Principles of salinity control

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 (saline) soils (see also land reclamation) Soil reclamation is also called soil improvement, rehabilitation, remediation, recuperation, or amelioration.

 

 

Salinity control relates to controlling the problem of soil salinity and reclaiming salinized agricultural land.

The figure shows an example of declining crop yields caused by high soil salinity. It was made with the SegReg program.

 

 

 

 

 

 

 

 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. ^[2]

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, drainage may be operated to the wet season, and closed during the dry season. This practice of checked 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. The environmental issues will not be further discussed here.

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 carbondioxide (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 watertable is subject to four main hydrological inflow and outflow factors:

• Infiltration of rain and irrigation water into the soil through the soil surface (Inf)
• 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

and the salt balance is

• Inf.Ci + Cap.Cc = Evap.Fc.Ce + Perc.Cp + Ss

where Ci is the salt concentration of the infiltrating 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 salt sotorage in the unsaturated soil.
The amount of salts removed by plants (Evap.Fc.Ce) is usually negiligibly small:

• Evap.Fc.Ce = 0

The salt concentration Cp can be taken as a part of the salt concentration of the soil in the unsaturated zone (Cu):

• Cp = Le.Cu

where Le is the leaching efficiency. The leaching efficiency is often in the order of 0.7 to 0.8 ^[3] , but in poorly structured, heavy clay 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 ^[4] .
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

Hence the salt balance can be simlified to:

• Perc.Le.Cd = Inf.Ci + Cap.Cc

Setting the amount percolation water required to fullfill this salt balance equal to Lr (the leaching requirement) it is found that:

• Lr = (Inf.Ci + Cap.Cc)/Le.Cd

In irrigation projects in (semi)arid zones and climates it is important to check the leaching requirement, whereby the field irrigation efficiency (a parameter indicating the percolation losses of irrigation water to the underground) is to be taken into account.
More elaborate water and salt balances can be dowloaded here.

[edit] Soil salinity models Please help improve this article or section by expanding it. Further information might be found on the talk page or at requests for expansion. (October 2007)

The majority of the computer models available for water and solute transport in the soil (e.g. Swatre ^[5], DrainMod ^[6] ) 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 charac­teristics like the relation between unsaturated soil moisture content, water tension, hydraulic conductivity and dispersivity.
These relations [variation|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.

 

[edit] Soil salinity model: SaltMod

Saltmod is 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 (geo)hydrologic conditions, varying water management options, including the use of ground water for irrigation, and several cropping rotation schedules. The water management options include irrigation, drainage, and the use of subsurface drainage water from pipe drains, ditches or wells for irrigation.