The Indus is justifiably among the most famous watercourses of the world – the ‘Nile of the Indian Subcontinent’ that supports the world’s largest contiguous irrigated land in the provinces of the Pakistani Punjab & Sindh and India’s Punjab, Haryana and Rajasthan states. A very large part of these lands is arid or semiarid in nature. The Indus derives its name from the Sanskrit word ‘Sindhu’ which was modified to the Persian ‘Hindu’ and Greek ‘Indos’ from which both the words ‘Hindu’ and ‘India’ are derived from. The mainstream rises from the northern slopes of the Kailas range in Tibet at an altitude of about 16,000 ft above mean sea level. The river flows for about 3000 km before entering the Arabian Sea in Pakistan through a delta 30,000 sq km in area. The Indus along with its tributaries drains a vast area of 1,165,000 sq km, which is larger than the basins of both the other Himalayan Rivers the Ganga and the Brahmaputra. However, much of this area is arid plains or high desert plateau where precipitation is low. It is for this reason that the volume of water carried by the Indus is considerably less than that of either the Ganga or the Brahmaputra.

Three main headstreams form the main stem of the river in Tibet – the Sengge Chu, Gartang Chu and the Lang Chu. Among these, the Sengge Chu or Lion River is regarded as the main source of the Indus. More than 80% of Pakistan’s agricultural wealth and food production is derived from the river. Three dams and about 19 barrages corral the waters of the Indus and its tributaries in Pakistan alone and divert them into 56,000 km of major irrigation canals irrigating approximately 35 million acres of land. This huge dependence on the great rivers’ water is what makes the Indus such a valuable asset to NW India and Pakistan. The bulk of the Indus waters are derived from the river’s mountain regions so that the maximum flow of the river and its tributaries occur at the point just below the entry into the plains. Within the plains proper, the rivers actually lose some water due to evaporation and seepage and very few tributaries augment the flow in this stretch. Historically the flow of the Indus and the five rivers of Punjab have been measured at their entry into the plains. These are the well-known ‘rim stations’. For the purpose of analysis of the hydrography of the Indus, it can be thought to consist of the Indus main stem along with the six major tributaries. These are the Kabul River on the right bank and the five rivers of Punjab (Jhelum, Chenab, Ravi, Beas & Sutlej) that join and flow into the Indus as a single stream (the Panjnad) below Mithankot in Pakistan.

The annual average flow of the Indus for the purpose of the Indus Water Treaty was taken as 208 cubic km based on the rim station readings. Out of this, the Indus main stem (including the Kabul) contributes about one half of the flow (110 cubic km). The Panjnad contributes about 98 cubic km to the Indus. The Jhelum and Chenab make up the major flow of the Panjnad contributing about 28 and 29 cubic km annually. Because both the Indus and the Brahmaputra rise beyond the main Himalayan chain they have a very large portion of their courses as well as catchment in the mountains as compared with the Ganga which debouches into the plains only 300 km from its source. The mountain catchment of the Indus main stem alone is 2.5 lakh sq km out of which around 37,134 sq km is glaciated area. A large part of the catchment of the Upper Indus derives its waters from the Karakoram and Western Himalayas where monsoonal rain is light.

Western disturbances originating in the Mediterranean and Caspian Sea areas provide the snowfall in the Trans Himalayan areas during the post-monsoon winter months. This snowfall accumulates at the higher elevations (5000-6000 msl) to feed the massive Glaciers of the Karakoram. Due to the very high average elevations in the Karakoram, the valley glaciers here grow to enormous lengths and are among the largest non-polar areas of permanent snow and ice on Earth. Five glaciers – all part of the Indus basin – are over 50 km in length! These are from East to West the Siachen (72kms), Baltoro (58km), Biafo (63km), Hispar (61km) and the Batura (58km). It is estimated that the glaciers of the Karakoram cover an area of about 16300 km². The Indus above Tarbela dam probably receives as much as 80% of its flow from snow and ice melt as do other western tributaries, the Kabul and Swat though in the case of the Kabul snowmelt predominates (Snow and Ice Hydrology Project – SIHP). Most of the flow of the left bank tributaries, traversing Punjab, is derived from snowmelt during spring and summer with a major monsoonal rain contribution in late summer.

