Chapter 15-1: River Systems 8th Grade Science: Pages 407- 410

Movement of Water on Earth More than 2/3 of the Earth is covered with water. Water is essential for life. Water is found in lakes, rivers, oceans, icecaps, underground, tissues of all living organisms, and in the atmosphere. Water Vapor- the invisible gas that water becomes in the atmosphere. Clouds and fog are other forms. Water cycle- continuous movement of water between the atmosphere, land and oceans

Water Cycle: Evapotranspiration Evaporation- process by which liquid water turns to water vapor. 500,000 km cubed evaporates each year. 86% of water evaporates from the ocean. Transpiration- When plants release water vapor into the atmosphere. Evapotranspiration- the total loss of water from an area, which = the sum of water lost from both evaporation and transpiration.

Water Cycle: Condensation and Precipitation Condensation- the change of state from a gas to liquid. As water vapor rises and cools, it condenses and changes into liquid water, forming clouds. Precipitation- when water falls from clouds. Includes rain, snow, sleet and hail. 75% falls on Earth’s oceans. The rest falls on land and becomes run-off or groundwater. Eventually, this water returns back to the atmosphere by evapotranspiration.

Water Budget Water Budget- the continuous cycle of evapotranspiration, condensation and precipitation. Just like a money budget (money going in and coming out), the water budget needs to be balanced. For Earth: Precipitation is the income, evapotranspiration and run-off are the expenses. The Earth’s water budget is always balanced; but the local water budget is usually not balanced.

What affects the water budget? Temperature, wind, amount of rainfall affects the water budget. When there is more precipitation, the result is moist soil and possible flooding. When there is more evapotranspiration, the soil is dry and irrigation may be necessary. Rain forests and deserts are examples of this. Vegetation reduces run-off but increases evapotranspiration. Wind also increases evapotranspiration. Most local water budgets also changes with the seasons. In cooler temps, evapotranspiration slows, so streams generally transport more water in cooler months.

Other factors… On average, each person in the USA uses 20, 890 gallons (95,000 L) of water each year. Bathing, washing clothes/dishes, watering lawns, carrying away wastes and drinking are the primary uses. Agriculture and industry also use a large amount of water. 90% of water used by cities and industry is returned to rivers and oceans as wastewater. This wastewater contains toxic chemicals and metals. These materials pollute rivers and harm plants and animals.

Conservation of Water Only a small amount of Earth’s water is freshwater (*not* salt). Conservation- the wise use of water. Desalinization- the process of removing salt from water. It is expensive and impractical for large populations. Conservation and desalinization are two ways to preserve fresh water.