You can get a wireless 12v switch at most farm stores and you could rig up a 12v valve.Another option is to take a piece of plastic pipe and make it to swivel so that when you let it down and it's below valve level water comes out and when you pull up with a rope it will stop once above valve level.I would use a generator for a pump and just shut off valve but iof you do that you would have to hose that kept pumping back into the pump as a bypass.You could hook up a 12v pump to your tail light and just turn off and on with light switch,I do this on 4wheeler or PU with trailer light plug.Do you have buckets at each tree