Somewhere nearby, you likely have a ball valve that controls flow to the autofill. The pool builder had to tap into the house plumbing someplace, and would have certainly installed a valve to control it. The first place to check would be at the nearest outside hose spigot, but it could be elsewhere. If you can't find a valve or lower the float, in a pinch you could probably bend a coat hanger and use it to prop up the float high enough that the flow stops. That's what I used till I figured it out, since my float valve was corroded and couldn't easily be adjusted.
There are a couple possibilities as to why it runs all the time, but that much evaporation is unlikely, even in NV; in Tucson, my autofill runs a little each day, but nothing I would describe as constant. Do you have an overflow drain on your pool? If the float is set too high, as Scott suggested, you could be adding water via the autofill and losing it via the drain simultaneously. Or, as Flyboy320 mentioned, you could have a leak. Either one of these scenarios would knock your chemistry all out of whack, since you'd constantly be replacing water. Are you seeing that?
