2.3 The Indus River

The Indus, one of the mightiest rivers of the world and the second longest in western Asia, has a mean annual discharge of 207.5 x 10⁹ m³ (ECAFE 1966). The Indus River system has ten times the volume of flow of the Colorado River in the United States and Mexico, and more than three times that of the Nile.

Alluvial plains in the middle to lower reaches of the Indus system have been developed to form the largest irrigation scheme in the world. Approximately 16,000 km of canals have been constructed to irrigate over 9 million ha (Buras 1967). This irrigation project covers the greater part of a vast plain covered with fine-textured alluvial soil overlying coarser sediments extending deep (ten to hundreds of metres) downward to the bedrock of an ancient valley.

The development of the irrigation project was mostly carried out after 1850, but elements of an ancient flood irrigation channel can still be found. Most of the canals were excavated through the surface soil to the more pervious underlying fine sand, so that a large proportion of the surface water diverted through these canals seeped underground. The sediment, which forms an extensive aquifer, was continuously recharged by the leaking irrigation canals, so that the groundwater table rose continuously. In many areas, this rise was estimated to be approximately 30 cm per year. As a result, in much of the Indus plain the groundwater level was near the soil surface. Proper aeration of the soil could not take place, and, with the prevailing arid climatic conditions, capillary action and evapotranspiration moved salts from the subsurface up to the root zone of crops and to the land surface. Thus once-fertile lands have so deteriorated that crops can no longer be grown. In the 1950s and 1960s, the salinity and waterlogging problems became so serious that intensive research was carried out by international agencies, headed by the Harvard University Water Resources Group, focusing on aquifer utilization and management, including the mining of groundwater.

During successive stages of development of the irrigation systems in the Indus valley, little attention was given to land drainage, and emphasis was put on maximizing the extent of irrigated land. Integrated management of the underlying aquifer system in line with the surfacesubsurface drainage is now being used successfully to control salinity and reclaim irrigated land.

2.3.1 The river basin

The Indus rises in Tibet, in the snow-clad Kailas range of the Himalayas, about 5,500 m above mean sea level. The catchment area extends over four countries-China, India, Pakistan, and Afghanistanwith the portion in Pakistan accounting for more than 50% of the total. The Indus basin lies in the subtropical zone; the Tropic of Cancer passes through its southernmost part, while its northern edge reaches the latitude 37°N (see fig. 2.16).

The Indus cuts through mountain ranges forming a narrow gorge and deep channel from the headwaters until an important tributary, the Kabul River, joins it from the west near Attock. A few kilometres above the town of Mithankot, the Indus is joined by its most important tributary, the Panjnad River, which carries the waters of five main tributaries-the Jhelum, the Chenab, the Ravi, the Beas, and the Sutlej. The river slope from the headwaters to Attock is approximately 1/ 300; from Attock to Mithankot it is 1/4,000; and from Mithankot to the sea it averages 1/7,000. The total length of the Indus is about 2,900 km. The drainage area of the whole system is approximately 970,000 km².

Fig. 2.16 The Indus River basin

The northern region of the basin, where the Indus and its main tributaries originate, is fully covered with rugged sky-high mountains and glaciers, comprising an area of about 452,000 km². Because of the steep and barren slopes of these mountains, the erosion is very heavy, ranging according to locality from about 400 to over 4,000 tons per km² annually, with an average of about 1,500 tons. Lower down, the basin comprises vast plains formed and separated from each other by the five main tributaries. Further down from Mithankot, the Indus valley is covered with alluvium built up by the deposition of silt carried down by the river. The delta begins from Kotri, about 185 km from the sea, where the land is almost level and the soil is generally infertile. Closer to the sea is marshy land and mangrove forests which are generally flooded during high tides.

