Solar net radiation over water in a class “A” evaporation pan at Giza, estimated on the base of its close relationship with global radiation

Abstract

Measurements of net radiation over water in a class “A” pan at Giza during daytime, nighttime, and daily periods for different seasons were compared with the corresponding simultaneous values over short vegetation and to the global radiation. Close linear relationships were obtained in all cases. The close linear relationships between daytime and daily net radiation and global radiation were applied to monthly averages of daily global radiation for a 10-year period to estimate average monthly values of daily and daytime net radiation over pan water. The annual variations of these values were described. Seasonal and annual values of these and other relevant items were given. An idea of the diurnal variation of net radiation over pan water in the different seasons was given by the seasonal variation of the ratio daytime/daily net radiation, as well as by information on the highest and lowest values of net radiation and their times of occurrence, depending on the hourly measurements. The relationships between pan evaporation and potential evapotranspiration, and between net radiation over pan water and pan evaporation were also obtained. The close relationship of radiation and other parameters could be explained by Linacre's formula[1], the energy budget equation, and the characteristics of the surfaces concerned. Such relationships could be applied to other desert climates like Egypt.