46. CULTIVATION OF THE INDUS VALLEY

by Dr Roger Revelle

In 1961, President Ayub Khan of Pakistan asked President Kennedy to send a group of scientists to study the problem of waterlogging and salinity in West Pakistan, which was throwing large areas of farmland out of cultivation or greatly reducing the yields. A panel of twenty specialists was appointed accordingly, and reported as follows.

The flood plain of the Indus and its five great tributaries - the Jhelum, the Chenab, the Ravi, the Beas and the Sutlejbearing the melted snows and monsoon rains from the Himalayas to water green ribbons of floodland, was naturally a place where farmers would settle and which conquerors would covet. The last of these, the British, worked a transformation of the land. There had been some great irrigation works before, but nothing compared with the barrages and 10,000 miles of canals which, beginning in the mid-19th century, the British engineers created in the Punjab and the Sind. British administrators encouraged hundreds of thousands of farmers and their families to immigrate into the newly watered lands; each farmer was allocated fifty acres, and a grid-like pattern of villages was built to house the new "canal colonies ".

The development of irrigation is continuing today. Under the Indus Settlement between Pakistan and India, the entire flow of the Sutlej and Ravi rivers will be diverted to India. The area previously irrigated from these rivers will be provided with water by new barrages and canals which will carry some of the flow of the Indus, Jhelum and Chenab to the eastward. Meanwhile, hundreds of thousands of acres a year are being brought newly under cultivation.

Yet something has gone badly wrong. One obvious problem was the damage to cultivated land by waterlogging and the accumulation of salt - twin evils that have afflicted ancient and modern irrigation schemes in many other parts of the world. It is believed, for example, that the ancient civilization of Sumeria crumbled because its soils gradually became saline.

The effects are plainly visible in the Indus Plain. As we flew into Lahore we saw salt lying like snow on the surface and villages of the canal colony grid here and there derelict, like holes in a punched card. The area of severe waterlogging and salinity in the culturable lands of West Pakistan is about 6.5 million acres and is increasing at a rate of 50,000 to 100,000 acres a year.

It is usual to suspect inadequate drainage in an irrigation scheme as the chief cause of waterlogging and salinity. As we shall see, this suspicion is justified in the Punjab and northern Sind. However, it should be noted that most of the salt in the soils of the Ghulam Mohammed area was there before irrigation began.

The hydrology of the Indus Plain is dominated by the great size and flatness of the area. In the 700 miles from Lahore to the Arabian Sea, the drop is only 700 feet. Drainage is therefore extremely difficult; even if one is conscious of the problem and prepared to spend money, there is nowhere for the water to go.

The hydrological regime can be divided into two parts. The Punjab (the " land of five rivers ") has, in the northern part, 10 to 20 inches of rainfall a year; it also has an immense underground sea of fresh water, equivalent in volume to ten years' flow of the Indus system, a resource of immense potential importance. The lower Indus Plain has much less rainfall (less than 5 inches in the Sind) and the underground water is salty.

In the Punjab most of the underground water is fresh, in the sense that it is quite suitable for irrigation or as drinking water. Nevertheless, like all waters in arid regions, it contains some dissolved salts. The underground water table formerly came close to the surface only near the rivers; otherwise it lay as much as 100 feet underground. The construction of canals, from which about 40 per cent of the water percolated into the ground, gradually brought the water table very close to the surface over large regions of the plain. When this happens, low-lying areas become water logged, drowning the crops; in other places, the water creeps to the surface continually from the water table by capillary action and evaporates, leaving its dissolved salts behind. These salts accumulate in the top few feet of the soil and poison the crops. Only part of the soil salt has accumulated in this way. Part of it has resulted from evaporation of irrigation water, which is customarily spread so thinly over the fields that none is left for leaching. At present, in the irrigated regions, 11 feet of water is used, on average, each year for irrigation, whereas twice this amount is needed to allow sufficient downward percolation to prevent salt accumulation.

The picture in the lower plain is similar but more complicated and less well understood. The fact that the underground water is salty greatly aggravates the problem.

The answer, at least for the major part of the Punjab, is by "vertical drainage " which will increase evaporation and at the same time allow some of the water to percolate downward from the surface, carrying dissolved salts back under ground. A network of deep tube wells can be sunk into the underlying water, which is then pumped to the surface for irrigation purposes, using electricity generated from Pakistan's abundant natural gas as well as from hydro-electric works. Some of the water seeps back underground, leaching away the salt in the process, but some of it evaporates. In this way, the level of the underground water is lowered. The 1 tube well solution was first proposed some fifteen years ago. But the results until very recently were discouraging and one of the principal problems before our panel was whether in fact the scheme would work at all. Our calculations showed that the difficulty came from using the method on too small a scale. In a region west of Lahore, a small tube well scheme covering 60,000 acres had no effect on the water level; a larger scheme covering 1.2 million acres has drawn down the level by about two feet - the first time that the water table in the Punjab has fallen since irrigation began.

(from New Scientist, 14th February, 1963)