Although oceans and ice caps contain some 99.3 percent of all the water on Earth, the fraction of 1 percent residing at any given time in the atmosphere, in lakes and streams, and in soil and subsurface layers plays unique and important roles. The flow of water on the surface is a major determinant of the configuration of the physical environment. Soil moisture is essential to most terrestrial plant life. The stocks and flows of ground and surface water are major links in the transport and cycling of chemical nutrients and important determinants of what kinds and intensities of human activity can be supported in what locations. And water in the atmosphere has several functions that are central to shaping climates.
The set of processes that maintain the flow of water through the terrestrial and atmospheric branches of the hydrosphere is called the hydrologic cycle. The cycle includes all three physical states of water--liquid, solid (ice and snow), and gas (water vapor). It also includes all of the possible transformations among these states--vaporization, or evaporation (liquid to gas); condensation (gas to liquid), freezing (liquid to solid); melting, or fusion (solid to liquid); and sublimation (gas to solid, or the reverse).
The principal flows in the hydrologic cycle are: (1) evaporation of water from the surface of the oceans and other bodies of water, and from the soil; (2) transpiration of water by plants, the result of which is the same as that of evaporation--namely, the addition of water vapor to the atmosphere; (3) horizontal transport of atmospheric water from one place to another, either as vapor or as the liquid water droplets and ice crystals in clouds; (4) precipitation, in which atmospheric water vapor condenses (and perhaps freezes) or sublimates and falls on the oceans and the continents as rain, sleet, hail, or snow; and (5) runoff, in which water that has fallen on the continents as precipitation finds its way, flowing on and under the surface, back to the oceans. Because it is difficult and not particularly useful to distinguish between the contributions of evaporation and transpiration on the continents, these two terms are often lumped together as evapotranspiration.
The magnitudes of these flows, averaged over all the continents and oceans and expressed in thousands of cubic kilometers of water per year, are shown in Figure 1.1 These magnitudes are based on the assumption that the various components of the hydrosphere are in equilibrium, which is at least a good first approximation. That is, on a yearround average, inflows and outflows for the atmosphere, the oceans, and the continents all balance. (For example, in thousands of cubic kilometers, the atmosphere receives 62 + 456 = 518 as evaporation from the surface and gives up 108 + 410 = 518 as precipitation).
The magnitude of the flows in the hydrologic cycle is more readily grasped if one thinks of the flows in terms of the equivalent depth of water, averaged over the surface area involved. In these terms, the world's oceans annually lose to evaporation a layer of water 1.26 meters deep (about 4 feet) over their entire surfaces, gaining back 1.14 meters from precipitation and 0.12 meters from the discharge of rivers and groundwater. The continents receive precipitation each year equivalent to a layer of water 0.73 meters (29 inches) deep over their entire surface areas, of which 0.42 meters is lost to evaporation and 0.31 meters makes up the runoff.
Combining the foregoing information on equilibrium flows with the information on stocks in the hydrosphere permits us to estimate the average residence time of water in the different parts of the cycle. These residence times, which are of great importance in analyzing the transport of pollutants, as well as nutrients, by the hydrologic cycle, are listed in Table 1. There is an enormous range, from the average nine days a water molecule spends in the atmosphere between being evaporated from the surface and falling again as precipitation, to the 10,000 years a molecule of water typically spends as ice between falling in a snowflake on the Antarctic ice sheet and rejoining the ocean with the melting of an iceberg. It is also important to remember that there are large deviations from the average in any given category--a water molecule may fall in a raindrop not nine days but an hour after being evaporated from Earth's surface; another may wander not two weeks but two years in the delta of the Amazon River before reaching the sea. Nevertheless, the average residence times can provide useful insights into a variety of important problems, and the approach can be refined whenever information more pertinent than global averages is available.
The balance between precipitation and evapotranspiration varies widely from continent to continent, as shown in Table 2. The size of the runoff (the difference between precipitation and evapotranspiration) is a measure of how much water is potentially available for domestic and industrial uses by society (including dilution and removal of wastes) and for the other functions that flowing water performs. Note in Table 2 the remarkable fact that South America has a runoff per unit of surface area almost three times that of North America, the continent with the next greatest runoff. It is perhaps not so surprising, then, that the discharge of the Amazon River, which drains the wettest third of South America, amounts to about a seventh of the runoff of the entire world.
Much of the runoff on the continents takes place not on the surface but beneath it. Although the quantities can only be estimated, it is clear that most rivers receive at least as much of their flow from seepage through the ground as from flow over the ground; and a certain amount of water reaches the oceans via flowing aquifers and seepage at the edges of the continents without ever joining a surface river at all (Strahler and Strahler, 1973; Kuenen, 1963). Water beneath the land's surface is called soil moisture, or soil water, when it is distributed in the first meter or so of soil (a zone defined by the depth of penetration of the roots of most plants.) Below the zone of soil moisture is an intermediate zone where the water percolates downward through open pores in the soil and rock; and below this is the water table, marking the surface of the body of groundwater that saturates the soil or rock in which it finds itself, filling all pores and spaces in the soil or rock completely. The groundwater extends downward until it is limited by an impermeable layer of rock. In some circumstances, there are successive layers of groundwater (aquifers) separated by impermeable layers of rock. The absolute lowest limit of groundwater is probably about 16 kilometers from the surface, where the pressure is so great that all pores are closed and any rock becomes impermeable.
Most groundwater is flowing, albeit very slowly (10 meters per day [m/d] in coarse gravel near the surface, more commonly 1 m/d, and much more slowly at greater depths). At 1 meter per day, of course, it takes almost three years to move 1 kilometer. Figure 2 is a schematic diagram of the zones and flows of subsurface water, showing the intersection of the water table and a surface river.
The energy that drives the hydrologic cycle is energy from the sun--indeed, this function is the largest single user of the solar energy reaching Earth's surface. The reason so much energy is required is that it takes a great deal of energy to evaporate water--2250 joules per gram at the boiling point of 100 degrees C and 2440 joules per gram at Earth's average surface temperature of 15 degrees C. (This is the highest heat of vaporization of any known substance.) It takes fifty times as much energy to evaporate a gram of water as it does to lift it to an altitude of 5 kilometers. The energy used to evaporate the water is stored as latent heat of vaporization, which is released to the environment as heat whenever and wherever the watervapor condenses into liquid. Thus, energy delivered by the sun at one point on Earth's surface may be released high in the atmosphere over a point 1000 kilometers away. This mechanism of redistributing energy by the transport and condensation of water vapor is a major determinant of Earth's climate.
As noted above, the energy the sun supplies at the time of evaporation reappears as heat at the time of condensation. Similarly, the smaller amount of solar energy that does the work of lifting the water vapor against the force of gravity appears as frictional heat when falling droplets of condensed vapor collide with molecules of air and when rushing mountain streams rub against their rocky beds. That all the energy the sun supplies to terrestrial processes comes back again in one form or another is not coincidence or quirk, but an illustration of the first law of thermodynamics?the law of conservation of energy. Further excursions into the machinery of the physical world--and of human technology--will require some familiarity with this law and with its companion, the second law of thermodynamics.
This article was reprinted with permission by the authors from Ecoscience: Population, Resources, Environment. Tables and figures omitted in on-line version.