Transpiration and the Ascent

202 7. Tlanspiration
Table 7.1 Relative Water Losses by Transpiration and Evaporation from an Illinois
Cornfield during the Period from Mid-June to Early September
Total evapo- Transpiration Tbnspiration Excess of -transpiration
from from covered as percentage Total evapotranspiration
uncovered plot plot of evapotran- precipitation over rainfall
Year (cm) (cm) spiration (cm) (cm)
1954 32.25 16.5 51% 18.5 1375
1955 34.50 17.5 51% 23.0 1L50
1957 3375 15.J 45% 24..0 975
aFrom Peters and Russell (1959)
The quantitative importance of transpiration is indicated by the fact that a
well-watered Kansas corn plant loses about 200 liters of water during a growing
season or nearly 1300 tons per acre (= 475 metric tons per hectare or about
28 cm of water) (Miller, 1938, p. 412). An Illinois corn field transpired an
amount of water equal to 60 to 90% of the precipitation during the growing
season (Peters and Russell, 1959) and the combined evaporation from the soil
and transpiration from the crop (evapotranspiration) exceeded the precipitation
during the growing season, as shown in Table 7.1. In contrast, a deciduous forest
in the more humid southern Appalachians transpired 40 to 55 cm per year,
which used only 25 to 35% of the annual precipitation (Hoover, 1944), as
shown in Table 7.2.
Several hundred grams of water are required to produce a gram of plant dry
matter (Table 7.3), but about 95% of this is lost in transpiration. If it were not
for the water dissipated by transpiration, a crop could be grown with the water
Table 7.2 Amounts of Water Lost in Various Ways by a North Carolina Watershed
Covered with a Deciduous Forest (1940-1941) and the Increase in Runoff
Which Followed Cutting of All Woody Vegetation and Elimination of
Transpiration (1941-1942)a
Process 1940-1941 1941-1942
Precipitation
Interception
Runoff
Soil storage
Evaporation
Transpiration
Note. Data in centimeters.
aFrom Hoover (1944),
158.0
16.6
53.4
-004
39.7
48.7
15804
9.5
93..0
9.7
46..0
00.0
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