Not so sure in the desert that an open loop is your best option. They waste a lot of water if you don't have a well to dump it down. Why not install a closed loop system ? you could drill a well to install the closed loop tube in to.
I have no idea about cooling loads in the desert but 18 GPM is 1080GPH. so I would think you need a several thousand gallon tank to make this worth while how would you refill thousands of gallons a day for an open loop system ?