Section 1: Water Resources • Some of the water we drink today has been around since water formed on Earth billions of years ago • Water is essential to life on Earth • Humans can only survive a for days without water
The Water Cycle • Earth has an abundance of water in all forms: solid, liquid, and gas • Water is renewable because it gets circulated through the water cycle where water molecules travel between the surface and atmosphere • Water vapor (a gas) rises into the air • Water vapor cools as it rises through the atmosphere and condenses into droplets forming clouds • Water falls back down to Earth as precipitation • Oceans are an important part because so much of water on Earth is in the ocean
Global Water Distribution • 71% of Earth's surface is covered with water, but 97% of water on Earth is salt water • Only a small percentage is liquid, fresh water that humans can use • It is found in lakes, rivers, and beneath Earth's surface
Surface Water • Surface water is fresh water on Earth's land surface • It's found in lakes, rivers, streams, and wetlands • Throughout history, people have built cities near reliable sources of surface water • Rivers, lakes, and streams provide drinking water, water to grow crops, food, power for industry, and transportation
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 • As streams and rivers move across land, they form a flowing network of water called a river system • The Mississippi, the Amazon, and the Nile are enormous river systems • The Amazon is the largest river system in the world
Watersheds • The area of land drained by the river is called a watershed • Pollution anywhere in a watershed may end up polluting a river • The amount of water entering a watershed changes throughout the year • Rapidly melting snow, spring and summer rains, etc. can dramatically increase the amount of water in a watershed • Communities depending on these watersheds can be severely affected by changes in the river system
Groundwater • Most of the freshwater available to humans cannot be seen because it exists underground • Water beneath Earth's surface in sediment and rock formations is called groundwater • The point where rocks and soil are saturated with water is called the water table • The water table can be at the surface or way below the surface
Aquifers • An aquifer is an underground formation that contains groundwater • The water table forms the upper boundary of an aquifer • Most aquifers are rock, sand, and gravel that have lots of space for water to accumulate • Groundwater can dissolve limestone rock formations filling vast caves with water creating underground lakes • Aquifers are an important water source for many cities and agriculture
Porosity and Permeability • Although rocks appear to be solid, many kinds of rocks contain small holes or pores • Porosity is the percentage of the total volume of a rock that has pores • The more porous a rock is, the more water it can hold • The ability of a rock or soil to allow water to flow through it is called permeability • Materials that allow water to flow through are permeable; materials that do not let water flow through are impermeable
Wells • If you go anywhere on Earth 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. • People dig wells because groundwater may be a more reliable source of water than surface water in some areas and because water is filtered and purified as it travels underground.
The Recharge Zone • To reach an aquifer, surface water must travel down through permeable layers of soil and rock • The area of the Earth’s surface from which water percolates down into an aquifer is called a recharge zone • Recharge zones are environmentally sensitive areas because any pollution in a recharge zone can also enter the aquifer • Size of recharge zone is affected by the permeability of the surface above the aquifer • Structures such as buildings and parking lots can act as impermeable layers to reduce the amount of water entering an aquifer, so recharge zones must be managed carefully.
Copy now, answer later • Describe the distribution of water on Earth. Where is most of the fresh water located? • Explain why fresh water is considered a limited resource. • Explain why pollution in a watershed poses a potential threat to the river system that flows through it. • Describe how water travels through rock. • Why is an underground lake an aquifer?