With the Geisenheim Irrigation Scheduling it is very easy to calculate the water demand of vegetable crops.
To irrigate vegetable crops following the "Geisenheimer Irrigation Scheduling" method you need to have:
FAO56 PENMAN-Monteith equation
Through continous observation of the development of the plants, it is possible to see which development stage should currently be considered for the calculations.
The following pdf-files (english) shows crop coefficients for many vegetables.