In this work were studied the correlation and the proportionality coefficients between the mesured evapotraspiration and Class A pan evaporation data, in a potato field-crop under three soil water regimes. The values obtained were significantly correlated, except when the soil moisture content reached a value corresponding to a capillary potential of about -2.0 bars from the stage of tuber growth to maturity. To practical purposes the evapotraspiration/Class A pan evaporation relationship were simplifield, so that only three average values, representatives of three stages of plant water requeriments, could be a useful tool to predict the irrigation requirements of the crop, even in relatively short periods.