Uneven Distribution

Why is water distributed unevenly across the Earth's surface?

As you probably know, things are far more interesting than a hypothetical case of evenly distributed precipitation! Both precipitation and evaporation vary widely over the Earth's surface. This unequal distribution of water on the planet drives a diversity in climate and ecosystems (or biomes); water availability for human life, industry, and agriculture; and is fundamentally and intimately tied to the history of politics, economics, food production, population dynamics, and conflict – both in the U.S. and globally.

The abundance of water in some areas and scarcity in others follows systematic and predictable patterns. As part of this module, we'll explore the physical processes that shape the overall distribution of precipitation - and thus water resources.

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Figure 7 Map of average annual precipitations for the U.S. for 1981-2010
Source: PRISM Climate Group, Oregon State University, map created 2013.

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Figure 8. MODIS satellite image renderings of vegetation coverage for July 2013 (top) and January 2013 (bottom). Gradations of green indicating leafy vegetation with the darkest green reflecting highest coverage of plants. Brown colors are ice or desert and black is "no data" (largely the oceans). Note the seasonal differences most pronounced in the northern hemisphere.
Source:NASA Earth Observations (NEO)

Learning Checkpoint

Note: The questions below are not graded. They may show up as summative evaluation questions on mid-term or final exams.

1) Look at Figure 7 above. What is the annual mean precipitation in Southern Nevada?

1. 32 in/yr
2. greater than 80 in/yr
3. 0 in/yr
4. 4-8 in/yr

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2) Look at Figure 7 above. What is the annual mean precipitation in Coastal Washington State?

1. a. 32 in/yr
2. greater than 80 in/yr
3. 0 in/yr
4. 4-8 in/yr

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3) Why do you think Nevada and Eastern Oregon are deserts?

1. a. They are far North of the equator.
2. They are far from the ocean.
3. They are in the rain shadow of mountains.
4. They are subject to large annual temperature fluctuations.
5. They are at high elevation.

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4) Look at Figure 8. What do you think is the global pattern of precipitation?

1. a. It rains most South of the equator.
2. There is East-West "banding" of climate/precipitation.
3. There is North-South "banding" of climate/precipitation.
4. There is snow in the Southern Hemisphere year-round.

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Note the contrasting patterns in the two images in Figure 8 above, based on global satellite coverage. Vegetation in the southern hemisphere, which has relatively more ocean area (and less land area) than the northern hemisphere, changes little seasonally, whereas vegetation distribution in the northern hemisphere undergoes large changes. Why is that? There are probably two impacts on vegetation distribution—precipitation and temperature. Examine the figure below that illustrates the available moisture seasonally (summer vs. winter) and compare to the distribution of vegetation for the same seasons. Think about the role of temperature, precipitation, and soil moisture (water availability to plants), as well as the availability of sunlight for photosynthesis. Yes, there is a more complex relationship between plant growth and other factors, but the hydrologic cycle plays a major role.

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Figure 9. Contours of average atmospheric water vapor calculated as equivalent rainfall in millimeters on the basis of satellite observations for July 2003 (top) and January 2003 (bottom). Again note the strong seasonal differences in the northern hemisphere in particular. Can this explain the vegetation differences entirely?
Source: NASA AIRSThe Encyclopedia of Earth