How to calculate the water requirements for trees, shrubs and ground covers?

You need to get the ETCo( Crop water requirement + Evaporation 

Evaporation on daily basis for peak period i.e. June, July months

kC values of different crops

Soil moisture holding capacity

Age of the crop/ trees i.e., growth stage

 Depending on the trees, temperature of the atmosphere and degree of soil cohesion

ETC=ET0+evaporation and you need to conisder the plant type

Through ETo and Kc Values.

Trees (80 L/D)

sHRUBS (20 L/D)

Ground Cover (12 l/m2)

Plants water requirement + Evapotranmission rate + Soil Water holding capacity.

Consider the avergage plants water requirementd and location climate.