You do it as it's partially set up and while it finishes, not right after the pour.
Also we don't have that problem here, at least in the summer. It evaporates off faster than it rises.
You send them out there with water bottles at a minimum, splash a little here and there as the surface dries too fast. Once in a while you'll end up hosing down a whole slab or timing it to finish overnight when it's cooler and the sun is gone.
Concrete makes its own heat as it cures. Add the sun to that when it's 100+ in the shade and we have the opposite problem.