Hydrologic Cycle

The water cycle—technically known as the '''hydrologic cycle'''—is the continuous circulation of water within the Earth 's Hydrosphere , and is driven by Solar Radiation . This includes the Atmosphere , land, Surface Water and Groundwater . As water moves through the cycle, it changes State between Liquid , Solid , and Gas phases. Water moves from compartment to compartment, such as from River to Ocean , by the physical processes of Evaporation , Precipitation , Infiltration , Runoff , and subsurface flow. Movement of water within the water cycle is the subject of the field of Hydrology .

MOVEMENT OF WATER WITHIN THE WATER CYCLE

There is no definable start or finish to the water cycle. Water molecules move continuously among different compartments, or reservoirs, of the Earth's hydrosphere, by different physical processes. Water evaporates from the oceans, forms clouds, which precipitate and the water falls back to Earth. However, water does not necessarily cycle through each compartment in order. Before reaching the ocean, water may have evaporated, condensed, precipitated, and become runoff multiple times.

The physical processes

The major physical processes involved in the water cycle are the evaporation of water from the oceans and land, the transport of water in the atmosphere, condensation, precipitation over the oceans and land, and the flow of water from land to the oceans.

Evaporation is the transfer of water from bodies of surface water into the atmosphere. This transfer entails a change in the physical nature of water from liquid to gaseous phases. The source of energy is primarily Solar Radiation . Evaporation is closely related to Transpiration from Plant s, as well as, to a lesser degree, Perspiration from land Mammal s and Marsupial s. Thus, this transfer is sometimes referred to as ''' Evapotranspiration '''. 90% of atmospheric water comes from evaporation, while the remaining 10% is from transpiration.

Condensation is the transformation of water vapor to liquid water droplets in the air, producing Cloud s and Fog .

Advection is the movement of water—in solid, liquid, or vapour states—through the atmosphere. Without advection, water that evaporated over the oceans could not precipitate over land.

Precipitation is water vapour that has condensed into clouds and falls to the Earth's surface. This mostly occurs as Rain fall, but also includes Snow , Hail , Fog Drip , Graupel , and Sleet .

Runoff includes the variety of ways by which water moves across the land. This includes both Surface Runoff and Channel Runoff . As it flows, the water may infiltrate into the ground, evaporate into the air, become stored in lakes or reservoirs, or be extracted for agricultural or other human uses.

Less fundamental processes involved in the water cycle are:

Sublimation is the state change directly from solid water (snow or ice) to water vapour.

Canopy Interception is the precipitation that is intercepted by plant foliage and eventually evaporates back to the atmosphere rather than falling to the ground. The amount of water that it intercepted depends on the duration of the storm, the windspeed and temperature, and the amount of foliage present.

Infiltration is the flow of water on the ground surface into the ground. The speed of infiltration depends on how moist the ground already is, and on its Infiltration Capacity . Having infiltrated, water comprises Soil Moisture within the Vadose Zone , or Groundwater in an Aquifer .

Snowmelt refers to the runoff produced by melting snow.

Subsurface flow is the flow of water underground, in the vadose zone and aquifers. Subsurface water may return to the surface (eg. as a spring or by being pumped) or eventually seep into the oceans. Water returns to the land surface at lower elevation than where it infiltrated, under the force of Gravity or gravity induced pressures. Groundwater tend to move slowly, and is replenished slowly, so it can remain in aquifers for thousands of years.

Conservation of mass

The total amount, or mass, of water in the water cycle remains essentially constant, as does the amount of water in each reservoir of the water cycle. This means that rate of water added to one reservoir must equal, on average over time, the rate of water leaving the same reservoir.

The adjacent table contains the amount of water that falls as precipitation or rises as evaporation, for both the land and oceans. The runoff and groundwater discharge from the land to the oceans is also included. From the law of the conservation of mass, whatever water moves into a reservoir, on average, the same volume must leave. For example, 107 thousand cubic km (107 × 10³ Km³ ) of water falls on land each year as precipitation. This is equal to the sum of the evaporation (71 × 10³ km³/year) and runoff (36 × 10³ km³/year) of water from the land.

Water that cycles between the land and the atmosphere in a fixed area is referred to as Moisture Recycling .

RESERVOIRS

In the context of the water cycle, a reservoir represents the water contained in different steps within the cycle. The largest reservoir is the collection of oceans, accounting for 97% of the Earth's water. The next largest quantity (2%) is stored in solid form in the ice caps and glaciers. The water contained within all living organisms represents the smallest reservoir.

The volume of water in the fresh water reservoirs, particularly those that are available for human use, are important Water Resources .

RESIDENCE TIMES

The residence time is the average time a water molecule will spend in a reservoir. It is a measure of the average age of the water in that reservoir, though some water will spend much less time than average, and some much more. Groundwater can spend over 10,000 years beneath Earth's surface before leaving. Particularly old groundwater is called Fossil Water . Water stored in the soil remains there very briefly, because it is spread thinly across the Earth, and is readily lost by evaporation, transpiration, stream flow, or groundwater recharge. After evaporating, water remains in the atmosphere for about 9 days before condensing and falling to the Earth as precipitation.

(''See the adjacent table for residence times for the other reservoirs.'')

Residence times can be estimated in two ways. The more common method relies on Conservation Of Mass , and may be expressed by the following equation:

\mathrm{Residence} \mbox{ } \mathrm{time} =\begin{matrix} rac{\mathrm{Volume} \mbox{ } \mathrm{of} \mbox{ } \mathrm{reservoir}}{\mathrm{Rate} \mbox{ } \mathrm{water} \mbox{ } \mathrm{is} \mbox{ } \mathrm{added} \mbox{ } \mathrm{to} \mbox{ } \mathrm{reservoir}} \end{matrix}

An alternative method, gaining in popularity particularly for dating groundwater, is the use of isotopic techniques. This is done in the subfield of Isotope Hydrology .

Example: Calculating the residence time of the oceans

As an example of how the residence time is calculated, consider the oceans. The volume of the oceans is roughly 1,370 km³. Precipitation over the oceans is about 0.398 km³/year and the flow of water to the oceans from rivers and groundwater is about 0.036 km³/year. By dividing the total volume of the oceans by the rate of water added (in units of volume over time) we obtain the residence time of 3,200 years—the average time it takes a water molecule that reaches an ocean to evaporate.

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Hydrologic Cycle