Environmental Engineering Reference
In-Depth Information
Plate 5.4
An evaporation tank and gauge.
Photo: E. M. Rollin.
Because of this problem of obtaining data, a large number of simpler, more empirical
formulas have been devised. They are much easier to use; they are based, however, not
on physical principles but on the observed relationship between evapotranspiration and
one or more climatological variables. The relationships have usually been obtained under
one particular climatic regime and they may not be applicable elsewhere, hence the
number of formulas.
Probably the best known of these empirical equations is that developed by
Thornthwaite to determine PE. Thornthwaite was trying to devise a climatic classification
that went beyond mere description and incorporated indices of heat and water availability
and how they were related to vegetation. In simple terms, PE is calculated from the
formula:
where PE is the unadjusted monthly value of potential evapotranspiration,
T
is the mean
monthly temperature in degrees Celsius, I is an annual heat index derived from the sum
of twelve monthly index values and
a
is a function that varies in relation to I. There are
other minor adjustments required to allow for day length variations. Fortunately
nomograms and tables have been prepared to simplify the calculations.
In this formula Thornthwaite is using temperature as a substitute for radiation, and it
therefore works reasonably
