FORMULE PERMETTANT D’INTEGRER L’EVAPOTRANSPIRATION ET LA PLUIE PAR UN INSTRUMENT
Irrigation needs have been estimated, in the last few years, by a method referred to as the "soil water budget" and based on meteorological factors such as evapotranspiration and rain. In order to eliminate the needs for daily recording and calculation of records, a new formula has been derived. An instrument capable of integrating both evapotranspiration and rain, and made up of an evaporimeter and an integrating cylinder, has been mounted according to it. The cylinder diameter (Dc) is determined by this formula Dc = De √K, when De is the diameter of the evaporimeter and K the ratio between evaporation and potential evapotranspiration of a crop.This instrument was made and tried in 1961. A Wright AP evaporimeter was used. The integrating cylinder, terminated by a funnel to collect rain, is equipped with a scale indicating the soil water deficit and an overflow pipe permitting the evacuation of water in excess of field capacity. The evaporation of a 4-foot sunken tank was taken as the basic value of potential evapotranspiration, since it is considered approximately equivalent to the evapotranspiration of a crop adequately provided with water.This instrument, after some minor corrections, will be an efficient means of predicting irrigation needs. A good correlation was established between this instrument and two other methods of estimating evapotranspiration. Its response to meteorological factors was good.