The hydrologic cycle is the continuous circulation of water through the environment, which can be thought of as a series of hydrologic compartments. The most important places in which water occurs are the ocean, glaciers, underground aquifers, surface waters, and the atmosphere. The total amount of water among all of these compartments is a fixed quantity. However, water moves readily among its various compartments through the processes of evaporation and transpiration (often combined and referred to as evapotranspiration), precipitation, and surface and subsurface flows. Each of these compartments receives inputs of water and has corresponding outputs, representing a flow-through system. If there are imbalances between inputs and outputs, there can be significant changes in the quantities stored locally or even globally. An example of a local change is the drought that can occur in soil after a long period without replenishment by precipitation. An example of a global change in hydrology is the increasing mass of continental ice that occurs during glacial epochs, an event that can remove so much water from the oceanic compartment that sea level can decline by more than 328 ft (100 m), exposing vast areas of continental shelf for the development of terrestrial ecosystems.

By far the largest quantity of water occurs in the deep lithosphere, which contains an estimated 27 × 10¹⁸ tons (27-billion-billion tons) of water, or 94.7 percent of the global total. The next largest compartment is the oceans, which contain 1.5 × 10¹⁸ tons, or 5.2 percent of the total. Ice caps contain 0.019 × 10¹⁸ tons, equivalent to most of the remaining 0.1 percent of Earth's water. Although present in comparatively small amounts, water in other compartments is important ecologically because it is present in places where biological processes occur. These include shallow groundwater (2.7 × 10¹⁴ tons), inland surface waters such as lakes and rivers (0.27 × 10¹⁴ tons), and the atmosphere (0.14 × 10¹⁴ tons).

The smallest compartments of water also tend to have the shortest turnover times because their inputs and outputs are relatively large in comparison with the mass of water contained in the compartment at any time. This is especially true of atmospheric water, which receives annual inputs equivalent to 4.8 × 10¹⁴ tons as evaporation from the oceans (4.1 × 10¹⁴ tons/yr) and terrestrial ecosystems (0.65 × 10¹⁴ tons/yr), and turns over about 34 times per year. These inputs of water to the atmosphere are balanced by outputs through precipitation of rain and snow, which deposit 3.7 × 10¹⁴ tons of water to the surface of the oceans each year, and 1.1 × 10¹⁴ tons/yr to the land.

These data suggest that the continents receive 67 percent more water as precipitation than is lost by evapotranspiration from the land. The difference, equivalent to 0.44 × 10¹⁴ tons/yr, is made up by 0.22 × 10¹⁴ tons/yr of water discharged to the oceans through rivers, and another 0.22 × 10¹⁴ tons/yr of subterranean runoff to the oceans.

The movement of water through the hydrologic cycle is driven by energy gradients. Evaporation occurs in response to the availability of thermal energy and water vapor concentration gradients. The ultimate source of energy for almost all natural evaporation of water on Earth is solar electromagnetic radiation. This solar energy is absorbed by surfaces, increasing their heat content, and thereby providing a source of energy to drive evaporation. In contrast, surface water and groundwater flow in response to gradients of potential energy.

The hydrological cycle of a watershed is a balance between water added by precipitation and upstream drainage, and water removed by evapotranspiration, surface water flow, infiltration into the ground, and any internal storage that may occur because of imbalances of the inputs and outputs. Hydrological budgets of landscapes are often studied on the spatial scale of watersheds, which are areas in which water flows into a stream, river, or lake.

The simplest watersheds are headwater systems that do not receive any drainage from watersheds at higher altitude, so the only hydrologic input occurs mainly as precipitation. In places where fog is common, wind can drive droplets of water vapor into the forest canopy and the direct deposition of cloud water can also be important. This effect has been measured for a foggy conifer forest in New Hampshire, where fog water deposition was equivalent to 33 in (84 cm) per year, compared with 71 in (180 cm) per year of rain and snow.

Vegetation can have an important influence on the rate of evaporation of water from watersheds. This hydrologic effect is especially notable for well-vegetated ecosystems such as forests because an extensive surface area of foliage supports large rates of transpiration. Evapotranspiration refers to the combined rates of transpiration from foliage, and evaporation from nonliving surfaces such as moist soil or surface waters. Because transpiration is such an efficient means of evaporation, evapotranspiration from any well vegetated landscape Page 4740  |  Top of Articleoccurs at much larger rates than from any equivalent area of nonliving surface.

In the absence of evapotranspiration, an equivalent quantity of water would have to drain from the watershed as seepage to deep groundwater or as stream flow. Studies of forested watersheds in Nova Scotia, Canada, found that evapotranspiration was equivalent to 15–29 percent of the hydrologic inputs with precipitation. Runoff through streams or rivers was estimated to account for the other 71–85 percent of the atmospheric inputs of water because the relatively impervious bedrock in that region prevented significant drainage to deep groundwater.

