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366 Chapter 11 Hydrology: Rainfall and Runoff
2. Obstructions—trees, bushes, fences, and buildings—cast a rain shadow. To avoid
this, gauges are placed in the clear by a distance greater than the height of the
tallest obstruction. If gauges must be put on a roof, they should be centered in the
largest available flat area.
Engineers should be fully aware of the limitations of precipitation records. Aside from
errors of measurement, areal variations in precipitation may be large, even over small
stretches of country. In such circumstances, a single gauge cannot yield representative
information.

11.4 EVAPORATION AND TRANSPIRATION
Evaporation raises the storage requirements of reservoirs and lowers the yield of lakes and
ponds. Swamps and other wet surfaces, too, return much water to the atmosphere.

11.4.1 Evaporation from Water Surfaces
Rates of evaporation from open water surfaces vary with the temperature or vapor pressure
of the water and the air in contact with it. They also vary with wind speed, barometric pres-
sure, and water quality. Because these factors are by no means independent, individual
effects are not clear cut. In general, evaporation and gas transfer have much in common:

1. For large differences between the maximum vapor pressure at water-surface tem-
perature and the actual pressure of aqueous vapor in the overlying air, evaporation
is rapid. For small differences, evaporation is slow; for negative differences, there
is condensation.
2. At the temperatures of natural waters, the vapor pressure is almost doubled for
every rise of 10 C (18 F). Hence, temperature affects evaporation profoundly, yet
the slow warming and cooling of deep bodies of water make for relatively even
evaporation.
3. Films of still air above a water surface soon become saturated with moisture, and
evaporation practically ceases. Within limits, wind stimulates evaporation by
displacing moisture-laden films with relatively dry air.
4. As pressures drop, evaporation rises, but altitude has little effect because of coun-
terbalancing changes in temperature. Fast rates of evaporation at high altitudes are
caused, in large measure, by greater wind velocities.
5. Rates of evaporation are decreased slightly when salt concentrations are high.

Rohwer’s formula (1931) illustrates the dependence of evaporation on the factors
just cited:
-2
E = 0.497(1 - 1.32 * 10 P )(1 + 0.268w)(P - P ) (U.S. Customary Units) (11.9)
a
d
w
Where E is the evaporation, in in./d; P is the barometric pressure, in. Hg; w is the wind
a
velocity, in mi/h; and P and P are the vapor pressures, in. Hg, at the water temperature
w
d
and dew-point temperature of the atmosphere, respectively.
The following is an equivalent evaporation equation using the SI Units:
E = 0.497(1 - 5.2 * 10 -4 P )(1 + 0.167w)(P - P ) (SI Units) (11.10)
w
a
d
Where E is the evaporation, in mm/d; P is the barometric pressure, mm Hg; w is
a
the wind velocity, in km/h; and P and P are the vapor pressures, mm Hg, at the water
w
d