April 23, 2014

Salting the earth: the problem of salinisation

Published as Chapter 11 of The Social and Environmental Effects of Large Dams: Volume 1. Overview. Wadebridge Ecological Centre, Worthyvale Manor Camelford, Cornwall PL32 9TT, UK, 1984. By Edward Goldsmith and Nicholas Hildyard.

The causes of salinisation

All soils contain salt. That salt is the result of what geologists call ‘weathering’ – the natural chemical, biological and physical processes which lead to the gradual breakdown of rocks and other geological formations. As those rocks are gradually worn down, so they release their natural salts into the soil, generally to be dissolved in rainwater.

That water either percolates into the underlying groundwater or is washed away into streams and rivers. It follows that all water, like all soil, contains traces of salt. Indeed, even a fresh mountain stream will contain up to 50 parts per million (ppm) of salt – admittedly a minute amount compared with the 35,000 ppm found in seawater, but significant nonetheless. [1]

When the concentration of salts in soil reaches 0.5-1.0 percent, land becomes toxic to plant life. [2] In the dry tropics, that problem is particularly acute since there is not enough rainfall to flush out the salts which accumulate in the soil. Soils in those areas can thus have a natural salt content as high as twelve percent. Equally important, groundwater can contain salts at levels approaching the concentration of those in seawater. [3] Although such natural salt levels are the exception rather than the rule, the generally high salt burden of arid and semi-arid lands renders them particularly vulnerable to salinisation.

As Professor Victor Kovda of the Soviet Academy of Sciences is at pains to point out, groundwater provides “the main reserve and source of salts circulating in the soil profile” and, for that reason, it is essential that the water table under potentially saline soils should be kept at the appropriate level below the surface. [4]

If the water table is permitted to rise to within 2.5 metres of the surface, then the groundwaters are drawn upwards through capillary action – adding still further to their own salt burden on the way, by dissolving the salts in the soils near the surface. [5] In effect, the land becomes waterlogged with increasingly saline water.

Even before that water reaches the surface, it starts affecting crop yields by interfering with the capacity of plants to take up moisture and oxygen. Thus, in China’s Shaanxi Province (where the impact of waterlogging on wheat and coffee production has been carefully recorded) it was found that normal yields could still be obtained when the water was 2 to 3 metres below the surface.

When it rose to 1 metre below the surface, and hence into the root zone, wheat yields fell to one fifth of the norm, and cotton yields to one half of the norm. When the water table rose to 0.5 metres and higher, wheat production fell to zero and cotton production fell to between one-fifth and one-twentieth of the norm. [6]

Worse still, once they approach the surface, the by now increasingly saline groundwaters quickly evaporate. The salts they contain are thus left behind to accumulate on the surface. Once again, the dry tropics are particularly vulnerable, evaporation rates in hot arid and semi-arid lands being four to five times higher than those in temperate areas. Under such conditions, it is not long before the whole area becomes covered with a white saline crust.

Where the salts in the water contain sodium or sodium bicarbonate – the result of sodium alumino-silicate minerals being released by the weathering of volcanic rocks – the destruction goes one step further. The lands become alkaline. As Kovda explains,

“The soil undergoes intensive hydrophilisation, losing its structure and permeability. An intensive cementation process gets under way in dry conditions. High alkalinity, cementation and impermeability in the aggregate leads to loss of fertility. The soil turns barren and is very difficult to reclaim”. [7]

Such alkalinisation is already affecting parts of Northern India, Pakistan, Armenia, Afghanistan and Iran. Effectively, those lands are now dead forever.

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Irrigation and salinisation: the intimate connection

If arid lands are not to become salinised, it is clearly essential to maintain the ‘water-salt balance’ of the soil. That is to say, the amount of water leaving the soil must be at least equal to the amount entering it. The water should not be allowed to accumulate. So, too, with the salt balance: salt must not be added to the soil – for example, by using irrigation water with a high salt content – unless an equal amount of salt can be flushed out of the land.

Irrigation schemes throw that delicate water-sale balance dangerously out of kilter. Firstly, perennial irrigation invariably raises the water table. According to Professor Gilbert White of the University of Colorado at Boulder, for instance, there are “numerous cases” throughout the world where the water table under irrigated land “has risen within ten years from about 25-30 metres below the soil surface up to 1-2 metres depth”. [8] Indeed, in some areas, groundwater tables are rising at a rate of 3 to 5 metres a year.

That rise in groundwater levels is caused, primarily, by the water lost through seepage from irrigation channels. Such seepage losses can be considerable – in some instances, up to 60 percent of the water transported through the canals is lost to seepage. Where the irrigation water is provided by large-scale dams, the problem is compounded by the seepage of water from the dam’s reservoir: in some cases, such seepage has raised the level of ground-waters up to 20 kilometres away. Finally, the over-use of irrigation water (a problem common to irrigation schemes the world over) helps to raise the water table and hence further increase waterlogging. As waterlogging sets in, so the inevitable process of salinisation begins.

