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A statistical estimate model for the anisotropic diffuse fraction as a function of clearness index K T is proposed to estimate hourly, daily and monthly diffuse irradiations. Global, diffuse and direct solar irradiances were provided by the Laboratory of Solar Radiometry of Botucatu-UNESP (latitude 22.9º South, longitude 48.45º West, altitude 745 m). The period assigned for the study comprised the years 1996 to 2002. Global solar irradiance was measured by an Eppley PSP pyranometer, direct irradiance by an Eppley Nip pyrheliometer and diffuse irradiance by an Eppley PSP pyranometer under the Melo-Escobedo-Oliveira shadowring (radius of 40cm and width of 10cm). Isotropic and K T corrections were applied in diffuse irradiance. The proposed model was compared to classic models reported in the literature, with good results according to the MBE and RMSE statistical indicators for hourly, daily and monthly partitions, respectively. The results showed that the inclusion of K T correction improved the performance of the shadowring MEO according to MBE values for the three partitions: hourly, reduction from -7.21% to -1.74%; daily from -4.70% to 0.88% and monthly from -6.58% to -1.18%.

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FAO proposed the reference evapotranspiration to compare evapotranspiration in different areas, because most equations employed in estimating evapotranspiration are empirical and limited to be used in local and climates similar to those in which they were determined. The objective of this study was to analyze the difference in calculated values of ETo for Botucatu and Piracicaba (Brazil) and Güira Melena (Cuba), based on 30-year data. Seasonal differences between years and locations were assessed and subsequently the influence of local climates on these ETo values differences. Climatic elements maximum (Tmax) and minimum temperature (Tmin), relative humidity (RH), wind speed at 2 m height (U2) and solar radiation (n) were obtained from conventional meteorological stations. Evapotranspiration values in Cuba were higher during summer months in the Northern Hemisphere. The highest evapotranspiration values in Botucatu and Piracicaba occurred in summer months in the Southern Hemisphere. Correlation exists between the three locations with the same distance between them. It was concluded from the methodology used that the three localities have different evapotranspiration values and despite being in different hemispheres the evapotranspiration among Piracicaba and Güira Melena are closer than the one verified between Botucatu and Piracicaba.