Site description and data collection:
The study site was Mymensingh ( 24 0 43" N, 90 0 26 " E, and 19 m above Mean Sea Level (MSL)), Bangladesh. Solar radiation and other climatic data were collected from Bangladesh Meteorological Department.
Multiple Regression Model:
The multiple regression model tried was that of the form:
Rg = a0 + a1x1 + a3x2 + a3x3 + .............+ anxn ............................... (1)
where, Rg is the global radiation (MJ.m-2.d-1); x1, x2, x3,.... xn are the climatic variables; and a0 , a1, a2, a3,.... an are the regression coefficients.
The selected form was:
Rg = a0 + a1x1 + a3x2 + a3x3 + a4x4 .............................................. (2)
where, Rg = global radiation, MJ.m-2.d-1
x1 = maximum temperature of the day, 0C
x2 = minimum temperature of the day, 0C
x3 = bright sunshine hour for the day
x4 = photo-period or day-length, hr
a0 , a1, a2, a3 and a4 = coefficients, which should be determined for the specific site
Calculation of photo-period ( day-length)
The declination d, defined by Hunt (1982), as the angle of the sun north or south of the equatorial plane, can be found from the approximate equation of Cooper (1969), cited by Duffie and Beckman(1980).