2.3.2 Hydrology

As noted, the Indus basin lies in the subtropical zone. In the plains, the average temperature during winter is about 21°C in Karachi and about 15°C in Lahore. Summer, the hottest season, is from May to August, and average temperature ranges from about 29°C in May to about 34°C towards the end of June or early July, when the maximum temperature often rises above 27.7°C. Despite the mighty Indus, the Indus plain is semi-arid. There are significant extremes of rainfall in the basin. The area around Sukkur and Mithankot receives only about 100 mm of rain a year, while, at Murree, a hill station at 2,280 m elevation, the annual precipitation is about 1,270 mm. The precipitation, inclusive of snow, is many times heavier in the hilly region than in the plains. The mid-hill area, with elevations from about 1,200 m to 2,500 m, where the southwest monsoon generally strikes the mountain mass, has the heaviest rainfall, averaging about 1,250-1,500 mm a year. At higher and lower elevations, the rainfall decreases rapidly and the air is correspondingly drier and clearer. The rainfall further decreases rapidly from north to south in the plains, from about 550 mm of annual rainfall in the foothills to only about 100 mm at Mithankot and Sukkur. Below Sukkur, the rainfall increases a little owing to the maritime air, reaching about 200 mm along the sea coast. The mean annual rainfall over the Indus plains is less than 250 mm.

Due to the uneven distribution of precipitation over the basin, the Indus and its tributaries receive most of their flow from the mountains. The flows are subject to extreme variations; the maximum summer discharge is over 100 times the winter minimum. During July and August, all the rivers attain their peaks, discharging a considerable volume of water to the sea (see fig. 2.17).

Fig. 2.17 Average monthly flow of the Indus River and its two largest tributaries

The mean annual run-off of the Indus is 207 x 10⁹ m³, with a yearly maximum of 264 x 10⁹ m³ and minimum of 171 x 10⁹ m³. The Indus main stream carries about 110.3 x 10⁹ m³ per year, while the major tributaries Jhelum, Chenab, Beas, Sutlej, Ravi carry 27.85 x 10⁹ m³, 29 x 10⁹ m³, 15.65 x 10⁹ m³, 16.8 x 10⁹ m³, and 7.9 x 10⁹ m³ respectively. Thus the main stream alone carries a little more than half of the total discharge of the system. When combined with the Jhelum and Chenab, it carries a little more than fourthfifths of the overall total, while the Ravi, Beas, and Sutlej together deliver a little less than one fifth. The run-off coefficients are as high as 58%-82% (ECAFE 1966).

2.3.3 Water-resources development

As rainfall is scarce in the plains, where the cultivable areas lie, agriculture on the Indus plains has to depend almost exclusively on an irrigation system utilizing the river flows.

Many weir and canal systems were built on the Indus and its tributaries from the middle of the nineteenth century onwards, the first of which was the Upper Bari Doab canal, built between 1850 and 1859 to bring water from the Ravi River at Madhopur to the upper half of the doub, or inter-river land, in the vicinity of Lahore. By 1947, when India and Pakistan achieved independence as separate countries, the Indus water system had already been developed to provide irrigation to about 10.9 million ha, but it is remarkable that no storage reservoirs had yet been built in a system serving the world's largest irrigation area. Among the major hydraulic works built before 1947 were the following (see fig. 2.18):

Fig. 2.18 Indus River development

• on the Indus: Paharpur canal, Sukkur barrage (North-West, D. Unhar, Begari, Ghoti canals);
• on the Jhelum: Upper Jhelum canal (Rasul barrage);
• on the Chenab: Upper Chenab canal (Marala barrage), Lower Chenab canal (Khanki barrage):
• on the Jhelum and Chenab: Trimmu barrage, Rangpur canal, HaveliSidhnai canal, Panjnad barrage and canal, Abasia canal;
• on the Ravi: Madhopur barrage (Upper and Central Bari Doab canals), Balloki barrage (Lower Bari Doab canal);
• on the Sutlej and Beas: Ferozepore barrage (Eastern and Bikaner canals), Suleimanki barrage (Pakpattan, Fordwah, and E. Sadifia canals), Islam barrage (Mails), Qaimpur, and Bahawal canals);
• on the Swat: Upper and Lower Swat canals;
• on the Kabul: Kabul River canal.