Forested watersheds in seasonal climates display large variations in their rates of evapotranspiration and stream flow. This effect can be illustrated by the seasonal patterns of hydrology for a forested watershed in eastern Canada. The input of water through precipitation is 58 in (146 cm) per year, but 18 percent of this arrives as snow, which tends to accumulate on the surface as a persistent snowpack. About 38 percent of the annual input is evaporated back to the atmosphere through evapotranspiration, and 62 percent runs off as river flow. Although there is little seasonal variation in the input of water with precipitation, there are large seasonal differences in the rates of evapotranspiration, runoff, and storage of groundwater in the watershed. Evapotranspiration occurs at its largest rates during the growing season of May to October, and runoff is therefore relatively sparse during this period. In small watersheds in this region, forest streams can become seasonally dry because so much of the precipitation and soil water is utilized for evapotranspiration, mostly by trees. During the autumn, much of the precipitation input serves to recharge the depleted groundwater storage, and once this is accomplished stream flows increase again. Runoff then decreases during winter because most of the precipitation inputs occur as snow, which accumulates on the ground surface because of the prevailing subfreezing temperatures. Runoff is largest during the early springtime when warming temperatures cause the snowpack to melt during a short period of time, resulting in a pronounced flush of stream and river flow.

Some aspects of the hydrologic cycle can be utilized by humans for a direct economic benefit. For example, the potential energy of water elevated above the surface of the oceans can be used to generate electricity. Hydroelectric resource development, however, generally causes large changes in hydrology. This is especially true of hydroelectric developments in relatively flat terrain, which require the construction of large storage reservoirs to retain seasonal high-water flows, so that electricity can be generated at times that suit the peaks of demand. These extensive storage reservoirs are artificial lakes, sometimes covering tens of thousands of acres. These types of hydroelectric developments cause changes in river hydrology, especially by reducing variations of flow and sometimes by unpredictable spillage of water at times when the storage capacity of the reservoir is full. Both of these hydrologic influences have significant ecological effects, for example, on the habitat of salmon and other aquatic biota. In one unusual case, a large release of water from a reservoir in northern Quebec, Canada, drowned 10,000 caribou that were trapped by the unexpected cascade of water during their migration. The construction of dams and flooding needed to create water reservoirs has complex impacts that can be assessed in terms of their benefit to human society, effect on wildlife, the geology of a specific area, and the overall disruption to human lives as populations need to be relocated. However, construction of dams has historically been one of the most effective ways to retain water in reservoirs with the earliest known dam, the Jawa Dam in Jordan, dating back to 3000 BCE.

Where the terrain is suitable, hydroelectricity can be generated with relatively little modification to the timing and volumes of water flow. This is called run-of-the-river hydroelectricity, and its hydrologic effects are relatively small. The use of geologically warmed groundwater to generate energy also has small hydrological effects because the water is usually reinjected into the aquifer.

Human activities can alter the hydrologic cycle in other ways. The volume and timing of river discharges can be affected by channeling to decrease the impediments to flow. Changing the character of the watershed by paving, compacting soils, and altering the nature of the vegetation generally amplifies the intensity of runoff after rainstorms. Risks of flooding can be increased by increasing the rate at which water is shed from the land, thereby increasing the magnitude of peak flows. Risks of flooding are also increased if erosion of soils from terrestrial parts of the watershed leads to siltation and the development of shallower river channels, which then fill up and spill over during high-flow periods. Massive increases in erosion are often associated with deforestation, especially when natural forests are converted into agriculture.

The quantities of water stored in hydrologic compartments can also be influenced by human activities. One example is the mining of groundwater for agriculture, industry, and municipal consumption. The Ogallala aquifer in the central part of the United States, which has been drawn down mostly to obtain water for irrigation in agriculture, contains so-called fossil water that accumulated in the aquifer during wetter times.

Sometimes industrial activities lead to large emissions of water vapor into the atmosphere, producing a local hydrological influence through the development of low-altitude clouds and fogs. This effect is mostly associated with electric power plants that cool their process water using cooling towers.

One of the most important human-induced effects on the hydrological cycle comes from urbanization, i.e., the shift of human population from rural to urban areas of much higher density. One of the main differences between urbanized environment and uninhabited natural areas is the abundance of impervious surfaces in the former case. Such surfaces are highly artificial, and they include parking lots, pavements covered by water-resistant material such as asphalt and concrete, rooftops, etc. To counteract the detrimental effects of urbanization in the hydrologic cycle—including factors related to stormwater, groundwater, drinkable water supply, and wastewater management—urban developers have increasingly started incorporating water-sensitive urban design techniques into their planning and development. These relatively lower impact techniques focus on water conservation, and they aim to replicate the natural, pre-development elements of the water cycle. Typical practices include the installation of green roofs, bioretention cells (also known as rain gardens) that mitigate the flow of stormwater from impervious surfaces, the use of more porous materials for pavement, etc.

A more substantial hydrologic influence on evapotranspiration is associated with large changes in the vegetation within a watershed. This is especially important when mature forests are disturbed, for example, by wildfire, clear-cutting, or conversion into farms. Disturbance of forests disrupts the capacity of the landscape to sustain transpiration because the amount of foliage is reduced. This leads to an increase in stream flow volumes, and sometimes to an increased height of the groundwater table. In general, the increase in stream flow after disturbance of a forest is roughly proportional to the fraction of the total foliage of the watershed that is removed by logging or burning. The influence on transpiration and stream flow generally lasts until regeneration of the forest restores another canopy with a similar area of foliage, which generally occurs after about 5 to 10 years. However, there can be a longer-term change in hydrology if the ecological character of the watershed is permanently changed, as occurs when a forest is converted to agriculture.