When Bill talks about the sonoran desert, he asks the question what does the word sonoran mean.
It comes from sonorous- full of sound, and the sound was of the many rivers rushing to the sea. removing trees did in the rivers in the recent past. He uses examples- the last giant beaver was taken in Tucson(1890 I think), the Pima Indians ate huge cutthroat trout.....
I believe one of the main tree species was the cottonwood, and he points out that the saguaro cactus needs the shade of the forest to regenerate, and without a forest around it it will go away also, so the remnants of that forest are still there..
The point is, that to undo the damage is possible without worrying about the idiots pumping water faster than it is being returned. and wherever you start will become "headwaters, but ridges with tall tree bands are especially effective in creating the orographic lift mentioned earlier, so such a massive undertaking would get good results starting there. Planting trees everywhere ("the man who planted trees") is also a good idea, but arroyos at their heads would likely be the easiest place to start. rock walls(gabions) to slow the flood and if you can keep the young trees alive- planted right after the flood-- in a few years, they will become a biological barrier that will start to spread out from there. It would likely be good to study those water flows and and find the defensible places where trees could be started.
He also points out that at this stage small actions will generate large returns. walking around in the rain watching water flows, moving a rock to divert water down a gopher hole, or digging a small ditch to move water to natural storage areas are strategies that will give large returns for energy put forward.
One other likely answer may come from Bill's ideas around associations and incorporations. Establish your non profit so you can take tax deductible contributions, get like minds organized. Buy up distressed land and organize communities around it.
Bill's course from 1983 (on mp3 files for about 40$) has lots of inspirational directives for organizing at this level. Become a political and economic force that persists after you are gone to secure a project of this magnitude.
remember , his actual trust documents and procedures will need some adjustments according to local laws, but it is never too early to start to organize and figure out a path. If you are serious about this goal, it is certainly a worthy one, but it will not happen overnight, although if the stars align it may happen faster than you expect.
Just remember the axiom, if you're not having fun you've got the design wrong.