Coeficientes do tanque classe "A" para estimativa da evapotranspiração de referência na região do Vale do Submédio São Francisco, estado da Bahia Class-A pan coefficients for estimating reference evapotranspiration in the San Francisco Sub-Medium River Valley, state of Bahia

The use of class-A pan as instrument for determination of water requirements for irrigated crops has been widely used, and it is one of the methods recommended in the FAO-Paper 56. To convert water evaporation from class-A pan to reference evapotranspiration there is a need of a multiplying factor known as class-A pan coefficient (Kp), which is dependent on installation conditions of the equipment and local weather . In the literature there are many approaches to determine Kp, from tables to mathematical functions. This research's objective was to evaluate the performance of different approaches to estimate Kp to obtain ETo. To do so, a series of meteorological data (1998- 2006) from Mandacaru irrigated district, in the city of Juazeiro (state of Bahia, Brazil), was used to estimate Kp and