The natural water cycle

Although water is the most widely occurring substance on earth, only 2.53 percent is freshwater while the remainder is salt water.

Some two thirds of this freshwater is locked up in glaciers and permanent snow cover. The available freshwater is distributed regionally as shown in figure 1. In addition to the accessible freshwater in lakes, rivers and aquifers, man-made storage in reservoirs adds a further 8,000 cubic kilometres (km3).

Water resources are renewable (except some groundwaters), with huge differences in availability in different parts of the world and wide variations in seasonal and annual precipitation in many places.

Precipitation is the main source of water for all human uses and for ecosystems. This precipitation is taken up by plants and soils, evaporates into the atmosphere via evapotranspiration, and runs off to the sea via rivers, and to lakes and wetlands.

The water of evapotranspiration supports forests, rainfed cultivated and grazing lands, and ecosystems. We withdraw 8 percent of the total annual renewable freshwater, and appropriate 26 percent of annual evapotranspiration and 54 percent of accessible runoff.

Humankind’s control of runoff is now global and we are significant players in the hydrological cycle.

Per capita use is increasing (with better lifestyles) and population is growing.

Water crisis

Thus the percentage of appropriated water is increasing. Together with spatial and temporal variations in available water, the consequence is that water for all our uses is becoming scarce and leading to a water crisis.

Freshwater resources are further reduced by pollution. Some 2 million tons of waste per day are disposed of within receiving waters, including industrial wastes and chemicals, human waste and agricultural wastes (fertilizers, pesticides and pesticide residues).

Although reliable data on the extent and severity of pollution is incomplete, one estimate of global wastewater production is about 1,500 km3. Assuming that 1 litre of wastewater pollutes 8 litres of freshwater, the present burden of pollution may be up to 12,000 km3 worldwide.

As ever, the poor are the worst affected, with 50 percent of the population of developing countries exposed to polluted water sources. The precise impact of climate change on water resources is uncertain.

Precipitation will probably increase from latitudes 30oN and 30oS, but many tropical and sub-tropical regions will probably get lower and more erratic rainfall.

Extreme weather

With a discernable trend towards more frequent extreme weather conditions, it is likely that floods, droughts, mudslides, typhoons and cyclones will increase.

Stream flows at low-flow periods may well decrease and water quality will undoubtedly worsen, because of increased pollution loads and concentrations and higher water temperatures.

Recent estimates We have made good progress in understanding the nature of water in its interaction with the biotic and a biotic environment. We have better estimates of climate change impacts on water resources.

Over the years, our understanding of hydrological processes has enabled us to harvest water resources for our needs, reducing the risk of extreme situations.

But pressures on the inland water system are increasing with population growth and economic development. Critical challenges lie ahead in coping with progressive water shortages and water pollution. By the middle of this century, at worst 7 billion people in sixty countries will be water-scarce, at best 2 billion people in forty-eight countries.

(Courtesy: UN World Water Development Report)