Availability of Water on Earth

The hydrological balance of different parts of the earth's surface provides the overall conditions for plant growth (Ward and Robinson

1990). Disregarding changes of water storage in the soil, the water balance consists of precipitation (P), evaporation (E) and river discharge (F), which is fed by surface runoff and seepage.

The global distribution of precipitation is determined by the position of the sun and the global circulation of air masses; the result is high precipitation in the tropics, a minimum of precipitation around the tropics and increased precipitation in the temperate latitudes, also determined by distance from the oceans (oceanity) and the size of the continents (continentality). The gulf stream also provides favourable conditions for the eastern part of the Americas, as well as for Europe. In the higher latitudes of the Arctic and Antarctic precipitation decreases again.

Surface evaporation (EG), where there is sufficient moisture, depends primarily on the available energy (IG) at the ground surface (Philip 1957).

where X is the latent heat of evaporation of water (2.454 J kg"1 at 20 °C) and s the change in amount of water vapour relative to the heat content in water vapour saturated air at air temperature (1.27 at 10 °C). With drying of the ground surface an additional boundary layer resistance must be considered (see Chap. 2.2.7). If the ground surface is covered by plants, free evaporation only occurs after precipitation, when the so-called intercepted water (the

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