The Water Cycle Chapter 11-1

The Water Cycle

Water on the Earth H2O Almost all the water on Earth is salt water (97%). Of the three percent that is fresh water, 76% is frozen in the ice caps at the poles.

Groundwater Most of the fresh water that is not frozen is underground. The water that falls as precipitation sinks into the ground (permeable) and eventually reaches a layer it cannot pass through (impermeable).

The Water Cycle Continuous process by which water moves through the living and nonliving parts of the environment. The water cycle is driven by the sun.

Evaporation The process when the sun heats up the water in rivers, lakes, and oceans and turns it into vapor or steam. The water vapor goes into the atmosphere. Most of the water vapor that evaporates into the air comes from the oceans. The salt does not get enough energy to become a gas, so it stays behind. So rain is always fresh water!

Clouds Water vapor cools as it rises in the air. As it reaches the dew point, the water vapor will condense around dust particles and form clouds.

Precipitation Condensed water vapor that gets heavy enough to fall to the ground as rain, snow, sleet, or hail. Most water molecules only spend about ten days in the air before falling back as precipitation.

Surface Runoff Much of the precipitation runs off the surface and flows downhill into streams. These flow into larger streams, then rivers, and eventually flow back into the ocean.

Infiltration Infiltration is an important process where rain water soaks into the ground, through the soil and underlying rock layers. Some of this water ultimately returns to the surface at springs or in low spots downhill.

Groundwater Some of the water remains underground and is called groundwater.  As the water infiltrates through the soil and rock layers, many of the impurities in the water are filtered out. This filtering process helps clean the water.

Transpiration One final process is important in the water cycle. As plants absorb water from the soil, the water moves from the roots through the stems to the leaves. Once the water reaches the leaves, some of it evaporates from the leaves, adding to the amount of water vapor in the air. This process of evaporation through plant leaves is called transpiration. In large forests, an enormous amount of water will transpire through leaves.  The greatest living movers of water are plants. t

Earth’s Water Supply The total amount of water on the Earth has not changed much since early in its history. The same water is cycled over and over. The water you drink at lunch was probably drunk by a dinosaur millions of years ago!