As discussed in section 2.1.2, the water -or hydrological- cycle plays an important role in the energy cycle on Earth. It has a considerable impact on the radiative balance: water vapour is the most important greenhouse gas in the atmosphere (see section 2.1.2); the presence of snow and ice strongly modifies the albedo of the surface (see Table 1.3 and sections 2.1.4 and 4.2.3); and clouds influence both the longwave and shortwave fluxes (see sections 2.1.4, 2.1.6 and 4.2.2). Moreover, water is an essential vehicle for energy: the latent heat released during the condensation of water is a dominant heating source for the atmosphere (see section 2.1.6); the transport of water vapour in the atmosphere and of water at different temperatures in the ocean are essential terms in the horizontal heat transport (see section 2.1.5.2).
The hydrological cycle is also essential in shaping the Earth's environment, the availability of water being a critical factor for life as well as for many chemical reactions and transformations affecting the physical environment. Describing the various components of the hydrological cycle and analysing the mechanisms responsible for the exchanges of water between the different reservoirs are thus important elements of climatology.
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By far, the largest reservoir of water on Earth is located in the crust, with estimates of the order of 1022 kg of water (equivalent to 1019 m3 at surface pressure, i.e. about 10 times the amount of water in the oceans, the second largest reservoir). However, exchanges between deep Earth and other reservoirs are so slow that they only have a very weak impact on the hydrological cycle at the surface and are thus generally not taken into account in estimates of the global hydrological cycle (Fig. 2.21). Groundwater and soil moisture, which store a significant amount of water, interact much more quickly with the ocean and the atmosphere.
A large amount of water is also stored in form of ice, mainly on the Greenland and Antarctic ice sheets (see section 1.4.1). By contrast, the stock of water in the atmosphere is very low. If the 12.7 103 km3 of atmospheric water estimated in Fig. 2.21 all precipitated, it would correspond to about 2.5 cm of rainfall (=12.7 103km 3/(4 π R2)) over the whole Earth. As the actual precipitation on the Earth’s surface is of the order of 1 m per year (see section 1.2.3), the water in the atmosphere must be being replaced very quickly. This is achieved by evaporation over the ocean and other water bodies as well as by evaporation and transpiration over land. Most of the water that evaporates over the ocean falls back over the ocean (and similarly the water that evaporates over land, falls back over land), but there is also water transfer by the atmosphere from the oceanic area to the land area. This net transfer corresponds to roughly 35% of the total precipitation over land, and is compensated by a surface flow of water (mainly in rivers) from the land to the sea.
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According to the Clausius-Clapeyron equation (see section 2.1.6), intense evaporation occurs in the warm equatorial areas and in the tropics. In equatorial areas, because of the convergence at the surface and upward motions (see section 1.2.2), the moist air at low levels rises, reaching colder level. This induces condensation, the formation of clouds and high precipitation rates (see Fig. 1.7). Despite the high temperature and high evaporation rate, equatorial regions thus have more precipitation than evaporation at the surface (negative E - P, Fig. 2.22). In the subtropics, evaporation minus precipitation (E - P) is clearly positive, because of the general subsidence at these latitudes. At mid to high latitudes, E - P is again negative on zonal average because of the net moisture transport from tropical areas.
The imbalance of the water budget over land is generally small and is compensated by river runoff Rriv (in the long term mean, Rriv nearly equals P - E) which returns water to the sea. Because of the land topography, this river runoff is an important element of the water balance for some ocean basins. For instance, the Arctic Ocean receives about 10% of the total river runoff (mainly from the Russian rivers) although it only constitutes about 3% of the World Ocean. This partly explains why surface water in the Arctic is relatively fresh (see Fig. 1.11).
Over the oceans, the imbalance of the water budget at the surface is larger than over land because it can be compensated for by a net oceanic water transport that is much more efficient (and much larger) than that associated with river runoff. This net oceanic transport counter-balances the large atmospheric moisture transport out of the subtropics towards the equatorial regions and mid and high latitudes. In a way, the net meridional water transport and the associated energy transport in the atmosphere are only possible because the ocean transport is able to compensate for the imbalance at the surface due to the E - P fluxes.
Net oceanic water transport can also counterbalance the zonal transport by the atmosphere. In particular, because of high E - P rates, the Atlantic Ocean is a net evaporative basin and is thus more saline at the surface than the Pacific where the E - P balance integrated over the whole basin is negative (see Fig. 1.11). As a consequence, the global oceanic circulation must thus induce a net water transport from the Pacific to the Atlantic to achieve a water balance in both basins.