An essential part of moisture balance modeling is accurate real time weather data. The best option is a complete Et weather station such as MCS, Adcon, Automata or Davis that provides all the data required to do a Penmann-Monteith Et calculation. 

Depicted below left is one of the Davis stations selling for around R13000.00 (depending on exchange rate).

When none of these are available we supply  a simple instrument that registers daily maximum and minimum temperature and humidity similar to the one depicted below. with this it is possible to calculate a simplified Penmann-Monteith Et that works well enough. It sells for about R300.00 ($30.00)

   Request further information on weather stations by clicking here.