Page 284 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

248 part II The Water, Weather, and Climate Systems Arctic Circle
CANADA AND UNITED STATES ANNUAL PRECIPITATION cm
> 200 150–199 100–149 50–99 25–49 < 25
50°
50°
PACIFIC OCEAN
40°
30°
40°
30°
20°
r
ance
C
c of
120° 110° 90° 80°
▲Figure 9.7 Precipitation (P) in Canada, the continental United States, and northern Mexico— the supply. annual precipitation (water supply, P) in Canada and the United States. [adapted from nWS, US Department of agriculture and environment Canada.]
Water Storage: Soil Moisture As part of the water bud- get, the volume of water in the subsurface soil-moisture zone that is accessible to plant roots is soil-moisture storage (ST). This is the savings account of water that receives deposits (or recharge) and provides for with- drawals (or utilization). The soil-moisture environment includes three categories of water—hygroscopic, capil- lary, and gravitational. Only hygroscopic and capillary water remain in the soil-moisture zone; gravitational water fills the soil pore spaces and then drains down- ward under the force of gravity. Of the two types of water that remain, only capillary water is accessible to plants (Figure 9.9a).
Hudson Bay
ATLANTIC OCEAN
400 KILOMETRES
70°
Satellite
Global Water Balance Components
When only a small amount of soil moisture is present, it may be unavailable to plants. Hygroscopic water is inac- cessible to plants because it is a molecule-thin layer that is tightly bound to each soil particle by the hydrogen bond- ing of water molecules. Hygroscopic water exists in all climates, even in deserts, but it is unavailable to meet PE demands. Soil moisture is at the wilting point for plants when all that remains is this inaccessible water; plants wilt and eventually die after a prolonged period at this de- gree of moisture stress.
Capillary water is generally accessible to plant roots because it is held in the soil, against the pull of gravity, by hydrogen bonds between water molecules (that is, by
Gulf of Mexico
0
200
Tropi