## Vocabulary

biogeochemical cycle groundwater water cycle transpiration carbon cycle nitrogen cycle nitrogen fixation nitrogen-fixing bacteria ammonification nitrification denitrification phosphorus cycle

Water vapor (clouds)

Water vapor (clouds)

figure 18-12

Groundwater

### Groundwater figure 18-12

In the water cycle, water falls to Earth's surface as precipitation. Some water reenters the atmosphere by evaporation and transpiration. Some water runs into streams, lakes, rivers, and oceans. Other water seeps through the soil and becomes groundwater. Follow the pathways of the water cycle in the figure.

Materials disposable gloves, lab apron, 3 L plastic bottle (cut in half), small stones (250 mL), dry sod with grass, water, graduated cylinder, 500 mL beaker

V^ll

### Modeling Groundwater

Materials disposable gloves, lab apron, 3 L plastic bottle (cut in half), small stones (250 mL), dry sod with grass, water, graduated cylinder, 500 mL beaker

Procedure

1. Put on your lab apron, goggles, and disposable gloves.

2. Invert the top half of the plastic bottle, and place it inside the bottom half of the bottle to form a column.

3. Place the stones in the bottom of the inverted top half of the bottle. Place a chunk of dry sod with grass on top of the stones.

4. Pour 250 mL of water over the sod, and observe how the water penetrates the soil and moves through the column.

5. When the water is no longer draining, remove the top half of the column, and pour the water from the bottom of the column into a beaker. Measure the volume of liquid in the beaker.

Analysis What is the volume of the water that drained through the sod? How much of the water remained in the soil? Where does the water go when applied to a real lawn or crop? What might the fate of fertilizer or pesticides be that are applied to a lawn or crop?

figure 18-13

Carbon exists in the atmosphere as carbon dioxide. Cellular respiration, combustion, and decomposition of organic matter are the three major sources of carbon dioxide in the short-term carbon cycle. By burning large amounts of fossil fuels, humans are releasing carbon dioxide from a long-term reservoir and increasing the amount of carbon dioxide in the atmosphere.

Evaporation adds water as vapor to the atmosphere. Heat causes water to evaporate from bodies of water, from the soil, and from the bodies of living things. The process by which water evaporates from the leaves of plants in terrestrial ecosystems is called transpiration. Transpiration causes plants to take in water through their roots to replace the water that is being lost through their leaves. Animals also participate in the water cycle. Animals drink water or obtain it from their food. They release this water when they breathe, sweat, or excrete.

Water leaves the atmosphere through precipitation. The amount of water the atmosphere can hold depends on abiotic factors, such as temperature and air pressure. Once the atmosphere becomes saturated with water vapor, precipitation occurs in the form of rain, snow, sleet, hail, or fog.

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