Hydrological Cycle


Nature Gallery (Earth - Mountains) Hydrological Cycle
Whenever it rains, the water is part of the way along its circular journey from the ocean to the land and back to the ocean. This intricate cycle, which encompasses the whole globe, is the most important environmental system on Earth. Without it, all of our continents would become dry, lifeless deserts. The hydrological cycle is what is known as a closed system because there are no external inputs or outputs of water to or from the system. The same water has been continually circulating since the beginnings of life on Earth. There are also huge stores of water which do not contribute to the system today, although they may have done so in the past. The largest of these stores is present in the form of thousands of cubic kilometres of ice at the North Pole and South Pole. If global warming were to melt the ice, it would release enough water to cause an average rise in the sea level of about 5 to 6 metres (16 to 20 feet). However, in the last few million years these ice caps have not been part of the cycle. The other main long-term storage mechanisms are aquifers. These are permeable rock strata lying above impermeable rocks. Just as oil is found in such sedimentary rocks, so too are large amounts of water. In many desert regions of the world, aquifers provide the only source of water that people can rely on to drink.

Sometimes aquifers can be found only a few metres from the ground surface, while in other places it is necessary to bore a well hundreds of metres down in order to reach water. Some aquifers recharge over a long period of time as water slowly seeps down through the rock. Others are known as fossil aquifers, because the water is no longer being recharged. Ice caps and aquifers can be seen as long-term water storage mechanisms, while lakes, seas, and oceans provide short-term storage. The oceans, which cover more than 70% of the Earth's surface, control the hydrological cycle and indeed the whole ecosystem of the planet.

The Cycle
The Sun's heat drives the hydrological cycle. As wind blows across the surface of the ocean, this heat causes water to evaporate from the sea and become absorbed into the air as water vapour—the amount absorbed depends on the temperature of the water. If the ocean is dominated by a cold current, such as the Benguela Current off southwest Africa, or the Humboldt Current which flows up the western coast of South America, very little evaporation will take place. This is why the continental regions next to these cold currents are dominated by deserts. The Benguela Current, for example, is responsible for the formation of the Namib Desert. Conversely, if the ocean is dominated by a warm current, such as in the Gulf of Guinea off the coast of West Africa, the air becomes highly saturated with water. As a result, countries like Cameroon receive more than 5,000 millimetres (200 inches) of rain per year.

The water vapour in the atmosphere forms clouds. These clouds are blown across the land, and forced upwards by hills or mountains, or by two air streams meeting and pushing them upwards. Cooler temperatures higher up cause the water vapour to condense and fall to the Earth as rain, snow, sleet, hail, or frost. This is called precipitation. Some of the falling water is intercepted directly by trees and plants. Much of it infiltrates into the soil and is drawn up through the roots of plants. This water helps the plants to grow, and any excess is drawn out of the leaves and back into the atmosphere by a process known as evapotranspiration. Rainwater that is not used by plants runs through or over the surface of the soil until it reaches a river. Water that runs over the surface is called surface runoff. Throughflow is water that moves through the soil, and water that infiltrates further to the water-table is called groundwater flow. Eventually the water in the river flows back into the sea and the cycle begins again. The Human Effect on the Hydrological Cycle
Human beings play an important role in the hydrological cycle because they need large amounts of water to survive. Large reservoirs are constructed so that water can be pumped to big cities. Reservoir water is also used to create hydroelectric power. Every day people use many litres of water, which is then treated to remove dangerous materials before being returned to the rivers or sea. However, humans also contaminate the hydrological cycle, causing acid rain and river pollution. Acid rain is created by power stations and other industrial factories pumping waste material into the atmosphere, where it mixes with the moisture in the clouds. Various chemical reactions take place in the atmosphere, so that when it rains the water is slightly acidic. Acid rain is responsible for killing large numbers of trees. Farmers treat their soil with fertilisers that contain chemicals. Although these chemicals help plants to grow, the chemicals themselves are dissolved by rainwater and end up in streams and rivers. Particularly harmful are nitrates, which accumulate in lakes where they cause the number of nutrients in the water to eutrophy. As a result, the surface of the water becomes covered in a thick layer of algae, known as algal bloom, which removes dissolved oxygen from the water. Deprived of oxygen, the fish eventually die. Farmers also spray their crops with pesticides and herbicides, which may pollute groundwater before it enters rivers, and this also harms the fish. Farming the land can cause soil erosion. Rain hitting the bare soil is not intercepted first by plants, and there are no plant roots in the soil to bind it together, so the water runs off the surface into the river, carrying with it a layer of soil. A clean supply of water is essential for healthy living. However, its uneven distribution throughout the world, combined with the careless and inefficient use of this valuable resource, means that while many people in developed countries have access to water 24 hours a day, approximately 1 billion, mostly in developing countries, struggle to find enough clean drinking water to survive. Around 80 countries currently face serious water shortages, and meanwhile world demand for fresh water grows yearly. Some countries are even in dispute over water resources. For example, Egypt is concerned that Sudan and Ethiopia will take possession of the Nile waters. For these reasons, and more, it is vitally important that water supplies are managed carefully for the future.