Availability of Water on Earth

The hydrological balance provides the overall conditions for plant growth on the Earth (Ward and Robinson 2000). If changes of water storage in the soil are disregarded, the hydrological balance consists of precipitation (P), evapotranspiration (E) and river discharge (F: river flow) (Eq. 10.1), which is fed by surface run-off and seepage (groundwater recharge).

The global distribution of these variables, as shown in Fig. 10.3, indicates tropical and temperate regions with a high surplus of rainwater (P > E + F) and arid regions with a rainwater deficit (P < E + F).

Fig. 10.3. Global distribution of components of the hydrological balance of the Earth. Global distribution of a precipitation (based on Schneider et al. (2014)), b actual evaporation (Miralles et al. 2011) and c groundwater recharge, equalling river flow (Doll 2009). d Climatic classification scheme, after Koppen and Geiger. (Kottek et al. 2006)

Precipitation is the main input into the natural water cycle (neglecting the use of fossil groundwater for irrigation (Fig. 10.3a)). It is determined by the position of the sun, the global circulation of air masses and the recirculation of local evapotranspiration. All of these processes result in high precipitation over the tropics, a minimum of precipitation in the subtropics and increased precipitation at temperate latitudes. Precipitation over the continents is also determined by the distance from the oceans (oceanity) and the size of the continents (continentality). The Gulf Stream, with its northern extension towards Europe (the North Atlantic Drift), also provides exceptionally favourable conditions of temperature and rainfall for the eastern part of the Americas, as well as for Europe. Precipitation decreases in the Arctic and Antarctic.

Surface evaporation (E) includes evaporation from surfaces and transpiration of vegetation. If the ground surface is covered by plants, free evaporation occurs only after precipitation, when the intercepted water (the amount of precipitation captured by the canopy and not reaching the ground) evaporates. In spring, before plant cover is achieved, arable fields lose water similarly to a wet surface until the topsoil layer dries off. The rate of evaporation decreases as the crop grows (Greenwood et al. 1992). Transpiration describes the amount of water lost from the plant by evaporation, and it is thus subject to physiological control, in addition to energy-driven evaporation. In addition, evaporation from the ground occurs in any stand, depending on the leaf area index (LAI) (Schulze et al. 1995).

The sum of free evaporation and transpiration is called evapotranspiration (Fig. 10.3b). One distinguishes between potential evaporation, which is a function of the meteorological conditions, and actual evapotranspiration, which is additionally regulated by the plant cover. The global distribution of actual evapotranspiration shows a maximum in the tropical regions and roughly follows the distribution of precipitation. However, evapotranspiration is additionally influenced by the available solar radiation and mean wind speed. Thus, evapotranspiration decreases with decreasing available solar energy in the higher latitudes (north and south).

Fig. 10.4. Relations between river discharge, evapotranspiration and precipitation, based on river catchments. Evapotranspiration reaches a maximum that depends on the net radiation and surface conductance. Thus, the proportion of water that runs off or penetrates into the soil increases with increasing rainfall. (After Schulze and Heimann (1998))

Surface run-off, water storage in the soil profile and river discharge of water close the hydrological balance (Fig. 10.3c). Evapotranspiration does not increase with precipitation without limits. Fig. 10.4 shows a saturation curve, with river discharge making up a larger proportion of the total precipitation as precipitation increases. Even at low precipitation, vegetation does not consume the total amount of precipitation. In arid climates, precipitation occurs as heavy rainstorms, with sometimes massive surface run-off. This water is thus not available for plants at the site of rainfall but may be stored at greater soil depth (e.g. in the Kalahari sands), serve plant growth downhill (e.g. in dry valleys) or reach the ocean via rivers.