How to get the solar zenith from the longitude and latitude? I have real data of “Direct Normal Irradiation” and “Diffuse Horizontal Irradiation” and I should calculate “Global Horizontal Irradiation”. I have also longitude, latitude and elevation. 
Could you advise me how I can get zenith from these data, then to calculate the GHI?
Many thanks,
Direct Normal Irradiance (DNI) is the the amount of solar radiation from the direction of the Sun. 

Diffuse Horizontal Irradiance (DHI) is the amount of solar radiation that does not arrive on a direct path from the Sun. 

Global Horizontal Irradiance (GHI) is the total amount of solar radiation. It is the sum of Direct Normal Irradiance (DNI), Diffuse Horizontal Irradiance (DHI) and ground-reflected radiation. Ground-reflected radiation is usually insignificant (compared to DNI and DHI) so for all practical purposes GHI is the sum of DNI and DHI.

Thus $GHI = DHI + DNI \times cos (Z)$ where $Z$ is the solar zenith angle.

Solar zenith angle is derived from latitude. 

Solar zenith occurs at local solar noon, which is derived from longitude.