Earth’s Fresh Water

Three States of Water solid (ice or snow) liquid (water) gas (water vapor)

The Hydrologic (Water) Cycle

Steps of the Cycle Evaporation Condensation Precipitation

Energy The Sun provides the energy that drives the water cycle. Energy needed for evaporation, changing liquid to gas (water vapor).

Evaporation The Sun’s energy can evaporate water from the ocean surface or from lakes, streams, or puddles on land. Only the water molecules evaporate; the salts remain in the ocean or a fresh water reservoir. Water vapor is made of fresh water only.

Condensation The second step is condensation. Condensation is the process by which water vapor changes back into a liquid.

Condensation For 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.

Precipitation The third step is precipitation. Precipitation 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.

Precipitation After precipitation falls, some returns to the atmosphere through evaporation and the cycle is complete. Water Cycle Song http://www.youtube.com/watch?v=Zx_1g5pGFLI Monthly Mean Precipitation Around the World http://en.wikipedia.org/wiki/File:MeanMonthlyP.gif

Transpiration Plants and animals depend on water to live, they also play a part in the water cycle. Plants take up water from the soil and release large amounts of water vapor into the air through their leaves, a process known as transpiration.

Stored Water Oceans, glaciers, ponds and the atmosphere are examples of reservoirs. The amount of time a water molecule remains in a reservoir is know as its residence time. Reservoir=storage location for water

Surface Water Rivers, streams, ponds, and lakes are extremely important to a variety of organisms. Streams are bodies of water that have a current. Geologists recognize many categories of streams depending on their size, depth, speed, and location. Creeks, brooks, tributaries, bayous, and rivers

Stream Headwaters A stream originates at its source, and at high elevations. A source could be springs or snow melt Water always flows downhill

When Waters Join Two streams come together at a confluence. The smaller stream is a tributary of the larger stream. As water slows and becomes deeper, natural pools can form. The mouth of a stream is where it enters into a large body of water (ocean or lake). If the fresh water stream is mixing with saltwater, an estuary will form.

Parts of a Stream

Standing Water Within a watershed, some of the surface runoff gets caught in low places. Standing bodies of fresh water are formed there. Ponds are small bodies of fresh water that usually have no outlet. Lakes are larger bodies of water that usually drains out through a river or a stream. Usually fresh water, although the Great Salt Lake in Utah is just one exception.

Reservoirs The 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.

Groundwater Some 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. Groundwater is the largest reservoir of liquid fresh water on Earth and is found in aquifers, porous rock and sediment with water in between.

Surface Runoff The amount of runoff is affected by several factors. type of soil (Some soils are more porous and soak up more water.) condition of the soil (Dry soil will soak up more, Wet soil will accept less water.) Amount of vegetation (Plant roots absorb water.)

Groundwater Ground water moves slowly downward through pores in the rocks and soil. Material through which water can move quickly is described as permeable. Water moves slowly through impermeable material.

Aquifers Water moves through permeable rock layer, until it 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. http://water.usgs.gov/ogw/aquiferbasics/carbrock.html The Castle-Hayne Aquia aquifer yields large volumes of water in North Carolina where it consists of limestone.

The zone of saturation is the rock layer in which pores are filled with water. Above this water-filled zone, the ground is not as wet. Pores in the soil and rocks are filled mostly with air. This drier region is called the zone of aeration.

Water Table The 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.

Depth of the Water Table The depth of the water table varies based on Location Climate Weather Man-made structures.

Location In 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.

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.

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.

Man-made Structures The 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 water to the surface from the water table. It is easier and less expensive to dig wells in areas where the water table is high.