The water will simply run down the panel, onto the roof and into the roof drain. Water shortage was not mentioned when I was asked to make this device, although, you make a valid point. California had its share of water shortages in past summers and this issue may come up again in the next water shortage. Mind you, this device will mostly be used during non-rainy season, the other time the panels are washed by the rain.
This trolly is 24" long and 36" wide. Each row will get an up-wash and a down-wash, then the operator simply pick it up, place it on the next panel and repeat. It would be nice to haveNASA's budget to create one that operates with the push of a button and clean the whole farm in five minutes, but, this is county mindset, penny wise and pound foolish