Secondly, irrigation adds directly to the salt load of soils through increasing the rate of ‘evapo-transpiration’. As plants ‘breathe’, so much of the water taken up by their roots is lost through their leaves to the atmosphere – a process known as ‘transpiration’. So, too, a large proportion of the water applied to plants is lost to direct evaporation. Because it is practically impossible to quantify how much water is lost to transpiration and how much to evaporation, the two causes of water loss are treated as a single phenomenon, known technically as ‘evapo-transpiration’.

Where land is irrigated, the losses due to evapo-transpiration are particularly high. Not only does irrigation increase the extent of vegetative cover – and hence the rates of transpiration – but it also requires water to be spread thinly over a wide area, thus raising direct evaporation losses.

The inevitable result of high evapo-transpiration is that the natural salt in water becomes concentrated in the soil. On that score, the research of Arthur Pillsbury, Professor of Engineering and (until he retired) Director of the Water Resources Centre at the University of California, Los Angeles, is particularly relevant. Writing in Scientific American, Pillsbury estimates that three-quarters of the water applied each year to irrigated land in the US is lost to evapo-transpiration.

“If, as seems reasonable, the average annual amount of water applied in irrigation in the Western US is equivalent to 3 feet covering the area cultivated, about 120 million acre-feet of water is applied annually to some 40 million acres of land. Roughly 90 million acre-feet of the total volume is lost by evapo-transpiration. The remaining 30 million acre-feet holds essentially all the original salts: a four-fold concentration.”

Such water frequently contains more than 2,000 ppm of salt. [9]

That problem is exacerbated when irrigation water is drawn directly from rivers or from the reservoirs impounded by large dams. Thus, in the US, evaporation at Lake Mead (behind the Hoover Dam) and at lake Powell (behind the Glen Canyon Dam) is reported to have increased the salinity of the Colorado River by 100 milligrams per litre.

So, too, Waterbury reports that high evaporation rates at the Aswan High Dam reservoir “have led to 10 percent increases in salinity: that is, water entering the reservoir has about 200ppm, and when it leaves, 220ppm”.

He goes on to note:

“Because Upper and Middle Egyptian lands drain back into the main Nile, salinity around Cairo and in the Delta is in excess of 300 ppm. In itself, this is no cause for alarm, but agricultural intensification in Egypt and the Sudan cannot fail to aggravate the problem. Moreover, in developing new water resources from the equatorial lakes and the Jongiei scheme, the White Nile, with a higher salt content than the Blue Nile, will figure prominently in downstream discharge.” [10]

That rising salt burden in the waters of reservoirs and rivers leads Kovda to warn that irrigation water is itself now a significant factor in the spread of salinisation.

“Precipitation water has 10 to 30 mg/l, and sometimes 50 mg/l of salts and the water may still be considered practically fresh. The best irrigation water from large rivers contains 200 to 500 mg/l of salts. Supplying 10,000 cubic metres of water on 1 hectare of land during the irrigation season deposits 2 to 5 tons/ha of salts in soils. After 10-20 years of irrigation, this amount becomes enormous – amounting to dozens and even hundreds of tons per hectare.” [11]

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The extent of the problem

The devastation caused by waterlogging and salinisation is hard to quantify – not least because there are considerable differences of opinion as to when land should be classified as ‘saline’. The Pakistan Department of Power and Irrigation, for example, suggests that land should be regarded as ‘saline’ when crop yields have been reduced by a fifth or more.

Nonetheless, various figures have been advanced. The FAO, for instance, estimates that at least 50 percent of the world’s irrigated land now suffers from salinisation. [12] Others put the figure even higher. Thus, Victor Kovda argues that, worldwide, salinisation affects 60 to 80 percent of irrigated land – between 1 and 1..5 million hectares succumbing every year. Significantly, much of that land is “in irrigated crop-lands of high potential production”. [13]

In Pakistan, 25 million acres of the 37 million acres under irrigation are estimated to be salinised, waterlogged or both. [14] Of that land, 5 million acres are classified as “severely affected with salinity; 10 million as suffering ‘patchy salinity’; and 10 million as being ‘poorly drained’ “. [15] Overall, 23 percent of the country’s land suffers to varying degrees from salinisation or waterlogging – that figure reaching 80 percent in the Punjab.

In the lower Indus, concentrations of salt in the groundwater have been found to reach 30,000 ppm – almost as salty as seawater. Indeed, a recent survey reported that the water from 18 percent of the tube-wells in the area was “unfit for use”; that 76 percent of the wells produced water which, if used, might “salinise the soil profile to a depth of 6 feet within 12 years”; and that only 6 percent had water which could be classified as being “of excellent quality”. [16]

All told, an estimated 100,000 acres are lost annually to waterlogging and salinisation in Pakistan – more than 100 hectares a day. [17] This was in fact, the figure provided locally to the Revelle Panel appointed by President Kennedy to study the problem of irrigation and salinisation in what was then West Pakistan. Since, at that time (1962), 23 million acres of agricultural land was irrigated, nearly 5 percent of it was going out of production each year.

