4The Hydrologic CycleMost of the fresh water on the Earth’s surface is found in moving and in stand water.Rivers, streams and springs are moving water.Ponds, lakes, and swampy wetlands are standing water.
5Water CycleThe hydrologic cycle or water cycle is the movement of water from the oceans and freshwater sources to the air and land and finally back to the oceans.The water cycle constantly renews the Earth’s supply of fresh water.
6Steps of the CycleEvaporationCondensationPrecipitation
7EvaporationThe first step involves the heat energy given off by the sun. This energy causes water on the surface of the Earth to change to water vapor.This process is called evaporation.
8Sources of Evaporation Enormous amounts of water evaporate from the oceans.Water also evaporates from freshwater sources and from the soil.Animals and plants release water vapor into the air.
9Condensation The second step is condensation. Condensation is the process by which water vapor changes back into a liquid.
10CondensationFor condensation to occur, the water vapor must be cooled.This happens when the warm air close to the surface of the Earth rises.As it moves farther from the Earth, it cools.Cool air holds less vapor and it condenses into droplets of water known as clouds.
11Water VaporWhen water evaporates from the ocean, it leaves the salts behind.Water vapor is made of fresh water only.
12Precipitation The third step is precipitation. Precipitation occurs when the water returns to the Earth in the form of rain, snow, sleet or hail.
13PrecipitationPrecipitation occurs when the water droplets that form in clouds become too numerous and heavy to remain afloat in the air.It returns to the Earth as rain, snow, sleet or hail.After it falls, some returns to the atmosphere through evaporation and the cycle is complete.
14GroundwaterSome of the water that falls as precipitation may run off into land, ponds, streams, river or oceans.Some may soak into the ground and become groundwater.At some point, the groundwater flows underground to the oceans.
15Frozen Water Snow is a frozen form of water. The pressure of piled-up snow causes some of the snow to change into ice and eventually glaciers form.A glacier is a huge mass of moving ice and snow.
16GlaciersGlaciers form in very cold areas. Glaciers hold 2% of fresh water.Because of the extremely cold temperatures, the snow that falls does not melt completely. As more snow falls it do covers the old snow. The pressure squeezes the snow crystals together and ice forms. When the layer of ice become very thick and heavy, the ice begins to move.
17Valley GlaciersLong, narrow glaciers that move downhill between the steep sides of mountain valley are called valley glaciers.Usually valley glaciers follow channels carved by running water. The movement of the glacier downhill causes cracks in the ice known as crevasses.
18Valley GlaciersAs valley glaciers slide downward, they tear rock fragments from the mountainside.The rock fragments become frozen in the glaciers and cut deep grooves in the valley walls. Mountains located anywhere from the equator to the poles can contain glaciers.Mount Rainier in Washington and Mount Washington in New Hampshire contain small glaciers.
19MeltwaterAs the valley glacier moves, some of the ice begins to melt, forming a stream of water, called meltwater.Meltwater is usually pure water and some cities such as Boulder, CO use it as a source of drinking water.Some meltwater is used to generate electricity although it is costly and could alter the environment.
20Continental GlaciersIn the polar regions, snow and ice have built up to form thick sheets, called continental glaciers.Continental glaciers cover million of square kilometers of the Earth’s surface.They move slowly in all directions.Continental glaciers are found in Greenland (85%) and Antarctica (98%).
21IcebergsAt the edge of the sea, continental glaciers form overhanging cliffs. Large chunks of ice, called icebergs, often break off from these cliffs and drift into the sea. Some icebergs are as large as the state of Rhode Island.Icebergs pose a major hazard to ships, as in the sinking of the Titanic in 1912.
22Fresh Water in Icebergs Much fresh water is frozen in icebergs.Attempts have been made to tow icebergs to areas that need fresh water.There are problems with transporting icebergs. First the effect of icebergs on local weather must be considered. Second, it would be costly and time-consuming. Last, ways are needed to stop melting.
23Running WaterRivers and streams are important sources of fresh water. Many towns and cities build near rivers and streams.The water is used to irrigate crops, generate electricity, drinking and other household purposes.Rivers are also used for recreational purposes.Rivers have been used to remove waste.
24Surface RunoffRain and melted snow that do not evaporate or soak into the soil flow into rivers and streams. The water that enters a river or stream after a heavy rain or spring thaw is called surface runoff.
25Amount of RunoffThe amount of surface runoff is affected by several factors. One is the type of soil the precipitation fall on. Some soils are more porous and soak up more water.The condition of the soil is also important. Dry soil will soak up more. Wet soil will accept less waterPlant roots absorb water. In areas where there are many plants, more water is absorbed.
26WatershedA land area in which surface runoff drains into a river or a system of rivers is called a watershed.Water sheds prevents floods and water shortages by controlling the amount of water that flows into streams and rivers.Watersheds also provide a steady flow of fresh water into the oceans.
27Usefulness of RiversMany rivers are sources of fresh water. The amount of water in a river and the speed at which the water flows affect the usefulness or a river as a source.Rivers that move quickly carry more water. They also carry a lot of soil, pebbles and sediments. Slow moving rivers appear more clear.Pollution also affects the usefulness of a river.
28Standing WaterWithin a watershed, some of the surface runoff gets caught in low places. Standing bodies of fresh water are formed there.Depending on their size, these bodies of water are called ponds or lakes, which provide fresh drinking water.Surface runoff keeps the ponds and lakes from drying up. The ponds and lakes control flooding and hold water in reserve.
