In agricultural meteorological studies, the major difficulty is the determination of evapotranspiration (latent heat flux). There are numerous methods for estimating its value. However, these approaches yield accurate and successful results proportionally with the complexity. The main purpose of this paper is to determine the effects of atmospheric, soil, and field measured plant quantities on the latent heat flux by using the fuzzy inference systems techniques. The necessary measurements are recorded during two different periods in the faba bean plant growth at an agricultural meteorology station by considering a Bowen Ratio Energy Balance System. Copyright (C) 1998 IFAC.