Water on Earth moves between the oceans and land through the processes of evaporation and condensation.

The standards in this set range from a global understanding of the water cycle to knowing the source(s) of water used in the local community. The global water cycle provides an important perspective on how limited the supply of fresh water is. This understanding combined with knowledge of local water sources/uses can motivate and inform people with respect to how they and their community can conserve and recycle water (Standard 3d). The CONTENT TIPS for Standard 3d and for Standard 3e provide information and resources focused on California with respect to sources of local water and water conservation. This California-specific information tends to be limited in the instructional materials.

The global water cycle is an example of a very important feature of our planet. Essentially all the matter on Earth has been here since the planet was formed. We don’t get new matter (except for very small amounts arriving from outer space); old matter does not go away into outer space. Earth is essentially a closed system with respect to matter.

The hydrogen and oxygen atoms in water molecules have been here for billions of years. They keep changing form as they transition from liquid water evaporating to the gas state which condenses back to liquid and may even freeze to the solid state. The water cycle traces how these water molecules change their physical state and location.

Here is another way to understand the water cycle. Think about one of our ancestors who lived in Africa a million years ago. Or think about a dinosaur that lived 70 million years ago. Or consider a buffalo that roamed the American Midwest millions of years before the arrival of humans. No matter which you choose to bring to mind, that organism drank water throughout its life. The water was present in every drink and in every grain, fish, or flesh that was consumed. The water molecules became part of that organism’s body, and then flowed back into the world as blood, sweat, urine, and exhaled water vapor.

Now fill a glass with water. The glass that you hold in your hand today has more than ten million water molecules that passed through that body of the buffalo, more than ten million water molecules that passed through the particular dinosaur, and more than ten million water molecules that passed through each one of our African ancestors. The water that we drink connects us intimately with the living beings that inhabited the planet before us, that inhabit Earth today, and that will inhabit it in the future.

As in all the strands, these standards can be taught in many ways and in many sequences. The order in which they were written does not imply that they are supposed to be taught in that sequence. The concept map below provides one way to organize these standards. The wording of some of the standards has been slightly changed for space reasons and to emphasize a particular conceptual flow.

[A different concept map for these standards is on page 50 of the book "Making Connections" available from the California Science Teachers Association (CSTA).]