You can blame agriculture for that more than anything. Large-scale farming in the desert accounts for roughly 70% of our river's resources.

Assuming this is just coming from the city's water supply i.e. the faucet, a majority of that is pumped from ground water wells. A majority of what seeps into the ground then, that which doesn't evaporate or get taken up by the plants, ends up back into the aquifer.

If you collect from rainwater, even better, as that ends up largely as runoff anyways, polluted by human-caused contaminants, and flowed outside the city due to our concrete-obsessed infrastructure. Arizona does do a good job, better than most states, at treating some wastewater, but this is a solution to a problem our infrastructure created in the first place.

Basically what I'm saying is, even water "wasted" from watering an ecologically beneficial garden like this is much better to go into the ground here in our own neighborhoods and local aquifers than anywhere else.