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The Hydrologic Cycle
- 1. WATER ON EARTH The Hydrologic Cycle
Notas del editor
- Even though we can list them as different water sources and account for how much water is in each of them, they do interact with one another, which makes for some very interesting science. Hydrologic cycle - The process by which water is continuously exchanged between earth's various water sources
- What happens when water vapor cools? It condenses!
- One of two things that can happen to water: infiltration or runoff
- Perhaps the largest aquifer in the world is the Ogallala aquifer located in the Midwestern part of the United States. This aquifer is named after a Sioux Indian tribe. It is estimated to be more than two million years old and to hold about 650 trillion gallons (2,500 trillion liters)! It underlies parts of 8 states, stretching about 800 miles (1,288 km) from South Dakota to Texas. The Ogallala aquifer supplies vast amounts of water to irrigate the crops grown in this vitally important agricultural area.
- One of two things that can happen to water: infiltration or runoff
- Now the first thing you have to realize when looking at Table 5.2 is that they are only approximations. After all, there is no way to measure residence time. These numbers are based on calculations that make a lot of assumptions and use current theories of how the processes in the hydrologic cycle work. Only if the assumptions and theories used in the calculation are good will these numbers be accurate. If not, then you cannot rely on them. Nevertheless, the numbers at least illustrate what I am trying to say. The nature of the water source determines the length of the residence time. The next thing you need to realize is that these numbers are averages. The residence time in a small lake will be much shorter than the residence time in a large lake. Averaged over all lake sizes, however, the typical residence time for water in a lake is 10 years. Finally, you need to realize that I don't want you to memorize this table. Instead, I want you to look at the numbers and understand why the residence times are so different. Water in the atmosphere, for example, has a short residence time. Why? Well, the water in the atmosphere is constantly forming clouds and precipitating. Thus, a drop of water doesn't spend much time there. That's the kind of thinking I want you to develop in reference to residence times.