Glossary:

Precipitation: is the act of moisture falling from the clouds in any way or form. The precipitation could be in rain, snow, sleet, or hail.

Transpiration: During photosynthesis, plants produce water vapor in their internal chemical reactions. The water they use is produced as a gaseous state so it does not need to evaporate; the water goes directly into the atmosphere.

Percolation: is the process of water continuing to move through the soil at lower levels. This is very similar to infiltration.

The hydrological cycle

Every day, over one thousand cubic kilometres of water evaporates from the land and sea. The water vapour condenses into the tiny droplets of liquid that make up clouds, and eventually it falls back to the earth as rain, hail or snow. Every three millennia, this overturning of water from ground to sky and back again recycles an amount of water equivalent to all the world's oceans.

It is called the hydrological cycle, and all life on the planet depends on it. The hydrological cycle creates fresh water from the saltwater of the oceans, because the salt is left behind when seawater evaporates . Much of the water vapour simply falls as rain (precipitation) back into the oceans. But some falls over the land surface, where it gathers into streams and rivers, making its gradual journey back to the sea.

Some water takes other routes through the hydrological cycle. If it falls as snow (precipitation) over the polar ice caps, it can become locked away in the ice sheets for hundreds of thousands of years. And some rainwater percolates through porous soil and rock until it meets an impermeable rock layer. Then it seeps down subterranean slopes as ground water, carried in aquifers that break through to the surface as springs or which can be tapped through wells.

Ground water can be pumped back into the air before it can find its way back to the sea. Plants suck it up through their roots and release it as vapour from their leaves -- a process called transpiration . A typical birch tree transpires about eighty gallons of water each day. It is because of this plant-induced humidity that tropical rain forests swelter under a moist climate.

So the hydrological cycle is really a set of wheels within wheels, each turning at a different pace - each ferrying water between land, sea and sky, between salt and fresh.

Quantitative indices of the different components of the global hydrological cycle are shown in the diagram. Every year the turnover of water on Earth involves 577,000 km 3 of water. This is water that evaporates from the oceanic surface (502,800 km 3 ) and from land (74,200 km 3 ). The same amount of water falls as atmospheric precipitation, 458,000 km 3 on the ocean and 119,000 km 3 on land. The difference between precipitation and evaporation from the land surface (119,000 - 74,200 = 44,800 km 3 /year) represents the total runoff of the Earth's rivers (42,700 km 3 /year) and direct groundwater runoff to the ocean (2100 km 3 /year). (source: Unesco).

To assess the total water storage on the Earth reliably is a complicated problem because of the mentioned dynamic nature of water. It is in permanent motion, constantly changing from liquid to solid or gaseous phase, and back again. What is usually used, as an estimate, is the total amount of water contained within the hydrosphere. This is all the free water existing in liquid, solid or gaseous state in the atmosphere, on the Earth's surface and in the crust down to a depth of 2000 metres. Current estimates are that the Earth's hydrosphere contains a huge amount of water - about 1386 million cubic kilometres. However , 97.5 % of this amount is saline water and only 2.5% is fresh water. The greater portion of this fresh water (68.7%) is in the form of ice and permanent snow cover in the Antarctic, the Arctic, and in the mountainous regions. Next, 29.9% exists as fresh groundwater . So, by using such estimates we come to the realization that only 1.4 % of the total amounts of fresh waters on the Earth are concentrated in lakes, reservoirs and river systems where they are most easily accessible for our economic needs and absolutely vital for water ecosystems.

Difference in the hydrological cycles between temperate and tropical/arid regions

The hydrological cycle has been fittingly described as the bloodstream of the biosphere. It acts as a lifeline to society by providing numerous freshwater services, and performs vital biological functions in ecosystems that provide society with ecological goods and services. The hydrological cycle can be divided into five main processes: condensation, precipitation, infiltration, runoff and evapotranspiration. By analyzing these five processes we can see the major difference in the hydro climate between temperate and tropical regions.

Tropical regions, as mentioned earlier, can be bluntly divided into humid tropical regions , semi-arid and arid regions. In temperate ecosystems, such as Sweden, the hydrological cycle is characterized by an annual precipitation that lies around 500mm/yr. When looking at semi-arid regions, one would expect that this figure would be much lower. However, this is not the case. Semi-arid regions receive an annual amount of rainfall that is equal to that of temperate ecosystems - somewhere around 500mm/yr. Thus the main difference between these two areas (concerning precipitation) lies not in the amount of absolute precipitation, but rather in the extreme variability of rainfall distribution in space and time that is observed in semi-arid ecosystems. The deviation that can exist from year to year may be so high, so that any attempt at referring to "normal annual rainfall" in such areas can be misleading. Another clear difference is that when the rainfall comes, it usually takes the form of sudden wet seasons that are extremely intensive. In contrast to this, humid tropical regions (such as tropical rainforest areas) have a much higher annual precipitation level than temperate areas, reaching levels of up to 3500mm/yr. Here the rainfall distribution is very predictable with almost no annual variability. In arid regions such as steppes and deserts, rainfall is minimal and there is generally no water surplus to speak of.

The evapotranspiration rates in tropical regions differ to those in temperate regions. Both in semi-arid and humid tropical regions the evapotranspiration demand is much larger than in the temperate areas. The average evapotranspiration demand in Africa is about 1500mm/yr (about 2000 in humid tropic areas and between 1500-2000 in semi-arid regions). This is due to the fact that in arid and semi-arid areas the atmosphere has a higher "thirst" for water due to drier air and higher temperatures.

Another difference in the hydro climate concerns the processes of infiltration and blue water runoff. In semi-arid regions, when the hard and sudden rains appear, much of the rain is lost as run-off. The soils in these areas have bad physical characteristics and do not allow infiltration of the rain in the soil. The high evapotranspiration rates also lead indirectly to a lowered infiltration. The poor management of the land (clearing of forests, destructive agricultural practice) has led to hard soil that again leads to bad infiltration and a high run-off.