Hydrological cycles

Fig. 7. The principal reservoirs in the hydrological cycle R, reservoirs in units of 10 metric tons (10 km ) E, fluxes in units of 10 km /yr T, residence time, yr. R/F = volume /input—output. Fluxes (flows) are approximate. For range of estimates, see Ref. 8.

As emphasized by Postel and co-workers (9), only freshwater flowing through the solar-powered hydrological cycle is renewable (Fig. 7). 

Water is omnipresent on the earth. Constant circulation of water from the ocean to the atmosphere (evaporation) and from the atmosphere to land and the oceans (precipitation, mnoff, etc) is generally known as the hydrologic cycle (see Fig. 1) (1 2). Within the hydrologic cycHc, there are several minor and local subcycles where water is used and returned to the environment. 

The balance between evaporation, precipitation, glaciers, and oceans, known as the hydrological cycle, is usually considered to be in rough equilibrium over the Earth, although there is evidence that the... 

Global warming would also be expected to influence surface waters such as lakes and streams, through changes induced in the hydrologic cycle. However, the last published report of the IPCC states no clear evidence of widespread change in annual streamflows and peak discharges of rivers in the world (IPCC, 1995, p. 158). Wliile lake and inland sea levels have fluctuated, the IPCC also points out that local effects make it difficult to use lake levels to monitor climate variations. 

The last published report of the IPCC acknowledges that the single largest uncertainty in determining the climate sensitivity to either natural or anthropogenic changes are clouds and their effects on radiation and their role in the hydrological cycle. .. At the present time, weaknesses in the parameterization of cloud formation and dissipation are probably the main impediment to improvements in the simulation of cloud effects on climate (IPCC, 1995, p. 346). 

This article begins with a description of how hydroelectricity works, from the beginning of the hydrological cycle to the point at which electricity is transmitted to homes and businesses. The histoiy of the dam is outlined and how dams evolved from structures used for providing a fresh water supply to irrigation and finally to providing electricity. The histoiy of hydropower is considered and the different hydroelectric. systems (i.e., conventional, run-of-... 

Land/atmospheric interfacial processes which impact climate and biological activity on earth are illustrated in Figure 3. Emissions of carbon dioxide, methane, nitrogen dioxide, and chlorofluorocarbons (CFCs) have been linked to the transmission of solar radiation to the surface of the earth as well as to the transmission of terrestrial radiation to space. Should solar radiation be an internal process or an external driver of the hydrologic cycle, weather, and air surface temperatures Compounds of sulfur and nitrogen are associated with acidic precipitation and damage to vegetation, aquatic life, and physical structures. 

The major reservoirs of water on Earth are the oceans. The hydrologic cycle is driven primarily by evaporation of water from the oceans, lakes. 

While the hydrosphere has long been appreciated as essential to life on Earth, only in the past couple of decades have scientists expanded their exploration of the global hydrologic cycle and its roles across the spectrum of Earth science... 

Figure 6-3 shows the hydrologic cycle as seven primary reservoirs interconnected by a number of water fluxes. The role of each reservoir in the hydrologic cycle and its connections with other cycles is briefly summarized below, in order of storage volume. 

The oceans are by far the largest reservoir in the hydrologic cycle, containing more than 25 times as much water as the rest of the reservoirs combined. As another means of comparison, the volume of water in the oceans is four orders of magnitude larger than that in the next most visible reservoir, the world s lakes and rivers. The oceans are also one of the Earth s primary... 

Robert Horton, an influential pioneer in the field of hydrology, developed one of the first comprehensive representations of the hydrologic cycle in 1931. His original diagram. Fig. 6-4, illustrates the processes by which water moves between the Earth s hydrologic reservoirs. Hydrologic fluxes can be summed up in four... 

Fig. 6-4 The fluxes of the hydrologic cycle, developed by Robert Horton (1931).

Fig. 6-5 Evaporation and transpiration from vegetation are among the complex land surface interactions in the hydrologic cycle. (From Dickinson, 1984.)...

The ability to predict runoff and water availability is critical to water resources planners. However, the complex non-linearities of the hydrologic cycle make this an extremely difficult process. Even where precipitation is fairly well known, runoff prediction is a non-trivial problem, as land surface response depends as much (or more) on precipitation patterns and timing as on precipitation amount. The historical record of monthly rainfall and inflow at the Serpentine Dam, near Perth, Western Australia, provides an illustration of this sensitivity (Fig. 6-11a and b). 

Five components of the hydrosphere play major roles in climate feedbacks - atmospheric moisture, clouds, snow and ice, land surface, and oceans. Changes to the hydrologic cycle, among other things, as a result of altered climate conditions are then referred to as responses. Interactions with climate can best be explored by examirung potential response to a climate perturbation, in this case, predicted global warming. 

---