Salinity control

Salinity control

Salinity control entails a combination of measures aiming at the prevention of soil salinization, or aiming at the reclamation (also called improvement, rehabilitation, remediation, recuperation, amelioration) of salty (saline) soils to ascertain sustained crop yields unaffected by salinity problems and an excessive salt content of the soil.
The figure shows safe values of soil salinity (EC<5). It was made with the SegReg program, see segmented regression.

Crop yield of mustard and soil salinity

Salty (saline) soils are a common feature in irrigated lands of the (semi)arid regions and have poor to no crop production. The problems are often associated with the occurrence of high water tables, indicating a lack of natural subsurface drainage to the underground owing to: (1) insufficient transport capacity of the aquifer or (2) lack of free outflow conditions of the aquifer because, for example, the waterlogged area is situated in a topographical depression.

The prime cause of salinization is the fact that irrigation water brought in from the rivers contains salts. All irrigation water, however "sweet", bring salts that remain behind in the soil after evaporation.
    For example, assuming irrigation water with a low salt concentration of 0.3 g/l (equal to 0.3 kg/m3 corresponding to an electric conductivity of about 0.5 dS/m) and a modest annual supply of irrigation water of 10000 m3/ha (almost 3 mm/day) already brings 3000 kg salt/ha each year. In the absence of sufficient natural drainage (as in waterlogged soils) and without a proper leaching and drainage program, this would lead in the long run to a high soil salinity and reduced crop yields.
    Many irrigation waters have higher salt contents and in many irrigation projects the annual supply of irrigation water is more than mentioned in the example (e.g. sugar cane needs about 20000 m3/ha per year), so that the import of salts is often more than 3000 kg/ha per year and it can go up to 10000 kg/ha/year.

The secondary cause of salinzation is the enormous change of the natural water balance of the irrigated lands. In irrigation projects it is impossible to achieve 100% irrigation efficiency (meaning that all the irrigation water is consumed by the plants). The maximum attainable efficiency is about 70% but usually it is less than 60%. This means that minimum 30%, but usually more than 40% of the irrigation water is not evaporated and it must go somewhere.
    The main part of the water losses are stored in the underground and they change the original aquifer hydrology considerably. Many aquifers are unable to cope with these quantities of water and, as a result, the water table rises and creates a problem of water logging.
    The adverse effect of water logging is two-fold: (1) it reduces the yield of most crops and (2) it is a symptom of insufficient natural drainage to the underground so that the salts brought in with the irrigation water cannot be removed and accumulate in the soil.

Normally, the salinzation of agricultural land affects a considerable part of the irrigation project, to the tune of 20 to 30%. When the agriculture in such a fraction of the land is abandoned, a new salt and water balance is attained, a new equilibrium is reached, and the situation becomes stable.
     In India alone, millions of hectares have been severely salinized. China and Pakistan do not lag much behind (perhaps China has even more salt affected land than India), and world wide the score is tens of millions of hectares.

Although the principles of the processes of salinization are fairly easy to understand, it is more difficult to explain why certain parts of the land suffer from the problems and other parts do not, or to predict accurately which part of the land will fall victim. The main reason for this is the variation of natural conditions in time and space, the usually uneven distribution of the irrigation water, and the seasonal or yearly changes of agricultural practices. Only in lands with undulating topography the explanation and prediction is pretty simple: the depression areas will degrade the most.
     The preparation of salt and water balances for distinguishable sub-areas in the irrigation project, or the use of agro-hydro-salinity models can be helpful in explaining or predicting the extent and severity of the problems.

The governing principle of salinity control is to establish a drainage system in the affected or to be affected parts of the land (see also Land drainage). 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.
    In a stable salinity situation, the salt concentration of the drainage water is normally 5 to 10 times higher than that of the irrigation water, so that with the given drainage fraction the salt import will equal the salt export and no salt accumulation will occur.

When reclaiming already salinized soils, the salt concentration of the drainage water will initially be much higher than that of the irrigation water (say 50 times higher) and the salt export will be much more than the 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 (rainy) seasons it is worth while to consider limiting the drainage function of the system to the wet season, and close the system 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 CO2 produced by the plant roots and the inhalation of fresh oxygen 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 reader is requested to be aware of the generality of the statements made above on the optimum depth of the water table, because in some instances the water table can 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 the agricultural drainage criteria.
See also Watertable control.

The FAO presents an article on salty soils in:
http://www.fao.org/docrep/R4082E/r4082e08.htm

A leading research institute is the US Salinity Laboratory at:
http://www.ars.usda.gov/main/site_main.htm?modecode=53-10-20-00

The following private website gives free downloads of articles and softwares on soil salinity:
http://www.waterlog.info/