Water hydrologic cycle

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. 

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. 

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-... 

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

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... 

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). 

The following sections summarize only the most prominent interactions between the elemental cycles and the links in the hydrologic cycle. Water also plays a role in many chemical and biological reactions that are beyond the scope of this discussion. The carbon, nitrogen, sulfur, and phosphorus cycles are discussed in detail in Chapters 11, 12, 13, and 14, respectively. 

To this point, direct human impacts on the hydrosphere have remained restricted to the regional scale. Although they can still be important, particularly in terms of water supply, these direct manipulations of the hydrologic cycle are unlikely to affect the global water balance significantly. However, this is not to suggest that the global water cycle is immune to human influence its close ties to other physical and... 

They deliver water from the atmosphere to the Earth s surface as rain or snow, and are thus a key step in the hydrologic cycle. 

Only two possibilities exist for explaining the existence of cloud formation in the atmosphere. If there were no particles to act as cloud condensation nuclei (CCN), water would condense into clouds at relative humidities (RH) of around 300%. That is, air can remain supersaturated below 300% with water vapor for long periods of fime. If this were to occur, condensation would occur on surface objects and the hydrologic cycle would be very different from what is observed. Thus, a second possibility must be the case particles are present in the air and act as CCN at much lower RH. These particles must be small enough to have small settling velocity, stay in the air for long periods of time and be lofted to the top of the troposphere by ordinary updrafts of cm/s velocity. Two further possibilities exist - the particles can either be water soluble or insoluble. In order to understand why it is likely that CCN are soluble, we examine the consequences of the effect of curvature on the saturation water pressure of water. 

The overall rainfall rate and amoimt depend on these microphysical processes and even more greatly on the initial amount of water vapor present, and on the vertical motions that transport water upward, cool the air, and cause supersaturation to occur in the first place. Thus the delivery of water to the Earth s surface as one step in the hydrologic cycle is controlled by both microphysical and meteorologic processes. The global average precipitation amounts to about 75 cm/yr or 750 L/(m yr). 

As a compound water is remarkable. It is the only inorganic liquid to occur naturally on earth, and it is the only substance found in nature in all three physical states, solid, liquid and vapour (Franks, 1983). It is the most readily available solvent and plays a vital role in the continuation of life on earth. Water circulates continuously in the enviromnent by evaporation from the hydrosphere and subsequent precipitation from the atmosphere. This overall process is known as the hydrologic cycle. Reports estimate that the atmosphere contains about 6 x 10 litres of water, and this is cycled some 37 times a year to give an annual total precipitation of 224 X 10 litres (Franks, 1983 Nicholson, 1985). 

The Water Cycle. The evaporation of water from land and water surfaces, the transpiration from plants, and the condensation and subsequent precipitation of rain cause a cycle of transportation and redistribution of water, a continuous circulation process known as the hydrologic cycle or water cycle (see Fig. 86). The sun evaporates fresh water from the seas and oceans, leaving impurities and dissolved solids behind when the water vapor cools down, it condenses to form clouds of small droplets that are carried across the surface of the earth as the clouds are moved inland by the wind and are further cooled, larger droplets are formed, and eventually the droplets fall as rain or snow. Some of the rainwater runs into natural underground water reservoirs, but most flows, in streams and rivers, back to the seas and oceans, evaporating as it travels. 

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