4.2 THE HYDROLOGICAL CYCLE

Surface and groundwater / Evaporation and transpiration / The hydrological regime

The Origins of Water--
Water falls on the land in the form of rain, hail and snow. Rain and hail fall at lower altitudes and snow is confined to higher, mountainous regions. Water falling as rain and hail immediately starts to flow to the sea, although the speed at which this occurs is influenced by many things. Water falling as snow may be stored on the mountains for many years, but a certain amount melts and starts to flow to the sea through the headwater streams every hot season. Water also sinks into the earth where it is stored and released into rivers from springs over the whole year. Most tropical rivers are fed mainly by rainfall.

Surface and Groundwater--
On land, water is present in three forms - as snow and ice in high mountains, as surface water in rivers, lakes and marshes and underground as groundwater.

Surface Water
The water in rivers flows downhill to the sea or to a lake. In places dams may interrupt this flow artificially, but the water continues to flow through. Water in natural lakes is normally stored for much longer periods, although even here water flows through where there is an out-flowing river. The amount of time water stays in a reservoir or lake is known as the residence time; the time it takes to change all the water in a lake or reservoir is known as the replacement rate. Residence times and replacement rates can range from years in large, natural lakes to weeks in large reservoirs and days in run-of-the-river dams.

Ground Water
Much of the water falling on the land filters through the surface soil and rock to form ground water. The amount of watre stored as ground water is third to the sea and to glaciers and icecaps. Most of this water lies up to 100 metres below the surface and is stored in aquifers. These are areas where porous rocks are underlain by waterproof rocks. They form a valuable resource that is reached by wells, and is used widely for drinking water and for agriculture. In some areas the porous layer comes to the surface and the stored water emerges as a spring. Water closer to the surface forms the water table upon which plants depend. Lakes, swamps and marshes occur where the water table is higher than the surrounding land, although they may also occur at other places in the rivers basin where channels are obstructed, or where there are impermeable layers in the bedrock. Water is withdrawn from the ground water by evaporation, filtration and human use; the water table lowers accordingly. Swamps and marshes may dry out when this occurs. This water is replenished during the floods.

Of the total amount of water in the world, the proportion stored in various areas is as follow:

• Oceans: 97%
• Icecaps and glaciers: 2%
• Ground and soil: 0.5%
• Stream, rivers and lakes: 0.02%
• Atmosphere: 0.0001%

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