The Panel, unfortunately, played down the problem and reported that “between 50,000 and 100,000 additional acres are being affected each year, some of which are passing out of crop production”. [17a]

Of the area earmarked to receive water via China‘s giant Yangtse Diversion scheme, 2.7 million hectares already suffer from salinisation. “Slightly and moderately saline soils prevail on 73.7 percent of the affected area”, Guo Huancheng and Xu Zhikang of the Institute of Geography of the Academy Sinica, told a recent conference held to discuss the diversion scheme.

“Here the salt concentration generally ranges between 0.1 percent and 0.7 percent, so it is still possible to grow crops. The remaining 26.3 percent of the affected area has a salt concentration exceeding 1 percent and a seedling retention rate below 30 percent. The land is used mostly for livestock and forestry. The extensive saline area in the region is not only unfavourable to current agricultural production, but also an important problem which must be taken into account in considering a south-to-north water transfer.” [18]

Xu Yuexian and Hong Jilian, also of the Institute of Geography, expressed similar concern about the extent of salinisation in the proposed transfer region. Although salinisation levels in the area had fluctuated dramatically over the last 30 years, they told the conference, there was little room for complacency. Overall, the amount of saline land was increasing and, moreover, much of the increase had taken place during the late 1970s – largely, they suggested, as a result of a rise in water tables alongside those rivers which have recently been dammed. In addition to the 2.7 million hectares already affected in the area, a further 4.7 million hectares consists of potentially saline soil “which is most vulnerable to secondary salinisation if affected by detrimental factors”. [19]

In Egypt, the problems of salinisation and waterlogging have been described as “grave”. John Waterbury writes,

“A few years ago, an FAO study contended that 35 percent of Egypt’s cultivated surface is afflicted by salinity and 90 percent by waterlogging. A USAID mission reported in 1976 that 4.2 million feddans (1 feddan=1.038 acres) were undergoing slight to severe effects from inadequate drainage, and unless something were done, all would be severely affected.” [20]

Waterlogging alone is estimated to have reduced agricultural productivity by at least 30 percent, although it is claimed (perhaps optimistically) that drainage will restore productivity.

More than 50 percent of the 3.6 million hectares under irrigation in Iraq suffer from salinisation and waterlogging. [21] Worst affected are the middle and lower Rafidain Plains. Indeed, Erick Eckholm – at the time a researcher with the Worldwatch Institute – reports that vast areas of South Iraq now “glisten like fields of freshly fallen snow”. [22]

500,000 acres in Syria – half of the country’s irrigated land – are waterlogged or salinised. According to M. M. El Gabaly,

“Due to the aridity of the climate, with evaporation exceeding precipitation in many locations, it is estimated that 70 percent of the soils put under irrigation are potentially saline.”

Nonetheless, plans are afoot to irrigate a further 1.5 million acres of part of the giant Euphrates Project. [23] Annual crop losses due to salinity and water-logging in the Euphrates Valley alone already amount to $300 million.

In Iran, 15 percent of the irrigated land degree by waterlogging, salinity and alkalinity. 16.8 million hectares of arable land, 7.3 million are estimated to be saline, and 8.2 million are waterlogged. [24]

In India, the amount of land devastated by water and salt has been variously estimated at between 6 million and 10 million hectares – almost a quarter of the 43 million hectares under irrigation. In Madhya Pradesh, affected areas are referred to as ‘wet deserts’. [25]

For the Near East as a whole, El Gabaly warns: “In all countries of the region, without exception, salinity is of prime concern in agricultural development”. [26] Elsewhere, it is estimated that more than 70 percent of the 30 million hectares of irrigated land in Egypt, Iran, Iraq and Pakistan is now “moderately to severely affected” by salinisation.

In the US, Jan Van Schilgaarde, Director of the US Salinity Laboratory, considers that 25 to 35 percent of the country’s irrigated land suffers from salinity – and that the problem is getting worse. “Today, about 400,000 acres of irrigated farmland in the San Joaquin Valley are affected by high, brackish water tables”. [27] If no remedial measures are taken, the Valley could lose over a million acres of farmland in the next hundred years.

Worldwide, the annual losses to salinisation and waterlogging are simply staggering. In a survey conducted for the UN Water Conference, Malin Falkenmark and Gunnar Lindh estimate that between 200,000 and 300,000 hectares of irrigated land are taken out of production every year due to the ravages of water and salt. Harold Dregne of Texas Tech puts the figure even higher – at 500,000 hectares. [28]

For its part, the FAO does not propose a precise figure: nonetheless, it admits that “several hundred thousand hectares are abandoned annually as a result of salinisation”. [29] Indeed, according to one recent study, as much irrigated land is now being taken out of production due to waterlogging and salinisation as new irrigation schemes are bringing into production. If this is correct, then the rate of salinisation is very much worse than even Dregne’s figures suggest.

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