Damn guys. This really sucks for car enthusiasts in Cali.
If it helps any. I have my Z in a garage that has no water access.
I still wash my car in my garage because I do not like going to car washes.
My system is this:
1. I am a little nuts so I carry about 3-4 gallons of water into my garage. I have a military style backpack with a 60-80 pound load weight. So I can actually fit the jugs in there and still have my hands free.
2. I bought a 2 gallon lawn sprayer that I keep in the garage. Like this one.
Amazon.com : Smith 190216 2-Gallon Max Contractor Sprayer With Heavy Duty 18-Inch Wand : Lawn And Garden Sprayers : Patio, Lawn & Garden
3. I usually can give my car a good wash with less than 3 gallons of water. Sprayer has adjustable nozzle and gives off low but still usable pressure.
4. 3-4 gallons will fill up both my wash bucket and sprayer sufficiently. I usually save a little water for an extra rinse cycle.
Overall, the process had served me well in my year plus of ownership. The only real pain in the *** is how many people want to lug around 4 gallons of water.
Since my garage is not next to where I live. I actually have to lug my gallons of water around on public transit, and scare people because I make sloshing sounds when I walk.
But F them I love my car and I like to wash it in peace and quiet so it is worth the effort.
I tried waterless wash but still wasn't comfortable with process and results.
My way would work for you if you could find a little privacy so you don't get ratted on by the neighbors. You also get to keep track of exactly how much water you are using so to some extent you aren't completely ignoring the water crisis.
The only downside to how I do things is that I can only do a nice two bucket wash when you want to transport extra water around, or make multiple water drop off trips on consecutive days prior to wash and detail day.