Hydrologie Cycle
Remember from Chapter 1 that the hydrosphere, crust, and atmosphere combined make up the biosphere. The hydrosphere includes all the water in the atmosphere and on the Earth's surface.
When the sun heats the oceans, the cycle starts. Water evaporates and then falls as precipitation in the form of snow, hail, rain, or fog. While it's falling, some of the water evaporates or is sucked up by thirsty plants before soaking into the ground. The sun's heat also keeps the cycle going.
The hydrologic cycle is made up of all water movement and storage throughout the Earth's hydrosphere.
Water is constantly circulating between the atmosphere and the Earth and back to the atmosphere through a cycle involving condensation, precipitation, evaporation, and transpiration. This is called the hydrologic cycle. Fig. 5-1 illustrates the many ways water is transported through the hydrologic cycle.
- Fig. 5-1 The hydrologlic cycle is a dynamic system.
Water vapor is carried by wind and air currents throughout the atmosphere. When an air mass cools down, its vapor condenses into clouds and eventually falls to the ground as precipitation in the form of snow, rain, sleet, or hail.
Water takes one route from the atmosphere to the ground, but can take a variety of paths and time periods to get back up into the atmosphere. These paths include the following:
Absorption by plants; Evaporation from the sun's heating; Storage in the upper levels of soil; Storage as groundwater deep in the earth; Storage in glaciers and polar regions; Storage or transport in springs, streams, rivers, lakes; and Storage in the oceans.
When water is stored somewhere for any length of time, it is called a water reservoir. A reservoir is a holding area. Nature's reservoirs are oceans, glaciers, polar ice, underground storage (aquifers), lakes, rivers, streams, the atmosphere, and the biosphere (within living organisms).
Surface water in streams and lakes returns to the atmosphere as a gas (vapor) through the process of evaporation.
Water held inside plants returns to the atmosphere as a vapor through a biological process called transpiration. When plants pull water up through their roots from the soil, use some of the dissolved minerals to grow, and then release the water back through the leaves, the entire cycle is known as evapotranspiration. This happens the most during times of high temperatures, wind, dry air, and sunshine. In temperate climates, this occurs during the summertime.
When air currents rise into the colder atmospheric layers, water vapor condenses and sticks to tiny particles in the air. This is called condensation. When a lot of water vapor coats enough particles (dust, pollen, or pollutant), it forms a cloud.
As a child, you may have lain on your back and stared up at the clouds while trying to spot different shapes. It's fun because clouds are always changing. Just when you see a lion or armadillo, the cloud drifts into the shape of a dog or a tree. Clouds are constantly reevaporating and forming new ones. Unless you live in a very dry climate with only one small cloud in the sky at a time, the cloud you see now will probably not be the one you see in 15 minutes. Unless, of course, it's a large storm system getting ready to dump a boat-load of water on your picnic! Even then, the cloud shapes would be changing.
As the air gets wetter and wetter (saturated), water droplets accumulating within the cloud get bigger and bigger. When these droplets get too heavy, gravity wins and they fall as precipitation.
Precipitation can take the form of rain, snow, sleet, or hail, depending on temperature and other atmospheric conditions.
[Note: Throughout this chapter, I often use the words "rain" and "rainfall" interchangeably with "precipitation." Just know that precipitation includes rain, drizzle, mist, fog, sleet, hail, snow, and any other wet stuff falling from the sky.] After rain hits the ground, it can evaporate quickly, be absorbed (by the land or the sea), or run off into storm sewers, streams, or rivers.
Even though the hydrologic cycle balances what goes up with what comes down, one part of the cycle gets stuck in polar regions during the wintertime. In cold climates, rain is stored as snow or ice on the ground for several months. In glacial areas, the time period can extend from years to thousands of years. Then, as the temperature climbs in the spring, the water is released. When this happens in a very short period of time, flooding occurs.
The hydrologic cycle is an endless loop. With transpiration and evaporation happening constantly, the cycle goes on and on and on. Every time a molecule of H2O goes through the hydrologic cycle, it is recycled and is ready to begin the adventure all over again.
Let's look at the different parts of the hydrologic cycle more closely. Because we are talking about water, each process has some variations.
Post a comment