Karnataka has a water potential of about 102 km3. The state accounts for about 6% of the country’s surface water resources. Around 60% of this is provided by the west flowing rivers while the remaining comes from the east flowing rivers. There are seven river basins in all formed by the Godavari, Krishna, Cauvery, the west flowing rivers, south Pennar and Palar.
Surface Water Rivers: Karnataka has seven river systems. Together with their tributaries they form a catchment of 191,773 km2. The state accounts for about 6% of the country's surface water resources. The Western Ghats are a major divide for river basins. Rivers flowing westward into the Arabian Sea carry 40% of the state’s surface water and those flowing eastward 60%. The availability of water from these river basins is estimated to be 7,663 thousand million cubic feet per annum (TMC/a) of which 45% (3,475 TMC/a (or) 98 billion m3/a) can be economically utilised. The state can utilise only 40% of the potential (1,690 TMC/a or 48 billion m3/a) because the west flowing rivers cannot be harnessed.
Rainfall: The state comprises of ten agro-climatic zones. Rainfall shows very high spatial and temporal variability. About 70% of the geographic area falls in arid and semi-arid zones. Annual rainfall varies between 500 mm in the northern region and around 4,000 mm in coastal regions with an average of 1,151 mm. About two thirds of the state receives less than 750 mm of rainfall. Agumbe in the Sahyadri hills is ranked as India’s second highest rainfall location receiving about 7,600 mm per year. Karnataka has on an average of 55 rainy days in a year during the monsoons. However, Monsoons have experienced some changes in recent years that are attributed to climate change and variations in the southern oscillation El Niño.
Lakes and Tanks: A portion of water demand is met from lakes and tanks. Karnataka has about 37,000 traditional tanks with a potential command area of 685,000 ha. About 60% of these are on the southern plateau, 25% in the Malnad region and 15% on the northern plateau. Most of them (50%) have a command area of 4 to 20 ha and 38% have less than 4 ha. Only 11.4% have a command area greater than 20 ha. Besides traditional tanks, there are about 20,000 irrigation tanks in the state with an irrigation potential of 6.5 lakh ha. Tanks alone serve nearly 80% of the minor irrigation potential. The state also has about 1,100 other minor surface irrigation structures that include barrages, bridges-cum-barrages, moles in river courses (anicuts) and pick-ups for lift irrigation. The relative contribution of tanks in the irrigation potential since 1951 has declined with the development of surface water irrigation and the shrinking of tanks owing to encroachment and siltation. Urban areas including Bangalore have a large number of lakes and tanks for impounding monsoon runoff to meet drinking water needs. The city alone has presently 201 water bodies, 96 of which are perennial.
Ground Water: Though groundwater is considered ubiquitous, it is not uniformly available. Weathered hard rocks account for 97% of the aquifers, the great majority being granite (90%) and the remainder Deccan trap basalts. Alluvial coastal flood plains account for only 2% of aquifers and other formations for just 1%. Tube and bore wells irrigate a net area of 12.5 lakh ha, contributing 37% to the state’s net irrigated area. The replenishable groundwater is 15.9 billion m3 and annual groundwater availability 15.3 billion m3. The groundwater draft stands at 10.7 billion m3 out of which 91% is for irrigation alone. The increasing dependence on groundwater has already led to a 70% of groundwater development against 58% in the country.
Water Balance: The Karnataka state receives 92% of water through rainfall and 8% from the upper riparian state Maharashtra, totalling to 236 billion/ m3 year. About 50% of is lost in evaporation from the soil and transpiration of plants. This is generally referred to as evapotranspiration. Input stands less than 18 billion/ m3 year over the aggregate output. This implies that with growing water uses, the state will face water deficits and is unlikely to meet future agricultural, industrial and domestic water requirements. However, a thought can be given to the possibilities of utilizing water of west flowing rivers; but, potential ecological costs must be considered. Construction of dams, irrigation projects, land use change, encroachment of river beds, deforestation, diversion of water from rivers, groundwater extraction, channel dredging have affected water balances of rivers and other water bodies in recent years.
Fig 1 Water resource at a glance
Source: SOER Report 2011