دراسة لاستصلاح الاراضى الصحراوية الساحلية(2)

Principles of salinity control
Drainage is the primary method of controlling soil salinity. The system should permit a small fraction of the irrigation water (about 10 to 20 percent, the drainage or leaching fraction) to be drained and discharged out of the irrigation project. [3]
In irrigated areas where salinity is stable, the salt concentration of the drainage water is normally 5 to 10 times higher than that of the irrigation water. Salt export matches salt import and salt will not accumulate.
When reclaiming already salinized soils, the salt concentration of the drainage water will initially be much higher than that of the irrigation water (for example 50 times higher). Salt export will greatly exceed salt import, so that with the same drainage fraction a rapid desalinization occurs. After one or two years, the soil salinity is decreased so much, that the salinity of the drainage water has come down to a normal value and a new, favorable, equilibrium is reached.
In regions with pronounced dry and wet seasons, the drainage system may be operated in the wet season only, and closed during the dry season. This practice of checked or controlled drainage saves irrigation water.
The discharge of salty drainage water problem may pose environmental problems to downstream areas. The environmental hazards must be considered very carefully and, if necessary mitigating measures must be taken. If possible, the drainage must be limited to wet seasons only, when the salty effluent does inflict the least harm.
[edit]Drainage systems


Parameters of a horizontal drainage system


Parameters of a vertical drainage system
Land drainage for soil salinty control is usually by horizontal drainage system(figure left), but vertical systems (figure right) are also employed.
The drainage system designed to evacuate salty water also lowers the water table. To reduce the cost of the system, the lowering must be reduced to a minimum. The highest permissible level of the water table (or the shallowest permissible depth) depends on the irrigation and agricultural practices and kind of crops.
In many cases a seasonal average water table depth of 0.6 to 0.8 m is deep enough. This means that the water table may occasionally be less than 0.6 m (say 0.2 m just after an irrigation or a rain storm). This automatically implies that, in other occasions, the water table will be deeper than 0.8 m (say 1.2 m). The fluctuation of the water table helps in the breathing function of the soil while the expulsion of carbon dioxide (CO2) produced by the plant roots and the inhalation of fresh oxygen (O2) is promoted.
The establishing of a not too deep water table offers the additional advantage that excessive field irrigation is discouraged, as the crop yield would be negatively affected by the resulting elevated water table, and irrigation water may be saved.
The statements made above on the optimum depth of the watertable are very general, because in some instances the required water table may be still shallower than indicated (for example in rice paddies), while in other instances it must be considerably deeper (for example in some orchards). The establishment of the optimum depth of the water table is in the realm of agricultural drainage criteria.
]Soil leaching


Water balance factors in the soil
The unsaturated zone or vadose zone of the soil below the soil surface and the watertableis subject to four main hydrological inflow and outflow factors:
 Infiltration of rain and irrigation water (Irr) into the soil through the soil surface (Inf) : Inf = Rain + Irr
 Evaporation of soil water through plants and directly into the air through the soil surface (Evap)
 Percolation of water from the unsaturated zone soil into the groundwater through the watertable (Perc)
 Capillary rise of groundwater moving by capillary suction forces into the unsaturated zone(Cap)
In steady state (i.e. the amount of water stored in the unsaturated zone does not change in the long run) the water balance of the unsaturated zone reads: Inflow = Outflow, thus:
 Inf + Cap = Evap + Perc or : Irr + Rain + Cap = Evap + Perc
and the salt balance is
 Irr.Ci + Cap.Cc = Evap.Fc.Ce + Perc.Cp + Ss
where Ci is the salt concentration of the irrigation water, Cc is the salt concentration of the capillary rise, equal to the salt concentration of the upper part of the groundwater body, Fc is the fraction of the total evaporation transpired by plants, Ce is the salt concentration of the water taken up by the plant roots, Cp is the salt concentration of the percolation water, and Ss is the increase of salt storage in the unsaturated soil. This assumes that the rainfall contains no salts. Only along the coast this may not be true. Further it is assumed that norunoff or surface drainage occurs.
The amount of salts removed by plants (Evap.Fc.Ce) is usually negligibly small: Evap.Fc.Ce = 0


