Biomedical Engineering Reference
In-Depth Information
Advection
Condensation
Condensation
Condensation
Sublimation
Snow
Rain
Advection
Glacier
and Snow
Melt
Runoff
Rain
Evaporation
Evapor
r tion
Rain
Evaporation
Evaporation
River
Lake
Ocean
Infiltration
Percolation
Soil moisture
Water table
Grou n dwater
FIGURE 15.5
A schematic of water cycle.
the air and thus moves water vapor from more concentrated area (i.e. above water bodies) to
more arid areas (e.g. over mountains). When air temperatures cool down or water vapor is
transported to cooler areas, water vapor precipitates into liquid droplets to maintain a vapor
pressure at the saturation level (
p
0
A
) as shown in
Table 15.2
. Liquid water is much denser (or
heavier) than air. As shown in
Table 15.2
, the specific volume of liquid water is orders of
magnitude greater than the specific volume of water vapor. Water droplets fall toward earth
surface, forming rain.
Tabl e 15 . 1
shows the approximate amount of water at each key nodes of along the
water transport cycle. The largest reservoir is the interconnected oceans that contain about
96.5%ofthetotalwater.
Table 15.3
shows the water fluxes. Most water vapor over the
oceans returns to the oceans, but winds carry water vapor over land at the same rate as
runoff into the sea, about 36 Eg (or 36
10
15
kg)/year. This is the sustainable level we
are used to. Any large fluctuation over or below the 36 Eg causes alarm, as it interacts
with (influences or is influenced by) the temperature change on Earth's atmosphere.
Over land, evaporation and transpiration contribute another 71 Eg/year. Precipitation,
at a rate of 107 Eg/year over land, has several forms: most commonly rain, snow, and
hail, with some contribution from fog and dew. Condensed water in the air may also
TABLE 15.3
Hydrological Cycle of Water on Earth
Geological location
Evaporation into atmosphere, Eg/year
Receiving from rainfall, Eg/year
Oceans
434
398
Land
71
107
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