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368 Chapter 11 Hydrology: Rainfall and Runoff
11.4.3 Transpiration

The amounts of water transpired vary with the kind and maturity of vegetation, conditions of
soil moisture, and meteorological factors. On the whole, the continents return more water to
the atmosphere by transpiration than by evaporation. The total areal expanse of the leaves in
a forest is very great in comparison with land and water exposures. Moreover, some plants
can draw water from considerable depths and transpire it. Estimates for the United States
place the proportion of precipitation lost to the atmosphere by evaporation and transpiration
from forests and land uses and from surface reservoirs and phreatophytes as high as 70%.

11.4.4 Measuring Evaporation and Transpiration
Evaporation from water surfaces is commonly measured by exposing pans of water to
the atmosphere and recording evaporation losses by systematic measurements or self-
registering devices. Both floating and land pans have been used (Fig. 11.4). Neither one
is fully satisfactory. The standard (Class A) measuring device of the Weather Bureau is a
4-ft (1.22-m) galvanized iron pan, 10 in. (254 mm) deep, supported on a grid of 2-in. by
4-in. (51 103 mm) timbers that raise the pan slightly above the ground to promote air
circulation. A hook gauge in a stilling well identifies changes in water level. Temperature,
rainfall, and wind speed are also recorded. The anemometer is placed next to and just
above the pan. Observed losses are not the same as for floating pans of different materials,
color, or depth; nor are they the same as for natural or impounded bodies of water.
Pan evaporation is translated empirically to lake evaporation by the following equation:
CE¿(P - P )
w
d
E = (U.S. Customary or SI Units) (11.11)
P¿ - P¿ d
w
where letters with primes pertain to pan conditions; the others to lake conditions. The co-
efficient C is said to average 0.7. In most of the United States, mean annual evaporation
from water surfaces equals or exceeds mean annual rainfall.
Together, transpiration and evaporation are referred to as evapotranspiration or con-
sumptive use. They are measured experimentally by agriculturalists in a number of ways.
In one of them evaporation is equated to the difference between rainfall on a plot of ground
and water collected by underdrains; in another, soil-water level is held constant in a tank
filled with representative materials growing representative plants. If the tank bottom is per-
vious, the resulting lysimeter measures consumptive use as the difference between (a) water

Anemometer
Hook gage

Stilling well Hook gage
Stilling well

(a) Land pan

Float

(b) Floating pan
Figure 11.4 Evaporation Pan