# Examining the Hydrologic Cycle

8.1 Examining the Hydrologic Cycle

1. Sketch, label, and discuss the hydrologic cycle.

Earth’s water is constantly moving between Earth’s surface and atmosphere. The hydrologic cycle describes the continuous movement of water from the oceans to the atmosphere, from the atmosphere to the land, and from the land back to the sea. Over most of Earth, the quantity of precipitation that falls on the land must eventually be accounted for by the sum total of evaporationtranspiration (the release of water vapor by vegetation), runoff, and infiltration.

A portion of the precipitation that falls on land will soak into the ground through a process called infiltration. If the rate of rainfall exceeds the ability of the surface to absorb it, the additional water flows over the surface and becomes runoff. Runoff initially flows in broad sheets that form tiny channels called rills. The rills merge to form gullies, which eventually join to create streams. Erosion by both groundwater and running water wears down the land and shapes Earth’s surface.

Figure 8.1 illustrates Earth’s water balance, a quantitative view of the hydrologic cycle. The figure implies a globally uniform exchange of water between Earth’s atmosphere and surface, but factors such as climate, steepness of slope, surface materials, vegetation, and degree of urbanization produce local variations.

SmartFigure 8.1 Earth’s water balance, a quantitative view of the hydrologic cycle.

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SmartFigure: The Water Cycle

Activity 8.1: Examining the Hydrologic Cycle

Use Figure 8.1 as a reference to complete the following:

1. Globally, from which source does more water evaporate into the atmosphere: oceans or land?

2. Approximately what percentage of the total water evaporated into the atmosphere comes from the oceans?

Percentage from oceans=Ocean evaporationTotal evaporation×100%=−−−−−−−−−−−−− %Percentage from oceans=Ocean evaporationTotal evaporation×100%=_ %

3. Notice in Figure 8.1 that more water evaporates from the oceans than is returned directly by precipitation. If sea level is not dropping, identify a source of water for the oceans in addition to precipitation.

4. Worldwide, about how much of the precipitation that falls on the land becomes runoff: 35, 55, or 75 percent?

5. Much of the water that falls on land does not immediately return to the ocean via runoff. Instead, it is temporarily stored in reservoirs such as lakes. In some mountainous and polar regions, what features serve as reservoirs to temporarily store water?

6. Label the drawing in Figure 8.2 with the letters that correspond to the following terms:

Figure 8.2Illustration (cross section) of the hydrologic cycle.

9.3 Glaciers and Ice Sheets

1. Contrast alpine (valley) glaciers and ice sheets.

Present-day glaciers cover nearly 10 percent of Earth’s land area. At the height of the Quaternary Ice Age, glaciers were three times more extensive than they are today. These moving masses of ice create many unique landforms and are part of an important link in the rock cycle in which the products of weathering are transported and deposited as sediment.

glacier is a thick ice mass that, over hundreds or thousands of years, forms on land as the yearly snowfall exceeds the quantity of ice lost by melting. A glacier appears to be motionless, but it is not; glaciers move very slowly. Thousands of glaciers exist in lofty mountain areas, where they usually follow valleys originally occupied by streams. Because of their settings, these moving ice masses are termed valley glaciers, or alpine glaciers.

Ice sheets (sometimes called continental glaciers) exist on a much larger scale than valley glaciers. These enormous masses flow out in all directions from one or more snow-accumulation centers and completely obscure all but the highest areas of underlying terrain. Presently each of Earth’s polar regions supports an ice sheet—Greenland in the Northern Hemisphere and Antarctica in the Southern Hemisphere.

Glacial erosion and deposition leave unmistakable imprints on Earth’s surface (Figure 9.5). In regions once covered by ice sheets, glacially scoured surfaces and subdued terrain dominate. By contrast, erosion caused by alpine glaciers accentuates the irregular mountainous topography, often producing spectacular scenery characterized by sharp, angular features. Glacial deposits are usually visible in both settings.

Figure 9.5Moving glacial ice, armed with sediment, acts like sandpaper, scratching and polishing rock and creating glacial striations.

(Photo by Michael Collier)

Activity 9.3: Glaciers and Ice Sheets

1. What percentage of Earth’s land surface do glaciers presently cover?

%

2. Identify two locations where ice sheets are currently found.

3. Briefly compare an ice sheet to a valley glacier.

Activity 9.6B: Identifying Glacial Features on a Topographic Map

Refer to Figure 9.13, a portion of the Holy Cross, Colorado, topographic map. This is a mountainous area that has been shaped by alpine glaciers.

(Courtesy of U.S. Geological Survey)

1. Identify the glacial feature indicated by each of the following letters. Use Figure 9.12C as a reference.

· Letter A:

· Letter B:

2. A tarn is a lake that forms in a cirque. Of the features labeled C, D, E, and F, which indicate(s) a tarn(s)?

Letter(s):

3. The feature marked G on the map is a depositional feature composed of glacial till. What type of glacial feature is it? How did it form?

4. Explain how Turquoise Lake likely formed.

5. Use Figure 9.14 to draw a topographic profile along the X–Y line from Sugar Loaf Mountain to Bear Lake and mark the position of the Lake Fork stream. (Use only index contours.)

Figure 9.14Topographic profile of the valley of Lake Fork on the Holy Cross, Colorado, map.

6. Describe the shape of Lake Fork Valley, based on your profile.

7. What glacial feature is Lake Fork Valley?

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