29LakesLakes are usually large, deep depressions in the Earth’s crust that have filled with fresh water. Rain , melting snow and water from springs and rivers fill these depressions.A lake is sometimes formed when there is a natural blockage of a river or stream.Lakes are usually found at high altitudes where glaciers were once present.
30PondsPonds are shallow depression in the Earth’s crust that have filled with fresh water.They are smaller and not as deep as lakes.Sunlight penetrates to the bottom of a pond so plants can be found throughout a pond.
31ReservoirsThe most frequent source of fresh water are artificial lakes known as reservoirs.A reservoir is built by damming a stream or river that runs through a low lying area.Reservoirs help control flooding and store water.They sometimes provide irrigation for crops.They can also be used to generate electricity.
32GroundwaterThere is more fresh water below the surface of the land than in all the lakes and reservoirs on the Earth’s surface.Ground water moves slowly downward through pores in the rocks and soil.Material through which water can move quickly is described as permeable.Water cannot move quickly through impermeable material.
33Zone of SaturationGroundwater continues to move downward through permeable rock and soil until it reaches an impermeable layer. This underground zone in which all the pores are filled with water is called the zone of saturation.
34Zone of AerationAbove the water-filled zone, the ground is not as wet. Pores in the soil and rocks are filled mostly with air. This drier region in which the pores are filled mostly with air is call the zone of aeration.
35Water TableThe surface between the zone of saturation and the zone of aeration is an important boundary. It marks the level below which the ground is saturated, or soaked, with water. This level is called the water table
36Depth of the Water Table The depth of the water table varies based on location, climate, weather and man-made structures.
37Water Table LocationIn general, the water table is not very deep near large bodies of water.In high areas nears hills or mountains, the water table may be deep within the ground.In low-lying areas such as valleys with swamps and marshes, the water table may be close to the surface.
38Water Table and Climate The depth of the water table varies with the climate of an area.It will be deep in dry areas such as deserts.It will be near the surface in low-lying forest areas.In very moist climates, the water table may come right to the surface and form a swamp, lake or spring.
39Water Table and Weather Even in the same area, the water table may change.Heavy rains and snow may make the water table rise.If there is a long dry period, the water table will fall.
40Water Table and ManThe depth of the water table will also change if wells are overused or if many wells are located in a small area. Wells are holes drilled to bring the water table to bring water to the surface.
41Problems with Water Table In areas where the water table is high, it may flood foundations for buildings.It is easier and less expensive to dig wells in areas where the water table is high.
42AquifersAs groundwater moves through a permeable rock layer, it often reaches an impermeable rock layer or the water table. At this point, the groundwater may move sideways through a layer of rock or sediment that allows it to pass freely. Such a layer is called aquifer. Aquifers are usually layers of sandstone, gravel, sand or cracked limestone.
43AquifersBecause rocks form in layers, sometimes a layer of permeable rock may become trapped between two layer of impermeable rock. Sandstone trapped between two layers of shale is an example.An aquifer is a source of groundwater that is very vulnerable to pollution.
44Artesian WellsIn some places where the underground rock layers slope, an aquifer carries water from a higher altitude to a lower altitude. If the aquifer is trapped between two layers of impermeable rock, pressure may build up at the lower altitude. A well drilled in to the aquifer at this point will provide water without pumping. It is called an artesian well.
45CavernsIn some areas, the underlying rock is limestone. As groundwater moves down through the soil, it combines with carbon dioxide to form carbonic acid that can dissolve limestone. When water enters the cracks in limestone, the carbonic acid causes the cracks to become wider until large underground caverns with passage are formed.
46Stalactites and Stalagmites These underground caverns have long stone icicles hanging from the ceiling (stalactites) and built up from the floors (stalagmites.)Stalactites and stalagmites are formed when dissolved substances in groundwater are deposited.
47Composition of WaterA water molecule is the smallest particle that has all the properties of water.A water molecule forms when two atoms of hydrogen and one of oxygen combine.In a water molecule the atom of oxygen has a slight negative charge. Each atom of hydrogen has a slight positive charge.These charged ends give a water molecule the property known as polarity.
48Water as a SolventIt is the property of polarity that gives water molecules that makes water solvent.A solvent is a substance in which another substance dissolves. The dissolving process produces a solution.A solution contains two or more substances mixed on the molecular level.Water is known as the universal solvent.
49Hardness of WaterThe taste, odor and appearance of water varies from area to areas. The differences depend on the amounts and types of materials dissolved in the water.Hard water contains larges amount of dissolved minerals, especially calcium and magnesium.Soft water does not contain these minerals.
50Soft WaterSome water is softened naturally as it passes through and reacts with rock formations that contain certain minerals.These minerals remove the calcium and magnesium from the water, making it soft.
51Quality of WaterWater is necessary to all life on Earth. It is therefore necessary to maintain the quality of our water.In nature, water is usually filtered as it passes through sand and soil. Because so many different substances can dissolve in water, it is becoming more and more polluted.
52Pollution of WaterWater pollution limits the amounts and kinds of wild life that can live in water. It also affects drinking water supplies. It destroys recreational areas. Nitrates and phosphates from fertilizers pollute groundwater. They must be removed before the water can be used for drinking or swimming.Federal laws prevent industries from dumping certain chemicals wastes.