Page 285 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

170°
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Bering Sea
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Gulf of Alaska
PACIFIC OCEAN
CANADA AND UNITED STATES ANNUAL POTENTIAL EVAPOTRANSPIRATION
cm
152 and over
137 122 107
91 76 61
46 and less
130°
ARCTIC OCEAN Beaufort
Sea
Baffin Bay
Hudson Bay
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Arctic Circle
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Global Water Balance Component
▲Figure 9.8 Potential evapotranspiration (PE) for Canada and the continental United States—
the demand. [From C. W. Thornthwaite, “an approach toward a rational classification of climate,” Geographical
Review 38 (1948): 64. adapted by permission from the american geographical Society. Canadian data from M. Sanderson, “The climates of Canada according to the new Thornthwaite classification,” Scientific Agriculture 28 (1948): 501–17.]
Satellite
surface tension), and by hydrogen bonding between water molecules and the soil. Most capillary water is available water in soil-moisture storage. After some water drains from the larger pore spaces, the amount of available water remaining for plants is termed field capacity, or storage ca- pacity. This water can meet PE demands through the ac- tion of plant roots and surface evaporation. Field capacity is specific to each soil type; the texture and the structure of the soil dictate available pore spaces, or porosity (discussed in Chapter 18).
Gravitational water is the water surplus in the soil body after the soil becomes saturated during a precipi- tation event. This water is unavailable to plants, as it
percolates downward to the deeper groundwater zone. Once the soil-moisture zone reaches saturation, the pore spaces are filled with water, leaving no room for oxygen or gas exchange by plant roots until the soil drains.
Figure 9.9b shows the relation of soil texture to soil- moisture content. Different plant species send roots to different depths and therefore reach different amounts of soil moisture. A soil blend that maximizes available water is best for plants (see the discussion of soil texture in Chapter 18).
When water demand exceeds the precipitation supply, soil-moisture utilization—usage by plants of the available moisture in the soil—occurs. As water is removed from
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90°
Chapter 9 Water resources
249
60° 50°
Labrador Sea
ATLANTIC OCEAN