Environmental Engineering Reference
In-Depth Information
rain during the night, yet how quickly it dries out during the day. Similar patterns occur
seasonally. Potential evapotranspiration reaches a peak during the summer months and
declines markedly during the winter (Figure 5.15). The magnitude of the peak depends
upon geographical location and climatic factors, especially cloud.
ENERGY INPUTS: WIND
The second important factor is the wind. The wind enables the water molecules to be
removed from the ground surface by a process known as
eddy diffusion
. This main-
Figure 5.15
Mean monthly precipitation in comparison with
mean monthly PE at Amarillo (Texas) and Cornwall. Note the
different scales to cope with the high PE values on the
continental plains of Texas. In each month PE is greater than
precipitation. In Cornwall, only in May-July is this found.
tains the vapour pressure gradient above the surface. Wind speed is obviously one of the
variables determining the efficiency of the wind in removing the water vapour, but it is
not the only one. The rate of mixing is also important, and that depends upon the
turbulence of the air and the rate of change of wind speed with height.
ENERGY INPUTS: THE VAPOUR PRESSURE GRADIENT
Third, evapotranspiration is related to the gradient of vapour pressure between the surface
and the air. Unfortunately the vapour pressure gradient has proved very difficult to
measure precisely in the layer immediately above the surface, so wherever possible,
methods of calculating PE use measurements of vapour pressure at one level only.
