Page 281 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 9 Water resources 245
Water in the Atmosphere
Figure 9.4 is a simplified model of the hydrologic sys- tem, with estimates of the volume of water involved in the main pathways (in thousands of cubic kilometres). We use the ocean as a starting point for our discussion, although we could jump into the model at any point. More than 97% of Earth’s water is in the oceans, and it is over these water bodies that 86% of Earth’s evaporation occurs. As discussed in previous chapters, evaporation is the net movement of free water molecules away from a wet surface into air that is less than saturated.
Water also moves into the atmosphere from land en- vironments, including water moving from the soil into plant roots and passing through their leaves to the air in a process called transpiration. During transpiration, plants release water to the atmosphere through small openings called stomata in their leaves. Transpiration is partially regulated by the plants themselves, as control cells around the stomata conserve or release water. On a hot day, a single tree can transpire hundreds of litres of water; a for- est, millions of litres. Evaporation and transpiration from Earth’s land surfaces together make up evapotranspira- tion, which represents 14% of the water entering Earth’s atmosphere in Figure 9.4.
Figure 9.4 shows that of the 86% of evaporation ris- ing from the oceans, 66% combines with 12% advected (moved horizontally) from the land to produce the 78% of all precipitation that falls over the oceans. The remaining
20% of moisture evaporated from the ocean, plus 2% of land-derived moisture, produces the 22% of all precipi- tation that falls over land. Clearly, the bulk of continen- tal precipitation comes from the oceanic portion of the cycle. The different parts of the cycle vary over different regions on Earth, creating imbalances that, depending on the local climate, lead to water surpluses in one place and water shortages in another.
Water at the Surface
Precipitation that reaches Earth’s surface as rain fol- lows two basic pathways: It either flows overland or soaks into the soil. Along the way, interception also occurs, in which precipitation lands on vegetation or other ground cover before reaching the surface. Inter- cepted water that drains across plant leaves and down their stems to the ground is known as stem flow. Pre- cipitation that falls directly to the ground, includ- ing drips from vegetation that are not stem flow, is throughfall. Precipitation that reaches Earth’s surface as snow may accumulate for a period of hours or days before melting, or it may accumulate as part of the snowpack that remains throughout winter and melts in the spring.
After reaching the ground surface as rain, or after snowmelt, water may soak into the subsurface through infiltration, or penetration of the soil surface (Figure 9.5). If the ground surface is impermeable (does not permit
94 20 Atmospheric advection
of water vapour
Cloud formation
14
69
2
Precipitation 106 22
12
57
Cloud 325 66 formation
86 419
Precipitation
382
78
Evaporation Transpiration
Evaporation
Ocean
Surface runoff
▲Figure 9.4 The hydrologic cycle model. Water travels endlessly through the hydrosphere, atmosphere, lithosphere, and biosphere. The triangles show global average values as percentages. note that all evaporation (86% 1 14% 5 100%) equals all precipitation (78% 1 22% 5 100%), and advection in the atmosphere is balanced by surface runoff and sub- surface groundwater flow when all of earth is considered.
Animation
Earth’s Water and the Hydrologic Cycle
8 37 flow
Groundwater
Continent
Percent (global average) Volume (thousands km3)