The volume of water in the fresh water reservoirs, particularly those that are available for human use, are important water resources.
The residence time of a reservoir within the hydrologic cycle is the average time a water molecule will spend in that 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.
In hydrology, residence times can be estimated in two ways. The more common method relies on the principle of conservation of mass and assumes the amount of water in a given reservoir is roughly constant. With this method, residence times are estimated by dividing the volume of the reservoir by the rate by which water either enters or exits the reservoir. Conceptually, this is equivalent to timing how long it would take the reservoir to become filled from empty if no water were to leave (or how long it would take the reservoir to empty from full if no water were to enter).
An alternative method to estimate residence times, gaining in popularity particularly for dating groundwater, is the use of isotopic techniques. This is done in the subfield of isotope hydrology.
The water cycle is powered from solar energy. 86% of the global evaporation occurs from the oceans, reducing their temperature by evaporative cooling. Without the cooling effect of evaporation the greenhouse effect would lead to a much higher surface temperature of 67 °C, and a warmer planet.
While the water cycle is itself a biogeochemical cycle, flow of water over and beneath the Earth is a key component of the cycling of other biogeochemicals. Runoff is responsible for almost all of the transport of eroded sediment and phosphorus from land to water bodies. The salinity of the oceans is derived from erosion and transport of dissolved salts from the land. Cultural eutrophication of lakes is primarily due to phosphorus, applied in excess to agricultural fields in fertilizers, and then transported overland and down rivers. Both runoff and groundwater flow play significant roles in transporting nitrogen from the land to water bodies. The dead zone at the outlet of the Mississippi River is a consequence of nitrates from fertilizer being carried off agricultural fields and funneled down the river system to the Gulf of Mexico. Runoff also plays a part in the carbon cycle, again through the transport of eroded rock and soil.
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III. Answer the questions:
1. What does the water cycle describe?
2. How many water states do you know? Make a list of them.
3. Is the amount of water on the Earth constant or not?
4. How does the water enter the atmosphere?
5. What happens to water vapor in the atmosphere?
6. What kinds of precipitations do you know? What happens to them on the Earth?
7. What is ground water? Does it get in to the oceans or stay beneath the surface?
8. How many processes of water cycle do you know? Make a list of them.
9. What is a water reservoir? What are the largest?
10. What is the residence time of a reservoir within the hydrologic cycle?
11. How many ways of estimation of residence time do you know? Describe them.
12. What is the driving force of the water cycle?
13. Does water cycle have any influence on biogeochemical processes on the Earth?
IV. Do you agree or disagree with the following statements? Explain why.
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