The tributaries of the upper Indus from the north (Shyok, Shigar and Gilgit-Hunza) drain the major glaciated areas of the Karakoram and a large volume of the flow is contributed by ice melt runoff. The glaciers though present in the basins of the southern tributaries of the upper Indus (Zanskar, Shingo, Astor) are much smaller and the contribution is skewed in favour of seasonal snowmelt. There is for example heavy winter snowfall in the Suru and Zanskar River valleys the latter’s contribution is greater than the Indus itself at their junction (at Nimmu). As we move towards the South and East in the Indus basin, the proportion of rainwater contribution to the overall flows increases. The frontal ranges of the Himalayas and foothills receiving the greatest proportion of the rainfall. Still, as per Hewitt (SIHP) about 65% of the inflow of the Jhelum into the Mangla reservoir is contributed by Ice and snowmelt. These figures would be less for the rivers further east like the Chenab, Ravi, Beas and Sutlej. Detailed studies of snow and ice melt contribution to the hydrology of the Himalayan Rivers is lacking. In fact, the hydrology data for the Himalayan rivers in India is classified data! Further complicating this is the fact that peak flows from snow and ice melt in late summer coincide with the peak monsoon period. Despite this, it is clear that for the overall Indus basin the contribution of seasonal snowmelt and ice melt forms the major portion (>50%) of the total runoff of the entire basin.

A look at the total hydrological picture of the Indus basin makes this quite apparent. The Indus main stem with the Kabul River contributes a little more than half of the total runoff (110 cubic km). The Jhelum and Chenab contribute more than a quarter of the entire flow. Thus, the three western rivers of the Indus system contribute about 80% of the total flow. (Incidentally, these three western rivers form Pakistan’s share as per the Indus Water Treaty). The Ravi, Beas and Sutlej together contribute just 20% of the total flow in the Indus basin and it is these rivers that show a proportionally greater contribution from monsoonal rainfall runoff. So, what is the significance of these figures? To understand this, it is to be known that all rivers receive their waters from the following three sources – rainfall-runoff, seasonal snow and ice melt (from glaciers) and groundwater. School children in the subcontinent study that Himalayan rivers usually carry a considerable volume of water all year round due to their rising in the Himalayas (thus being snow-fed) and that the peninsular rivers have large flows only in the monsoon months and dwindle in the summer months. However, what is not apparent is that even in the case of the Himalayan rivers like the Ganga and the Brahmaputra a major part of their flow is still derived from monsoonal rainfall. For example, the Ganga carries 80% of its annual flow in the four monsoon months from June to September. So herein lies the uniqueness of the Indus River – it may be the only river in the Indian subcontinent where a major quantity of its water (>50%) may actually be derived from seasonal snow and glacial ice melt as opposed to rainfall!

This fact brings to bear some important conclusions. Climate change implies that the proportion of precipitation falling as rain may increase as opposed to snowfall due to a rise in average temperatures. The Upper Indus basin sees extensive snow cover at the end of winter (up to 200,000 sq km with snow covers as high as 90% of catchment above Tarbela dam, though commonly more than 70% with even around 60% in a poor year). Due to climate change, a greater proportion of this may fall as rain and form part of instantaneous runoff. Winter snow that used to lie around and form part of runoff during spring and early summer may reduce. The Karakoram glaciers have generally bucked the global trend and some have actually increased in size whereas others have been constant (the so-called Karakoram anomaly). A decrease in snowfall and diminution of the Karakoram glaciers can have a major impact on the Indus runoff regime in the years and decades ahead with drastic effects on irrigation and water availability in Pakistan and India. Compared to the Ganga and the Brahmaputra basins, climate change may hence have a greater impact in the Indus basin. In addition, Himalayan snow cover is one of the most significant parameters for the feedback mechanism controlling the summer monsoon. The above factors call for greater care in the utilization of the precious water resources of the Indus basin.