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Distribution of the Earth’s water
Earth is known as the “Blue Planet” because 71 percent of the Earth’s surface is covered with water. Water also exists below land surface and as water vapor in the air. Water is a finite source. The bottled water that is consumed today might possibly be the same water that once trickled down the back of a wooly mammoth. The Earth is a closed system, meaning that very little matter, including water, ever leaves or enters the atmosphere; the water that was here billions of years ago is still here now. But, the Earth cleans and replenishes the water supply through the hydrologic cycle.
The earth has an abundance of water, but unfortunately, only a small percentage (about 0.3 percent), is even usable by humans. The other 99.7 percent is in the oceans, soils, icecaps, and floating in the atmosphere. Still, much of the 0.3 percent that is useable is unattainable. Most of the water used by humans comes from rivers. The visible bodies of water are referred to as surface water. The majority of fresh water is actually found underground as soil moisture and in aquifers. Groundwater can feed the streams, which is why a river can keep flowing even when there has been no precipitation. Humans can use both ground and surface water.
Distribution of the water on Earth
- Ocean water: 97.2 percent
- Glaciers and other ice: 2.15 percent
- Groundwater,: 0.61 percent
- Fresh water lakes: 0.009 percent
- Inland seas: 0.008 percent
- Soil Moisture: 0.005 percent
- Atmosphere: 0.001 percent
- Rivers: 0.0001 percent.
Surface water is far easier to reach, so this becomes the most common source of potable water. About 321 billion gallons per day of surface water is used by humans. About 77 billion gallons of groundwater are used each day. Problems also exist in contamination of the water supplies. This further limits the amount of water available for human consumption. Water is found in many different forms and in many different places. While the amounts of water that exist seem to be plentiful, the availability of the water for human consumption is limited.
Surface waters can be simply described as the water that is on the surface of the Earth. This includes the oceans, rivers and streams, lakes, and reservoirs. Surface waters are very important.
They constitute approximately 80 percent of the water used on a daily basis. In 1990, the United States alone used approximately 327,000 billion gallons of surface water a day. Surface waters make up the majority of the water used for public supply and irrigation. It plays less of a role in mining and livestock industries.
Oceans, which are the largest source of surface water, comprise approximately 97 percent of the Earth’s surface water. However, since the oceans have high salinity, the water is not useful as drinking water. Efforts have been made to remove the salt from the water (desalination), but this is a very costly endeavor. Salt water is used in the mining process, in industry, and in power generation. The oceans also play a vital role in the hydrologic cycle, in regulating the global climate, and in providing habitats for thousands of marine species.
Rivers and streams
Rivers and streams constitute the flowing surface waters. The force of gravity naturally draws water from a higher altitude to a lower altitude.
Rivers obtain their water from two sources: groundwater, and runoff. Rivers can obtain their water from the ground if they cut into the water table, the area in which the ground is saturated with water.
This is known as base flow to the stream. Runoff flows downhill, first as small creeks, then gradually merging with other creeks and streams, increasing in size until a river has formed. These small creeks, or tributaries, where the river begins are known as the headwaters. Springs from confined aquifers also can contribute to rivers.
A river will eventually flow into an ocean. A river’s length can be difficult to determine, especially if it has numerous tributaries. The USGS Web site defines a river’s length as “the distance to the outflow point from the original headwaters where the name defines the complete length.” In order for water to flow, there must be land upgradient of the river, that is land that is at a higher elevation than the river.
The land that is upgradient of any point on the river is known as the drainage basin or watershed. Ridges of higher land, such as the Continental Divide, separate two drainage basins. Flowing water is extremely powerful and plays an important role in creating the landscape and in humans’ lives.
rivers carry soil
Rivers carry soil and sediment that have been washed into the river when it rains or snow melts. The faster the water moves, the larger the particle size the river is capable of carrying. The USGS measures how much sediment a river carries by measuring the streamflow, or the amount of water flowing past a given site; and the sediment concentration.
Sediment in the river can be helpful and harmful. Sediment, when deposited on the banks and in the flood plain, makes excellent farmlands. However, sediment can harm and even destroy dams, reservoirs and the life in the stream.
The water cycle
Water molecules remain constant, though they may change between solid, liquid, and gas forms.
Drops of water in the ocean evaporate, which is the process of liquid water becoming water vapor. Evaporation can occur from water surfaces, land surfaces, and snow fields into the air as water vapor.
Moisture in the air can condensate, which is the process of water vapor in the air turning into liquid water. Water drops on the outside of a cold glass of water are condensed water.
Glaciers and ice caps
Glaciers and icecaps are referred to as storehouses for fresh water. They cover 10 percent of the world’s land mass. These glaciers are primarily located in Greenland and Antarctica. The glaciers in Greenland almost cover the entire land mass. Glaciers begin forming because of snowfall accumulation.
When snowfall exceeds the rate of melting in a certain area, glaciers begin to form.
This melting occurs in the summer. The weight of snow accumulating compresses the snow to form ice. Because these glaciers are so heavy, they can slowly move their way down hills.