FAO Irrigation and Drainage Paper #56 (Allen et al 1998) contains methodologies for estimating missing data required for the Modified Penman Monteith equation.  See the link below for more information.

http://www.fao.org/docrep/X0490E/x0490e07.htm#estimating missing climatic data

If you have only temperature and humidity, it 's not possible to calculate with acceptable precision ET0 because you need the wind velocity and the solar radiation for this. The wind velocity in particular has a considerable effect on the soil's evaporation. You can anyway try the Thornthwaite formulae (1944) which gives an approximation of monthly Evapotranspiration using only temperatures

Hergeraves 1985 method is applicable in this case. You dont need RH.

Thanks to everybody for your input. I think Kshitij Parajuli is right, Hergeraves mehod is most suitable for this case.

Hergreaves method also need radiation data with temperature. You may try for artificial neural network models using the climate data. 

Schendle equation,

ETo=16T/RH

you can use Schindel equation,

ETo=16T/RH

Hergeraves equation which is accurate method even for semi-arid region can help you.

I normally use ETo calculator developed by FAO.

يمكنك استخدام معادلة شندل ET o = 16 T / RH