Water Cycle and Watersheds

Every drop of water you have ever drunk has been on Earth for billions of years. The same water that filled a dinosaur’s puddle might today be in your glass. That is because water never gets used up — it just keeps moving in a giant loop called the water cycle. The Sun powers it, plants help it along, and the land directs where it goes.

The Six Big Stages

Scientists describe the water cycle as six connected stages. Each stage changes water from one form (liquid, gas, or solid) to another, or moves it from one place to another.

1. The Sun heats water on the surface (oceans, lakes, rivers, puddles).
2. Evaporation — heat turns liquid water into invisible water vapor (gas) that rises into the air.
3. Transpiration — plants release water vapor through tiny holes in their leaves.
4. Condensation — high in the cool atmosphere, water vapor turns back into tiny liquid droplets, forming clouds.
5. Precipitation — when droplets get heavy enough, they fall back to Earth as rain, snow, sleet, or hail.
6. Runoff and infiltration — water flows downhill into rivers and lakes (runoff) or soaks into the ground to become groundwater (infiltration).

Three Phase Changes to Remember

• Evaporation: liquid water -> water vapor (gas). Needs heat.
• Condensation: water vapor (gas) -> liquid droplets. Happens when air cools.
• Precipitation: liquid (or frozen) droplets fall from clouds back to Earth.

What is a Watershed?

A watershed (also called a drainage basin) is all the land that funnels rain and melted snow into the same body of water. If you picture a giant bowl tilted toward a river, every drop that falls inside the bowl will end up in that river. The high ground around the bowl — usually a ridge or mountain crest — is called the divide. Rain that falls on one side of a divide flows into one watershed; rain on the other side flows into a different one.

• Watershed = all the land that drains into the same river, lake, or ocean.
• Divide = the high ridge that separates two watersheds.
• Tributary = a smaller stream that joins a bigger river inside the watershed.
• Groundwater = water that has soaked into the soil and rock below the surface.

Why Watersheds Matter

Whatever happens on the land in a watershed ends up in the water. Fertilizer from a farm, oil from a parking lot, or trash on a sidewalk can all be carried by runoff into the river that everyone in the watershed drinks from. Healthy watersheds — with forests, wetlands, and clean ground — filter water and slow runoff. Damaged watersheds let pollution and floods reach the river fast.