"I love a sunburnt country,
A land of sweeping plains,
Of ragged mountain ranges,
Of droughts and not much rain."
(Apologies to Dorothea McKellar)
Reality No.1 Residents don't water street trees
Reality No.2 A young street tree needs a minimum of two waterings a week
Reality No.3 Councils don't water their trees twice a week
Reality No.4 Most young street trees will die from lack of water
WHAT TO DO?
Logically, there are two possibilities:
1. Double your watering budget
2. Plant fewer trees
Given that Option 1 is unlikely to happen any time soon, my suggestion is go for Option 2:
Only plant the number of trees you can water twice a week*
It's actually the quicker way home in the long run.
Plus you lose less skin.
* if the residents do water them, consider that just a bonus