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Explain the steps and reservoir interactions in the water cycle.

The Cycling of Water

Water is the only substance on Earth that occurs naturally as a solid, a liquid, and a gas. Water also has a high specific heat, which means that water is resistant to changes in temperature. About 97% of Earth’s water is in the oceans, which is about 1.3 billion km³ of water, and only about 1% of water on Earth is freely available as fresh water on the surface. Interestingly, the Earth’s mantle is the largest reservoir of water, which has 10 times the volume of the oceans in water.

Showing circulation of water between the evapotranspiration loop, groundwater loop, & surface water loop — ***The water cycle is also known as the hydrologic cycle and has three major loops: evapotranspiration, surface water, and groundwater.***

Evapotranspiration Loop

The graphic depicts evapotranspiration’s effect on the water table, with trees, soil, and water table cross-section. It shows the water cycle’s influence through arrows. — ***Water vapor becomes clouds and falls as rain, refilling land and oceans.***

The evapotranspiration loop is powered by the sun’s heat energy, which causes evaporation, when liquid water changes to become gaseous water (water vapor). You may have heard meteorologists talk about the humidity in an area. This is the amount of water vapor in the air. When water evaporates from plants, it is known as transpiration.

An important part of the evapotranspiration loop is that water is purified in this loop because water evaporates, but the salts and other nutrients in the water do not. Therefore, the evaporated water is just water, nothing else.

The atmosphere is a reservoir for water in the evapotranspiration loop. Water in our atmosphere tends to turn over, or cycle, every 10 days.

Essential Knowledge

The hydrologic cycle, which is powered by the sun, is the movement of water in its various solid, liquid, and gaseous phases between sources and sinks.

Groundwater Loop

The illustration simplifies evapotranspiration’s impact on the water table, showing a cross-section with labeled trees, soil, and water table. Green-leafed trees stand on brown soil above a blue water table, with arrows indicating precipitation and water movement. — ***Water in aquifers is groundwater and 99% of all Earth’s liquid freshwater.***

Infiltration is when water soaks into the ground and runoff is when water runs off the surface of the land.

Often, we refer to the infiltration-runoff ratio, which is the amount of water that soaks into the ground compared to the amount that runs off. Typically, natural areas have more infiltration than human areas, such as shopping malls and housing developments because soil is more porous than asphalt or concrete.

Some water that infiltrates stays in the soil. This water returns to the atmosphere by evaporation or transpiration by plants. This water is in the evapotranspiration loop. The rest of the water percolates through the soil to become groundwater. This is sometimes called gravitational water because it trickles down through pores or cracks under the pull of gravity.

Water exits the aquifer through springs, seeps, and wells. Springs are natural areas where a large amount of water flows out of a relatively small opening, seeps are places where water slowly seeps out of the aquifer, and wells are usually dug by humans to pump water out of the aquifer.

Surface Water Loop

rain falling on plant leaves — ***Precipitation can be rain, snow, or hail.***

Water in the surface water loop runs off the surface of the land into surface waters. These surface waters can be rivers, lakes, streams, ponds, or directly into the ocean. Almost all rivers and streams eventually flow into the ocean, so this water tends to end up in the ocean. Some of this water evaporates and becomes rain and some of this water is stored in the ocean, which is a reservoir for water.

The oceans absorb heat from the atmosphere. Then, when water evaporates from the oceans, it cools the oceans.

The surface water loop is important to other biogeochemical cycles because surface water dissolves or erodes nutrients on its way to the ocean, providing a vehicle to carry the other key nutrients.

Essential Knowledge

The oceans are the primary reservoir of water at the Earth's surface, with ice caps and groundwater acting as much smaller reservoirs.

Putting it all Together

The water cycle facilitates the continuous movement of water on Earth through evaporation, condensation, precipitation, and runoff. Sunlight drives the evaporation of water from various sources, forming water vapor that rises and condenses into clouds. Precipitation in the form of rain, snow, or hail then replenishes surface water and infiltrates the soil, contributing to groundwater recharge. In the surface runoff loop, water runs across the surface of the ground and becomes part of the surface water system. In this part, rivers and streams transport water back to the oceans. In the groundwater loop, water infiltrates and percolates down to join the groundwater and moves through the aquifers, exiting through seeps, springs, and wells.

AP Exam Tip

You should know how the water cycle affects the other cycles.

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