Maybe this solution may help...??????
If you have access to a gas powered pressure washer (or electric if an outlet is near by) you can find a drum and easily bring water to the site.
When I raced MX, I would wash my bike in-between motos. (Most racers did) Having limited access to water we would bring our own. Most guys got a 55/30 gallon drum (your local auto store/ lube shop should have one lying around that was used for washer fluid.) Just rinse out thouroughly (really good) and fill...... when you go to siphon the water, make sure all the water is out of the hose. Place end in drum and then give the hose a suck...... once you see it flow...pinch off and connect.