well field in a uniform unconfined (preactic) aquifer with an hydraulic conductivity that is isotropic is [1] :

  • Q = 2π K (Db - Dm) (Dw - Dm) / ln (Ri/Rw)

where Q = safe well discharge - i.e. the steady state discharge at which no overdraught or groundwater depletion occurs - (m3/day), K = uniform hydraulic conductivity of the soil (m/day), D = depth below soil surface, Db = depth of the bottom of the well equal to the depth of the impermeable base (m), Dm = depth of the watertable midway between the wells (m), Dw is the depth of the water level inside the well (m), Ri = radius of influence of the well (m), Rw=radius of the well (m), ln = natural logarithm, and π = the number pi.

The radius of influence of the wells depends on the pattern of the well field, which may be triangular, square, or rectangular. It can be found as:

  • Ri = sqrt (At/πN)

where At = total surface area of the well field (m2), N = number of wells in the well field, and sqrt=square root.

The safe well discharge (Q) can also be found from:

  • Q = q At / N Fw

where q is the safe yield or drainable surplus of the aquifer (m/day) and Fw is the operation intensity of the wells (hours/24 per day). Thus the basic equation can also be written as:

  • Dw - Dm = q At ln (Ri/Rw) / 2π K (Db - Dm) N Fw

Well spacing

With a well spacing equation one can calculate various design alternatives to arrive at the most attractive or economical solution for watertable control in agricultural land.

The basic flow equation cannot be used for determining the well spacing in a partially penetrating well-field in a non-uniform and anisotropic aquifer, but one needs a numerical solution of more complicated equations.[3]

The costs of the most attractive solution can be compared with the costs of a horizontal drainage system - for which the drain spacing can be calculated with a drainage equation - serving the same purpose, to decide which system deserves preference.

The well design proper is described in [1]

An illustration of the parameters involved is shown in the figure. The hydraulic conductivity can be found from an aquifer test.

Output of WellDrain program, well-spacing=920m

Software

The numerical computer program WellDrain [3] for well spacing calculations takes into account fully and partially penetrating wells, layered aquifers, anisotropy (different vertical and horizontal hydraulic conductivity or permeability) and entrance resistance.

Modelling

With a groundwater model that includes the possibility to introduce wells, one can study the impact of a well drainage system on the hydrology of the project area. There are also models that give the opportunity to evaluate the water quality.

SahysMod [4] is such a polygonal groundwater model permitting to assess the use of well water for irrigation, the effects on soil salinity and on depth of the water table.

See also

References

  1. ^ a b c Boehmer, W.K., and J.Boonstra, 1994, Tubewell Drainage Systems, Chapter 22 in: H.P.Ritzema (ed.), Drainage Principles and Applications, Publ. 16, International Institute for Land Reclamation and Improvement (ILRI),Wageningen, The Netherlands. pp. 931-964, ISBN 90 70754 3 39 . On line : [1]
  2. ^ ILRI, 1999, Drainage and Hydrology/Salinity: Water and salt balances, 29 pp. Lecture notes of the International Course on Land Drainage (ICLD), International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. On line : [2]
  3. ^ a b ILRI, 2000, Subsurface drainage by (tube)wells: Well spacing equations for fully an partially penetrating wells in uniform or layered aquifers with or without anisotropy and entrance resistance, 9 pp. Principles used in the "WellDrain" model. International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands [3]
    Download "WellDrain" software from : [4] , or from : [5]
  4. ^ SahysMod, Spatial Agro-Hydro-Salinity Model: Description of Principles, User Manual, and Case Studies. SahysMod working group of the International Institute for Land Reclamation and Improvement, Wageningen, the Netherlands. On line: [6] .
    Download the model from : [7] , or from : [8]

External links


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Drainage law — is a specific area of water law related to drainage of surface water on real property. It is of great importance in areas where freshwater is scarce, where flooding is common, or where water is in high demand for agricultural or commercial… …   Wikipedia

  • Drainage density — is the total length of all the streams and rivers in a drainage basin divided by the total area of the drainage basin. It is a measure of how well or how poorly a watershed is drained by stream channels. It is equal to the reciprocal of the… …   Wikipedia

  • Drainage model — may refer to: a surface drainage model or rainfall runoff model      see surface runoff, runoff model (reservoir) , or a software site providing free download of a conceptual rainfall runoff model (RainOff) based on a… …   Wikipedia

  • Drainage basin A — Drainage Basin A, main river in this system is the Limpopo River, but is subdivided into 9 sub drainage basins. A1 is situated along the Botswana border, and is again divided into 3 river basins A10A covers the Ngotwane River up to and including… …   Wikipedia

  • Well stimulation — is a well intervention performed on an oil or gas well to increase production by improving the flow of hydrocarbons from the drainage area into the well bore.Cleaning the formationThe assortment of chemicals pumped down the well during drilling… …   Wikipedia

  • Drainage — See also: Storm drain and Stormwater Deep inside a Sydney drain in New South Wales Drainage is the natural or artificial removal of surface and sub surface water from an area. Many agricultural soils need drainage to improv …   Wikipedia

  • Drainage basin — Example of a drainage basin. The dashed line is the main water divide of the hydrographic basin A drainage basin is an extent or an area of land where surface water from rain and melting snow or ice converges to a single point, usually the exit… …   Wikipedia

  • Drainage system (Agriculture) — An agricultural drainage system is a system by which the water level on or in the soil is controlled to enhance agricultural crop production.=Classification=Figure 1 classifies the various types of drainage systems. It shows the field (or… …   Wikipedia

  • Drainage system (agriculture) — An agricultural drainage system is a system by which the water level on or in the soil is controlled to enhance agricultural crop production. Contents 1 Classification 1.1 Surface drainage systems 1.2 Subsurface drainage systems …   Wikipedia

  • Drainage equation — A drainage equation is an equation describing the relation between depth and spacing of parallel subsurface drains, depth of the watertable, depth and hydraulic conductivity of the soils. It is used in drainage design. Parameters in Hooghoudt s… …   Wikipedia


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.