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India has an annual average precipitation of 1 170 mm and about 80 percent of the total area of the country experiences annual rainfall of 750 mm or more (Table 2). Owing to the large spatial and temporal variability in the rainfall, water resources distribution is highly skewed in space and time.
The two main sources of water in India are rainfall and glacial snowmelt in the Himalayas. Although snow and glaciers are poor producers of freshwater, they are good distributors as they yield at the time of need, in the hot season. Indeed, about 80 percent of the river flow occurs during the four to five months of the southwest monsoon season. Several important river systems originate in upstream countries and then flow to other countries: the Indus river originates in China and flows to Pakistan; the Ganges-Brahmaputra river system originates partly in China, Nepal and Bhutan, and flows to Bangladesh; some minor rivers drain into Myanmar and Bangladesh. However, no official records are available regarding the annual flows into or our of the country.
The rivers of India can be classified into four groups:
- The Himalayan rivers (Ganges, Brahmaputra, Indus) are formed by melting snow and glaciers as well as rainfall and, therefore, have a continuous flow throughout the year. As these regions receive very heavy rainfall during the monsoon period, the rivers swell and cause frequent floods.
- The rivers of the Deccan plateau (with larger rivers such as Mahanadi, Godavari, Krishna, Pennar and Cauvery draining into the bay of Bengal in the east, and Narmadi and Tapi draining into the Arabian sea in the west), making up most of the southern-central part of the country, are rainfed and fluctuate in volume, many of them being non-perennial.
- The coastal rivers, especially on the west coast, south of the Tapi, are short with limited catchment areas, most of them being non-perennial.
- The rivers of the inland drainage basin in western Rajasthan in the northwestern part of the country, towards the border with Pakistan, are ephemeral and drain towards the salt lakes such as the Sambhar, or are lost in the sands.
For planning purposes, the country is divided into 20 river units, 14 of which are major river basins, while the remaining 99 river basins have been grouped into six river units, as presented in Table 3. The spatial imbalance of water resources distribution can be appreciated by the fact that the Ganges-Brahmaputra-Meghna basin, which covers 34 percent of the country’s area, contributes about 59 percent of the water resources. The west flowing rivers towards the Indus cover 10 percent of the area and contribute 4 percent of the water resources. The remaining 56 percent of the area contributes 37 percent to the runoff.
The potential surface water resources is assessed as the natural runoff of the rivers. Looking at the Indus Water Treaty (1960) between India and Pakistan, however, these are an estimated 1 869.37 km3, of which only 690.31 km3 are considered usable or exploitable because of constraints related to topography, uneven distribution of the resource over space and time, geological factors and contemporary technological knowledge (Table 3).
Annual renewable groundwater resources are an estimated 432 km3, of which around 90 percent or 390 km3 are considered overlap between surface water and groundwater. Annual internal renewable surface water resources (IRSWR) have been estimated as 1 446.42 km3, of which 1 404.42 km3 surface water, 432 km3 groundwater and 390 km3 overlap. The IRSWR have been estimated by deducting the inflow from the total renewable surface water resources.
Under the Indus Water Treaty (1960) between India and Pakistan, it was estimated that 73.31 km3/year is available for India (Table 3). The following rules apply:
- Eastern rivers: All the waters of the eastern tributaries of the Indus river originating in India, i.e. the Sutlej, Beas and Ravi rivers taken together, shall be available for unrestricted use by India. Pakistan shall be under an obligation to let flow, and shall not permit any interference with, the waters (while flowing in Pakistan) of any tributary which, in its natural course, joins the Sutlej Main or Ravi Main before these rivers have finally crossed into Pakistan. This average annual flow in India before crossing the border is an estimated 11.1 km3. All the waters, while flowing in Pakistan, of any tributary which in its natural course joins the Sutlej main or the Ravi main, after these rivers have crossed into Pakistan, shall be available for the unrestricted use of Pakistan.
- Western rivers: Pakistan shall receive for unrestricted use all those waters of the western rivers, i.e. Chenab and Jhelum, which India is under obligation to let flow, except for restricted uses, related to domestic use, non-consumptive use, agricultural use and generation of hydroelectric power of which the amounts are set out in the Treaty. Annual flow from China to India in the Indus basin is 181.62 km3 and it is estimated that the flow generated within India is 50.86 km3, resulting in a flow from India to Pakistan in this part of 232.48 km3, of which 170.27 km3 reserved for Pakistan and 62.21 km3 available for India.
Besides this outflow to Pakistan from the Indus basin, 1 121.62 km3 flows annually to Bangladesh (525.02 km3 from the Ganges, 537.24 km3 from the Brahmaputra, 48.36 km3 from the Meghna and 11 km3 from other rivers into southeast Bangladesh), and 20 km3 flows to Myanmar.
In 1996, produced wastewater was an estimated 25.4 km3. In 2004, wastewater production in urban centres (rural areas with larger population have not been accounted) was an estimated 10.585 km3 and the treated wastewater was about 2.555 km3.
No reliable statistics are available on the number or the status of desalination plants, or on their capacities or technologies adopted. Estimates indicate, however, that there are more than 1 000 membrane-based desalination plants of various capacities ranging from 20 m3/day to 10 000 m3/day. There are few thermal-based desalination plants. In 1996, some 550 000 m3 of seawater were desalinated in the Lakshadweep Islands, mainly using electro dialysis and reverse osmosis (RO). Solar stills are also installed on the peninsula, as in Gujarat in the northwest. A 6 300 m3/day desalination plant is being set up at Kalpakkam, Chennai with a capacity of 4 500 m3 from multi stage flash (MSF) method, using low pressure steam from the Madras Atomic Power Station and 1 800 m3/day from RO method. While the Plant using the RO method is under operation, the MSF-based plant is to be commissioned soon.
The total constructed water storage capacity, up to 2005, was 224 km3. Another 76.26 km3 are estimated to be possible from dams under construction and 107.54 from dams under consideration. Seven dams have a reservoir capacity that exceeds 8 km3. They are the Nagarjuna Sagar dam on the Krishna river (11.56 km3), the Rihand dam on the Rihand river (10.6 km3), the Bhakra dam on the Sutlej river (9.62 km3), the Srisailam dam on the Krishna river (8.72 km3), the Hirakud dam on the Mahanadi river (8.1 km3), the Pong (Beas) dam on the Beas river (8.57 km3) and the Ukai dam on the Tapti river (8.5 km3).
India controls the flow of the River Ganges using a dam completed in 1974 at Farraka, 18 km from the border with Bangladesh. The Farakka barrage is a low diversion structure and not classified as a large dam.
India is endowed with rich hydropower potential, ranking fifth in the world. The gross hydropower potential was an estimated 148 700 MW as installed capacity, or 84 000 MW at 60 percent power load factor, of which the Brahamaputra, Ganges and Indus basins contribute about 80 percent. Further, small, mini and micro hydropower schemes (with a capacity of less than 3 MW) have been assessed as having almost 6 782 MW of installed capacity.