After this lesson, students should be able to:

Hydrology is the science that encompasses the occurrence, distribution, movement and properties of the waters of the Earth and their relationship with the environment within each phase of the hydrologic cycle. The hydrologic (water) cycle is a continuous process by which water is purified by evaporation and transported from the Earth's surface (including the oceans) to the atmosphere and back to the land and oceans. All the physical, chemical and biological processes involving water as it travels its various paths in the atmosphere, over and beneath the Earth's surface and through growing plants, are of interest to those who study the hydrologic cycle. Water may take many pathways in its continuous cycle of falling as rainfall or snowfall and returning to the atmosphere. It may be captured for millions of years in polar ice caps. It may flow to rivers and finally to the sea. It may soak into the soil to be evaporated directly from the soil surface as it dries, or it may be transpired by growing plants. It may percolate through the soil and rocks and collect in groundwater reservoirs (aquifers), or it may flow to wells or springs or back to streams by seepage. The cycle for a drop of water may be short, or it may take millions of years. See The Water Cycle Visual Aid for a diagram of the hydrologic cycle.

People tap the water cycle for their own uses. Water is diverted temporarily from one part of the cycle by pumping it from the ground or drawing it from a river, lake, well or aquifer. It is used for a wide range of activities, such as household, business and industry; for agricultural and parkland irrigation; and for the generation of electric power. After use, water is returned to another part of the cycle: perhaps discharged downstream or released to soak into the ground.

While evaporation is just one part of the hydrologic cycle, it is a crucial link between water on the Earth's surface and water vapor and clouds in the atmosphere. Many people do not realize how evaporation is just as important as precipitation to the water cycle.

Crashing waves aid water evaporation from the ocean. copyright

Water evaporates from streams, lakes, oceans and even the soil beneath our feet. Most people understand precipitation, which returns water to the Earth's surface, providing us with opportunities play in the rain and build snowmen. But, evaporation is less understood. To our eyes, it looks like water just disappears when it evaporates. Evaporation can be visualized by thinking of boiling water and the resulting vapor that you can see and feel. But, evaporation does not have to occur at such high temperatures (boiling); evaporation from lakes, rivers and oceans takes place at much lower temperatures. Evaporation only takes place at the surface of the water. The thin layer of water molecules that are exposed to the air and sun are the ones that evaporate. Therefore, an amount of water with a large, exposed surface area evaporates faster than one with a smaller surface area exposed at the same temperature and pressure.

The operation of a reservoir is impacted by evaporation. People who analyze the water budget of a reservoir, usually in the form of a computer model, take evaporation into account when determining how much water should be in the reservoir today, as well as how much is expected and needed to be in the reservoir next month. This budget, in turn, affects the supply of water available for irrigation, domestic and industrial use.

Extra Credit Questions: How is a reservoir with a large surface area different than one with a small surface area, in terms of evaporation? (Answer: The surface area is the very top layer of the water that is in contact with the air. A reservoir with a large surface area is one that is more spread out, with more of its water in contact with the air, so it has the potential for more evaporation than a reservoir with a smaller surface area.) How does surface area figure into engineers' calculations in planning a dam and reservoir? (Answer: More surface area increases the amount of evaporation, so to minimize evaporation, engineers look for ideal reservoir locations — deep, narrow canyons — so that as little reservoir water surface area as possible is created by the dam.)