If you want to develop more or larger cities in the southwest, that requires a megaproject--several, actually. You burrow a tunnel canal from the Gulf of California into Laguna Salada. From there, you burrow on to the Salton Trough. And you create a similar tunnel canal from the Pacific into Death Valley. That's part A.
For part B, you build some new nuclear plants whose energy output goes entirely into creating artificial clouds on the shores of those artificial inland seas, along with Mono Lake, Lake Crowley, Lake Tahoe, Goose Lake, Pyramid Lake, Great Salt Lake, Utah Lake, Sevier Lake--anywhere that has enough volume that the cloud generators won't dry it up between rains. For the rains needed to keep the freshwater lakes filled, you will have also build evaporators on the ocean coast.
The point is to "bounce" that rainwater over the Rocky Mountains, so that the vast rain shadow east of them gets more precipitation. Though it sounds like an insanely expensive project (it would be), by far the cheapest way to move large amounts of water over long distances is to build a cloud upwind of where you want the water to be, and then collect it from the river for that watershed.
The second cheapest way would be to move your water very slowly through an existing aquifer. So for part C, you also pierce the caliche in several places over the Ogallalla aquifer. The rain "bounced" over the mountains can then partially drain into one end of the aquifer.