Leaching curves, calibrating leaching efficiency
The salt concentration Cp can be taken as a part of the salt concentration of the soil in the unsaturated zone (Cu) giving: Cp = Le.Cu , where Le is the leaching efficiency. The leaching efficiency is often in the order of 0.7 to 0.8 [4] , but in poorly structured, heavyclay soils it may be less. In the Leziria Grande polder in the delta of the Tagus river in Portugal it was found that the leaching efficiency was only 0.15 [5] .
Assuming that one wishes to avoid the soil salinity to increase and maintain the soil salinity Cu at a desired level Cd we have:
Ss = 0 , Cu = Cd and Cp = Le.Cd. Hence the salt balance can be simplified to:
 Perc.Le.Cd = Irr.Ci + Cap.Cc
Setting the amount percolation water required to fulfill this salt balance equal to Lr (theleaching requirement) it is found that:
 Lr = (Irr.Ci + Cap.Cc) / Le.Cd .
Substituting herein Irr = Evap + Perc − Rain − Cap and re-arranging gives :
 Lr = [ (Evap−Rain).Ci + Cap(Cc−Ci) ] / (Le.Cd − Ci) [3]
With this the irrigation and drainage requirements for salinity control can can be computed too.
In irrigation projects in (semi)arid zones and climates it is important to check the leaching requirement, whereby the field irrigation efficiency (indicating the fraction of irrigation water percolating to the underground) is to be taken into account.
The desired soil salinity level Cd depends on the crop tolerance to salt. Data on crop tolerance can be found online: [2] .
More elaborate water and salt balances can be viewed online : [3] .
]Soil salinity models

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SaltMod components
The majority of the computer models available for water and solute transport in the soil (e.g. SWAP[6], DrainMod-S [7], UnSatChem [8]), are based on Richard's differential equation for the movement of water in unsaturated soil in combination with a differential salinity dispersion equation. The models require input of soil charac teristics like the relation between unsaturated soil moisturecontent, water tension, hydraulic conductivity and dispersivity.
These relations vary to a great extent from place to place and are not easy to measure. The models use short time steps and need at least a daily data base of hydrological phenomena. Altogether this makes model application to a fairly large project the job of a team of specialists with ample facilities.
Simpler models, like SaltMod [9] , based on seasonal water and soil balances and an empirical capillary rise function, are also available. They are useful for long-term salinity predictions in relation to water management and drainage practices.
Spatial variations owing to variations in topography can be simulated and predicted using salinity cum groundwater models, like SahysMod.
Watertable control is the practice of controlling the water table in agricultural land by subsurface drainage with proper criteria to improve the crop production.
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.


Figure 1. Drainage parameters in watertable control


Figure 2. Crop yield (Y) and depth of water table (X in dm)
Purpose
In agricultural land drainage, the purpose of water table control is to establish a depth of the water table (Figure 1) that does no longer interfere negatively with the necessary farm operations and crop yields (Figure 2, from SegReg model, seeSegmented regression).
In addition, land drainage can help withSoil salinity control.
The soil's Hydraulic conductivity plays an important role in drainage design.
The development of agricultural drainage criteria 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.


Figure 3. Positive and negative effects of land drainage
Optimization
Optimization of the depth of the water table is related the benefits and costs of the drainage system (Figure 3). 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 [2] .


Figure 4. Example of effects of drain depth
The optimization of drainage design and the development of drainage criteria are discussed in the article on drainage research.
Figure 4 shows an example of the effect of drain depth on soil salinity and various irrigation/drainage parameters [3] .
[History
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.


Figure 5. Controlled drainage
]Environment
As a result of large scale developments, many modern drainage projects were over-designed [4] , 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, the hardliner trend is reversed. Further, checked or controlled drainage systems were introduced, see also Figure 5 and Drainage system (agriculture) .
Drainage design


Figure 6. Geometry of a well drainage system
The design of subsurface drainage systems in terms layout, depth and spacing of the drains is often done using subsurface drainage equations with parameters like drain depth, depth of the water table, soil depth, hydraulic conductivity of the soil and drain discharge. The drain discharge is found from an agricultural water balance.
The computations can be done using computer models like EnDrain [5] .
Drainage by wells
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 the Salinity Control and Reclamation Program (SCARP) in the Indus valley of Pakistan. 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 areas 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).
A classification of drainage systems is found in the article Drainage system (agriculture).