Well drainage
Encyclopedia
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.
) 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.
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.
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.
The determination of the optimum depth of the water table is the realm of drainage research
.
, 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 :
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:
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:
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:
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.
Download "WellDrain" software from : http://www.waterlog.info/software.htm , or from : http://www.waterlog.info/weldrain.htm
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
An illustration of the parameters involved is shown in the figure. The hydraulic conductivity
can be found from an aquifer test
.
WellDrain 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.
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
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
.
Introduction
Subsurface (groundwaterGroundwater
Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...
) drainage for water table
Water table
The water table is the level at which the submarine pressure is far from atmospheric pressure. It may be conveniently visualized as the 'surface' of the subsurface materials that are saturated with groundwater in a given vicinity. However, saturated conditions may extend above the water table as...
and soil salinity in agricultural land can be done by horizontal and vertical drainage systems.
Horizontal drainage systems are drainage systems
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...
using open ditches (trenches) or buried pipe drains.
Vertical drainage systems are drainage systems using pumped wells, either open dug wells
Water well
A water well is an excavation or structure created in the ground by digging, driving, boring or drilling to access groundwater in underground aquifers. The well water is drawn by an electric submersible pump, a trash pump, a vertical turbine pump, a handpump or a mechanical pump...
or tube wells.
Both systems serve the same purposes, namely water table control
Watertable control
Watertable control is the practice of controlling the water table in agricultural land by subsurface drainage with proper criteria to improve the crop production.- Description and definitions :...
and soil salinity control
Salinity control
Soil salinity control relates to controlling the problem of soil salinity and reclaiming salinized agricultural land.The aim of soil salinity control is to prevent soil degradation by salinization and reclaim already salty soils...
.
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.
- The discharge is found from a water balanceHydrology (agriculture)Agricultural hydrology is the study of water balance components intervening in agricultural water management, notably in irrigation and drainage.-Water balance components:...
. - The depth is selected in accordance to aquiferAquiferAn aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
properties. The well filter must be placed in a permeable soil layer. - 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
Drainage research
Drainage research is the study of agricultural drainage systems and their effects to arrive at optimal system design.- Aspects to be covered :...
.
Flow to wells
The basic, steady stateSteady state
A system in a steady state has numerous properties that are unchanging in time. This implies that for any property p of the system, the partial derivative with respect to time is zero:...
, 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
Hydraulic conductivity
Hydraulic conductivity, symbolically represented as K, is a property of vascular plants, soil or rock, that describes the ease with which water can move through pore spaces or fractures. It depends on the intrinsic permeability of the material and on the degree of saturation...
that is isotropic
Isotropy
Isotropy is uniformity in all orientations; it is derived from the Greek iso and tropos . Precise definitions depend on the subject area. Exceptions, or inequalities, are frequently indicated by the prefix an, hence anisotropy. Anisotropy is also used to describe situations where properties vary...
is :
- 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
Natural logarithm
The natural logarithm is the logarithm to the base e, where e is an irrational and transcendental constant approximately equal to 2.718281828...
, and π = the number pi
Pi
' is a mathematical constant that is the ratio of any circle's circumference to its diameter. is approximately equal to 3.14. Many formulae in mathematics, science, and engineering involve , which makes it one of the most important mathematical constants...
.
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
Square root
In mathematics, a square root of a number x is a number r such that r2 = x, or, in other words, a number r whose square is x...
.
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 controlWatertable control
Watertable control is the practice of controlling the water table in agricultural land by subsurface drainage with proper criteria to improve the crop production.- Description and definitions :...
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.
Download "WellDrain" software from : http://www.waterlog.info/software.htm , or from : http://www.waterlog.info/weldrain.htm
The costs of the most attractive solution can be compared with the costs of a horizontal drainage
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...
system - for which the drain spacing can be calculated with a drainage equation
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....
- serving the same purpose, to decide which system deserves preference.
The well design proper is described in
An illustration of the parameters involved is shown in the figure. The hydraulic conductivity
Hydraulic conductivity
Hydraulic conductivity, symbolically represented as K, is a property of vascular plants, soil or rock, that describes the ease with which water can move through pore spaces or fractures. It depends on the intrinsic permeability of the material and on the degree of saturation...
can be found from an aquifer test
Aquifer test
An aquifer test is conducted to evaluate an aquifer by "stimulating" the aquifer through constant pumping, and observing the aquifer's "response" in observation wells...
.
Software
The numerical computer programComputer program
A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...
WellDrain for well spacing calculations takes into account fully and partially penetrating wells, layered aquifers, anisotropy
Anisotropy
Anisotropy is the property of being directionally dependent, as opposed to isotropy, which implies identical properties in all directions. It can be defined as a difference, when measured along different axes, in a material's physical or mechanical properties An example of anisotropy is the light...
(different vertical and horizontal hydraulic conductivity
Hydraulic conductivity
Hydraulic conductivity, symbolically represented as K, is a property of vascular plants, soil or rock, that describes the ease with which water can move through pore spaces or fractures. It depends on the intrinsic permeability of the material and on the degree of saturation...
or permeability) and entrance resistance.
Modelling
With a groundwater modelGroundwater model
Groundwater models are computer models of groundwater flow systems, and are used by hydrogeologists. Groundwater models are used to simulate and predict aquifer conditions.-Characteristics:...
that includes the possibility to introduce wells, one can study the impact of a well drainage system on the hydrology
Hydrology
Hydrology is the study of the movement, distribution, and quality of water on Earth and other planets, including the hydrologic cycle, water resources and environmental watershed sustainability...
of the project area. There are also models that give the opportunity to evaluate the water quality
Water quality
Water quality is the physical, chemical and biological characteristics of water. It is a measure of the condition of water relative to the requirements of one or more biotic species and or to any human need or purpose. It is most frequently used by reference to a set of standards against which...
.
SahysMod
SahysMod
SahysMod is a computer program for the prediction of the salinity of soil moisture, groundwater and drainage water, the depth of the watertable, and the drain discharge in irrigated agricultural lands, using different hydrogeologic and aquifer conditions, varying water management options, including...
is such a polygonal groundwater model permitting to assess the use of well water for irrigation
Irrigation
Irrigation may be defined as the science of artificial application of water to the land or soil. It is used to assist in the growing of agricultural crops, maintenance of landscapes, and revegetation of disturbed soils in dry areas and during periods of inadequate rainfall...
, the effects on soil salinity and on depth of the water table
Water table
The water table is the level at which the submarine pressure is far from atmospheric pressure. It may be conveniently visualized as the 'surface' of the subsurface materials that are saturated with groundwater in a given vicinity. However, saturated conditions may extend above the water table as...
.
External links
- Salinity Control and Reclamation Program (SCARP) using wells in the Indus valley of Pakistan.
- Website on waterlogging and land reclamation by horizontal and vertical drainage systems : http://www.waterlog.info