According to Penman - Monteith method, to estimate the net radiation (Rn), you should first calculate the extraterrestrial solar radiation (Ra). (Ra is the same in Penman - Monteith and Hargreaves methods).
Read the the radiation concepts on page 41, then follow the equations (21- 40) in FAO paper-56 to calculate both Ra and Rn.
According to Penman - Monteith method, to estimate the net radiation (Rn), you should first calculate the extraterrestrial solar radiation (Ra). (Ra is the same in Penman - Monteith and Hargreaves methods).
Read the the radiation concepts on page 41, then follow the equations (21- 40) in FAO paper-56 to calculate both Ra and Rn.