Probably the lack of water. I don't know why they need to continually pump it instead of doing a closed loop and cooling it through a few miles of pipe underground which is practically free, after you install the plumbing. Sure evaporative cooling is good, but certainly there are workable alternatives.

That or latent heat cooling, the other end of the phase-state spectrum: Instead of evaporating water during the day, make ice all night when ambient air temps drop and then use it to cool a closed-loop system during the day.