The Hydrologic Cycle

Figure ! represents a conceptual model of the hydro logic cycle and shows Earth's water movement between the ocean, land, and atmosphere. As with all cycles, it is ongoing and continuous, and there is no specific start or end point; however, because the main focus of this handbook is meteorology, precipitation is an appropriate place to begin an evaluation. Precipitation is water released from the atmosphere in the form of rain, snow, sleet or hail. During precipitation, some of the moisture is evaporated back into the atmosphere before ever reaching the ground. Some precipitation is intercepted by plants, a portion infiltrates the ground, and the remainder flows off the land into lakes, rivers, or oceans. An important difference between the roles of snow and rain is that runoff occurs relatively quickly following the rain event, whereas snow usually melts much more slowly over days, weeks, or months. The subsequent surge of snowmelt runoff can provide seasonal recharge to groundwater resources but can also trigger flood conditions if the snowmelt occurs too rapidly and in excessive amounts. In addition, the solid snow or ice may changc directly into a gas, skipping the liquid state, in the process called sublimation.

When precipitation is intercepted by plants, it is eventually evaporated back to the atmosphere. When it infiltrates the ground, it can be taken up by roots and transpired by plants, it can be evaporated from the soil, or it may recharge an aquifer. The water in an aquifer is called groundwater, and its rate of flow in the subsurface is such that water can reside in aquifers for days to centuries before discharging to a surface body of water (e.g., river, lake, occan). Once groundwater has discharged into a river, lake, or ocean, the surface of the water body is exposed for evaporation, causing moisture to collect and concentrate in the atmosphere, eventually returning to the earth as precipitation as the cycle begins again. In addition to natural discharge, groundwater can more rapidly discharge when an aquifer is pumped. With the advent of motorized pumps, the rapid removal of groundwater from aquifers is a relatively recent phenomenon that has greatly affected the depletion of the aquifers and the water balance of many catchments.

Figure I Schematic representation of the hydrologic cycle. (Courtesy B. Imam.)

While the hydrological cycle is a continuous process, it is by no means uniform throughout the globe: the residence time of water varies—often dramatically— among different portions of the cycle. For example, water is continuously evaporated from the surfaces of water bodies (such as oceans, lakes, and rivers). Similarly, precipitation that is intercepted by plants and other surfaces is often evaporated within a matter of hours. Once evaporated, it takes an average of 10 days for a water molecule to cycle through the atmosphere, but if it infiltrates to the water table, or if the precipitation occurs in a polar region, it may reside for hundreds of years before transferring to another step in the hydrologie cycle. In addition to variable residence times, the processes associated with the hydrologie cycle are not evenly distributed over the globe; they vary by climatic region. For example, évapotranspiration occurs readily in semiarid and arid regions, but subsequent precipitation may not occur within the same basin or region. The dramatic differences in how the cycle operates are especially evident when one evaluates the hydrologie cycle at the catchment scale.

Additional, variably detailed discussions of the hydrologie cycle may be found in Horden (1998), Maidment (1993), Driscoll (1986), and Freeze and Cherry (1979). Chahine (1992) offers a particularly thorough discussion of the hydrologie cycle in the context of climate studies and hydrologie modeling.


On a global scale, the important reservoirs in the hydrologie cycle are the ocean, atmosphere, polar ice, groundwater, and moisture from land surfaces. At the global scale, water is transferred between reservoirs via four fluxes: precipitation, évapotranspiration, sublimation, and runoff. On a catchment scale, the availability of fresh water is the focus. Critical reservoirs on this scale are the atmosphere, lakes, rivers, and groundwater. Oceans and polar ice are typically irrelevant at the catchment scale, although seasonal snowmelt can contribute significantly (or destructively, in the case of floods) to a basin's water resources. Fluxes within a catchment are more strongly weighted toward the recharge and withdrawal of potable groundwater, as well as the occurrence of surface water flows.

Fresh water comprises only 2.5% of the world's total water supply. Of this scant freshwater supply, 69.6% is immobilized in ice and snow, primarily in the polar regions; nonsaline groundwater accounts for 30.1%; and the remainder of fresh water (0.3%) is distributed among lakes, rivers, wetlands, atmospheric water, and biological water found in plants and animals. While groundwater is Earth's second largest source of fresh water, on the average it accounts for less than 1% of the earth's total water supply; however, freshwater availability varies greatly on a regional basis. Figure 2 shows the distribution of terrestrial water in terms of Earth's total water supply and Earth's freshwater supply.

The major reservoirs of the hydrologie cycle are described below, and their role in the global- and/or catchment-scale hydrologie cycle is discussed briefly.

let caps. gLatitrs, ulher anaw and i(C 1.75% (61.6%)


Figure 2 Distribution of terrestrial water partitioned in reference to Harth's total water supply (top percentage) and Earth's freshwater supply {bottom percentage, in parentheses).

let caps. gLatitrs, ulher anaw and i(C 1.75% (61.6%)


Figure 2 Distribution of terrestrial water partitioned in reference to Harth's total water supply (top percentage) and Earth's freshwater supply {bottom percentage, in parentheses).

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  • Jan
    What approximate percentage of the earth's freshwater is groundwater?
    2 years ago

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