Global solar irradiance (Qg) is an important physical variable that has been used constantly in agrometeorological models, either for climatic characterization or to give support to radiometric studies developed for irrigation planning and crop production modeling. The current study aimed to compare measured daily Qg values with estimates of this variable by means of four different methods. Thus, using daily data set for the period between March 28th of 2008 and August 8th of 2011 at Ponta Grossa, PR, Brazil, a simple linear regression between radiometric data, measured by a pyranometer and model estimates of Qg was done. Global transmittance was conditioned by atmospheric cloudiness. The models based on mean global transmittance on daily basis performed more satisfactorily and generated Qg values with accuracy and exactness at the study site, as confirmed by the statistical parameters used to validate the Angström-Prescott proposed method. However, the performance of the methodologies based on the determination of mean global transmittance under extreme atmospheric conditions, considering totally cloudy and sunny days, showed the highest Willmott coefficients, close to 1, reflecting the precision and reliability of the calculated Qg values, compared to the observed values at an automatic weather station.
Global irradiance; global transmittance; modeling; solar radiometry