Because the new international boundary cut across the common canal system of Punjab, leaving one part in India and the other in Pakistan, controversy on the use of the canal waters arose soon after the Partition. It took twelve years of patient negotiation before the controversy was settled by the Indus Waters Treaty in 1960. Between 1947 and 1960 intensive river development was carried out, including the following:

• on the Indus: Thal canals, Taunsa barrage (Dera Ghazi Khan and Muzaffargarh canals), Gudu barrage (Desert, D. Unhar, Begari, and Ghoti canals), Kotri barrage (Fuleli and Pinyari canals);
• on the Jhelum: Lower Jhelum canal (Rasul barrage);
• on the Ravi: Upper Bari Doab canal extensions;
• on the Beas: Shah Nehar and Hoshiarpur canals;
• on the Sutlej and Beas: Rajasthan canal;
• on the Kabul: Warsak dam;
• on the Kurram: Kurram Garhi weir.

The following major works were undertaken through the Indus Basin Development Fund (IBDF) and its successor, the Tarbela Development Fund:

• on the Jhelum: Mangla dam;
• on the Jhelum, Chenab, Ravi, and Sutlej: Trimmu-Sidhnai-MailsiBahawal link canal system, Rasul-Qadirabad-Balloki-Suleimanki link canal system;
• on the Indus and Jhelum: Chashma barrage, Chashma-Jhelum link canal;
• on the Indus and Chenab: Taunsa-Panjoad link canal;
• on the Indus: Tarbela dam.

It was originally intended that the IBDF would finance tube-well drainage works to compensate for leakage from the link canal systems, but at the choice of the Pakistan government the tube-well programme was financed by other means and the savings to the IBDF were put towards construction of the Tarbela dam.

2.3.4 Salinity and waterlogging problems

During successive stages of development of the irrigation systems, emphasis was put on maximizing the extent of irrigated land. In the Indus valley, as in all other flat valleys in the world, the natural surface and subsurface drainage is poor. Since there were not enough drainage channels, most of the rainwater and canal seepage percolated down to lower depths. As time passed, the groundwater table got higher and higher by steps, and finally, in the 1950s and 1960s it came close to the ground surface and has thus caused waterlogging in many large areas. Proper aeration of the soil could not take place and the capillary action and evapotranspiration moved salts from the subsurface up to the root zone of the crops and to the land surface. Thus, once-fertile lands so deteriorated that crops could no longer be grown.

In the former Punjab area in Pakistan, 5 million ha have already gone out of cultivation due to salinity caused by waterlogging, 690,000 ha are in an advanced stage of deterioration, and 2 million ha are affected to a lesser degree (fig. 2.16). Since 1954, extensive groundwater and salinity investigations have been undertaken. As a result of these investigations, recommendations were made to install a great number of deep wells to lower the groundwater level, and to use the pumped water for flushing the salts from the ground surface down to the drains provided as well as to the lower layer of soil. Moreover, the pumped water is used to supplement the existing canal supplies for irrigation.

In 1959 the Water and Power Development Authority, established in 1958 to take charge of all water and power development in Pakistan, launched its first reclamation project in Rechna Doab, in the districts of Gujarawala, Sheikhupura, and Lyallpur. The project comprised about 1,800 tube wells in a gross commanded area of 480,000 ha. The average volume of groundwater pumped was about 1.85 x 10⁹ m³ per year. In 1961 a second project was launched in Chai Doab, including 3,300 tube wells in a gross commanded area of 920,000 ha, with an average pumping volume of about 2.5 x 10⁹ m³ per year. The Authority has continued this salinity-control and reclamation work to remove the catastrophic threat to the well-being of the people of Pakistan.

The extent of salinity and a schematic diagram of salinity-control land reclamation in the lower Indus valley, using conjunctive aquifer management with surface and subsurface drainage measures, are shown in figs. 2.19A and 2.19B.

Fig. 2.19A Saline groundwater in the Indus valley (Source: Buras 1967)