دراسة لاستصلاح الاراضى الصحراوية الساحلية(3)

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


Classification of agricultural drainage systems.
Figure 1 classifies the various types of drainage systems. It shows the field (or internal) and the main (or external) systems.[1] .
The function of the field drainage system is to control the water table, whereas the function of the main drainage system is to collect, transport, and dispose of the water through an outfall or outlet. In some instances one makes an additional distinction between collector and main drainage systems.
Field drainage systems are differentiated in surface and subsurface field drainage systems.
Sometimes (e.g. in irrigated, submerged rice fields), a form of temporary drainage is required whereby the drainage system is allowed to function on certain occasions only (e.g. during the harvest period). If allowed to function continuously, excessive quantities of water would be lost. Such a system is therefore called a checked, or controlled, drainage system. More usually, however, the drainage system is meant to function as regularly as possible to prevent undue waterlogging at any time and one employs a regular drainage system. In literature, this is sometimes also called a "relief drainage system".
[Surface drainage systems
The regular surface drainage systems, which start functioning as soon as there is an excess of rainfall or irrigation, operate entirely by gravity. They consist of reshaped or reformed land surfaces and can be divided into:
 Bedded systems, used in flat lands for crops other than rice;
 Graded systems, used in sloping land for crops other than rice.
The bedded and graded systems may have ridges and furrows.
The checked surface drainage systems consist of check gates placed in the embankments surrounding flat basins, such as those used for rice fields in flat lands. These fields are usually submerged and only need to be drained on certain occasions (e.g. at harvest time). Checked surface drainage systems are also found in terraced lands used for rice.[2]
In literature, not much information can be found on the relations between the various regular surface field drainage systems, the reduction in the degree of waterlogging, and the agricultural or environmental effects. It is therefore difficult to develop sound agricultural criteria for the regular surface field drainage systems. Most of the known criteria for these systems concern the efficiency of the techniques of land leveling and earthmoving.[3]
Similarly, agricultural criteria for checked surface drainage systems are not very well known.
]Subsurface drainage systems


Mug and sole drain (Scotland, 18th century)
Like the surface field drainage systems, the subsurface field drainage systems can also be differentiated inregular systems and checked (controlled) systems.


Controlled drainage system
When the drain discharge takes place entirely by gravity, both types of subsurface systems have much in common, except that the checked systems have control gates that can be opened and closed according to need [4] . They can save much irrigation water [5] . A checked drainage system also reduces the discharge through the main drainage system, thereby reducing construction costs.
When the discharge takes place by pumping, the drainage can be checked simply by not operating the pumps or by reducing the pumping time. In northwestern India, this practice has increased the irrigation efficiency and reduced the quantity of irrigation water needed, and has not led to any undue salinization.[6]


Parameters of horizontal drainage


Parameters of vertical drainage
The subsurface field drainage systems consist of horizontal or slightly sloping channels made in the soil; they can be open ditches, trenches, filled with brushwood and a soil cap, filled with stones and a soil cap, buried pipe drains, tile drains, or mole drains, but they can also consist of a series ofwells.
Modern buried pipe drains often consist of corrugated, flexible, and perforated plastic (PE or PVC) pipe lines wrapped with an envelope or filter material to improve the permeability around the pipes and to prevent entry of soil particles, which is especially important in finesandy and silty soils. The surround may consist of synthetic fibre (geotextile).
The field drains (or laterals) discharge their water into the collector or main system either by gravity or by pumping.
The wells (which may be open dug wells or tubewells) have normally to be pumped, but sometimes they are connected to drains for discharge by gravity.
Subsurface drainage by wells is often referred to as vertical drainage, and drainage by channels as horizontal drainage, but it is more clear to speak of "field drainage by wells" and "field drainage by ditches or pipes" respectively.
In some instances, subsurface drainage can be achieved simply by breaking up slowly permeable soil layers by deep plowing (sub-soiling), provided that the underground has sufficient natural drainage. In other instances, a combination of sub-soiling and subsurface drains may solve the problem.
]Main drainage systems


Deep collector drain
The main drainage systems consist of deep or shallow collectors, and main drains ordisposal drains.
Deep collector drains are required for subsurface field drainage systems, whereasshallow collector drains are used for surface field drainage systems, but they can also be used for pumped subsurface systems. The deep collectors may consist of open ditches or buried pipe lines.
The terms deep collectors and shallow collectors refer rather to the depth of the water level in the collector below the soil surface than to the depth of the bottom of the collector. The bottom depth is determined both by the depth of the water level and by the required discharge capacity.
The deep collectors may either discharge their water into deep main drains (which are drains that do not receive water directly from field drains, but only from collectors), or their water may be pumped into a disposal drain.
Disposal drains are main drains in which the depth of the water level below the soil surface is not bound to a minimum, and the water level may even be above the soil surface, provided that embankments are made to prevent inundation. Disposal drains can serve both subsurface and surface field drainage sys