Environmental Engineering Reference
In-Depth Information
Plate 5.5
A data-collecting system called Hydra used to
measure evapotranspiration. The sensors measure wind
speed and direction, humidity and radiation.
Photo: H. R. Oliver.
well where the two are closely correlated. In the tropics, however, the equation
underestimates PE because temperatures lag behind radiation inputs. In addition, the
method takes no account of wind, even though it may be locally important. Nevertheless
the relative simplicity of the method makes it popular, and, despite its shortcomings and
its inevitable inaccuracies, it is one of the more widely used methods of assessing PE.
In the United Kingdom the Penman formula has also been widely used for calculating
PE. It is less empirically based than the Thornthwaite method, combining the energy
budget and the aerodynamic approaches to the estimation of evaporation, but requires
much more meteorological data. Different versions of the basic formula have been
developed. They are based on the duration of sunshine, or net radiation if measured,
