The Hydrologic Cycle

Essentials of Toxic Chemical Risk Science and Society 

FIGURE 2.3 Diagram of the hydrologic cycle. Groundwater, surface water, and water vapor in the atmosphere are interconnected. Courtesy of the U.S. Geological Survey, (http // 

FIGURE 2.4 The physical states of water. The hydrologic cycle depends on temperature-driven transitions among the three physical states of water solid (snow and ice), liquid, and vapor. The total mass (amount) of water on planet Earth is a constant. Global temperatures determine how much water is present as a liquid, a vapor, or a solid. Global warming shifts water from ice and snow to liquid water and water vapor. 

Aquifers vary enormously with respect to the volume of water they can hold and the speed with which water can flow through them. The volume of water in an aquifer depends on its porosity. In the case of unconsolidated aquifers, porosity is determined by the number and size of pores within and between soil particles. In bedrock aquifers, water is held, not in pores, but in fractures or holes in the rock. Spaces holding water in bedrock are referred to as secondary porosity, and the kinds of spaces depend on the geologic features of the bedrock. For example, a hard, crystaUine bedrock may hold water in cracks and fractures, whereas a soft bedrock such as karst limestone may have undergone erosion by groundwater over millions of years, resulting in a Swiss cheese network of large fissures and holes in which water is held. 

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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.

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. 

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. 

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... 

The Earth s history, and its future, are shaped not by independent events but by an intricately linked series of feedbacks and responses spanning the spectrum of physical, chemical, and biological cycles, of which the hydrologic cycle is only a part, albeit a central one. 

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). 

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