The Water Cycle

• Also called the hydrologic cycle• Describes the continuous movement of water

on, above, and below the surface of the Earth• The balance of water on Earth remains fairly

constant over time• Where does the water cycle start?

Where Is the Water?

• Surface water is fresh water on Earth’s land surface

• Lakes, rivers, streams, and wetlands• Distribution of surface water has played a vital role in

the development of human societies. Many cities and towns were built near reliable water sources, and today most large cities still depend on surface water for irrigation, power for industry, food, a means for

transportation by boat, and….. drinking water

Surface Water• River Systems:– Streams form as water from falling rain and

melting snow drains from mountains, hills, plateaus, and plains.

– As streams flow downhill they combine with other streams and form rivers.

Surface Water• River Systems:– Streams form as water from falling rain and

melting snow drains from mountains, hills, plateaus, and plains.

– As streams flow downhill they combine with other streams and form rivers.

• Watersheds:– The area of land that is drained by a river.– Pollution anywhere in the watershed may end up

polluting a river…

Watersheds

• Pollutants from the Mississippi watershed area are carried into the Gulf of Mexico

• Excess nutrients from agricultural fertilization during the growing season enter the gulf and cause the Gulf Dead Zone

• The dead zones are also called hypoxic zone, meaning ‘low oxygen’

Gulf dead zone

Dead Zones• Caused by an excess of nutrients, especially

nitrogen and phosphorous that are carried into oceans by rivers due to runoff from agricultural fertilization

• There are about 200 dead zones world wide• By 2030, the rivers of the world will pump 14% more

nitrogen into seas and oceans than that found in the mid-90s

• The number and size of oxygen-deprived zones has grown each decade since the 1970s

Where Is the Water?• Most of the water available for human use

cannot be seen– it exists underground.• Groundwater is fresh water that is stored

beneath the Earth’s surface in sediment and rock formations.

• An underground formation that contains groundwater is called an aquifer

Ogallala Aquifer• Holds about 4 quadrillion liters of water• Total water storage is about equal to that of

Lake Huron• Formed from glaciers that melted at the end

of the last Ice Age, 12,000 years ago• Today, it supplies about one-third of all the

groundwater used in the U.S.

Ogallala Aquifer

• It provides nearly all the water for residential, industrial, and agricultural use in the High Plains region

• Because of the Ogallala, the High Plains region is the leading irrigation area in the Western Hemisphere– Overall 13.6 million acres are irrigated in Ogallala

region– Nebraska uses the most water for irrigation followed

by Texas and then KANSAS

The Ogallala is being both depleted and polluted:

• Since large-scale irrigation began in the 1940s, parts of the Ogallala have lowered more than 100 feet

• Since the 1980s & 90s, water overdraft has averaged approximately 2.7 ft. per year

• Water is now being withdrawn from the aquifer 10 to 40 times faster than it is being replaced

Recharge of the Ogallala

• A Recharge Zone is an area of the Earth’s surface where water percolates down into the aquifer

• Compared to the area the Ogallala covers, it’s recharge zone is fairly small– Much of the Ogallala is overlain by a shallow layer

that is nearly impermeable– Much of the plains is semi-arid and has steady

winds that increases evaporation rates

The Ogallala is being both depleted and polluted:

• Groundwater contamination in the Ogallala has been an issue since the 90s

• Water samples have detected traces of pesticides and nitrates

• Main sources of pollution are irrigated agriculture and livestock feeding operations

Where Is the Water?

• If you go nearly anywhere in the world and dig a hole deep enough, you will eventually find water.– A hole that is dug or drilled to reach groundwater

is called a well