World oceans cover about three fourth of earth’s surface. According to the UN estimates, the total amount of water on earth is about 1400 million cubic kilometre which is enough to cover the earth with a layer of 3000 metres depth. However the fresh water constitutes a very small proportion of this enormous quantity. About 2.7 per cent of the total water available on the earth is fresh water of which about 75.2 per cent lies frozen in polar regions and another 22.6 per cent is present as ground water. The rest is available in lakes, rivers, atmosphere, moisture, soil and vegetation. What is effectively available for consumption and other uses is a small proportion of the quantity available in rivers, lakes and ground water. The crisis about water resources development and management thus arises because most of the water is not available for use and secondly it is characterized by its highly uneven spatial distribution. Accordingly, the importance of water has been recognised and greater emphasis is being laid on its economic use and better management.
Water on the earth is in motion through the hydrological cycle. The utilisation of water for most of the users i.e. human, animal or plant involve movement of water. The dynamic and renewable nature of the water resources and the recurrent need for its utilisation requires that water resources are measured in terms of its flow rates. Thus water resources have two facets. The dynamic resource, measured as flow is more relevant for most of developmental needs. The static or fixed nature of the reserve, involving the quantity of water, the length of area of the water bodies is also relevant for some activities like pisciculture, navigation etc. Both these aspects are discussed below.
Analysing the country-wise geographical area, arable land and irrigated area in the World, it is found that among the continents largest geographical area lies in the Africa which is about 23 per cent of the world geographic area. However, Asia (excluding erstwhile countries of USSR) with only 21 per cent of world geographical area has about 32 per cent of world’s arable land followed by North Central America having about 20 per cent of World’s arable land. Africa has only 12 per cent of world’s arable land. It has been seen that irrigated area in the World as about 18.5 per cent of the arable land in 1994. In 1989, 63 per cent of world’s irrigated area was in Asia, whereas in 1994 this percentage has gone upto 64 per cent. Also 37 per cent of arable land of Asia was irrigated in 1994. Among Asian countries, India has the largest arable land, which is close to 39 per cent of Asia’s arable land. Only United States of America has more arable land than India.