It is proposed to use Penman's theoretical expression for ET practically. A theoretical formulation of the factor f(u) is therefore given; the quantity EO, which easily leads to misunderstanding, is omitted, and the empirical factor a is replaced by another empirical factor e. It was found that e = 0.92 gave the best fit to the observations.--From author's summary. (Abstract retrieved from CAB Abstracts by CABI’s permission)