The way it was explained to me was that as the plants we’re putting on an incline grow and intertwine their roots, just normal water uptake from the roots closest to the water source (a spillway for us) will result in some water very slowly making its way uphill underground into the surrounding soil, via the roots. And then the next plants closest to the plant closest to the water will start taking up that water, and repeat the process. I have no idea of this is a local adaptation for Southwest US plants or something that all plants everywhere do, I just show up to haul shit around and listen to all the knowledge the project leaders have! If you feel like waiting a week and a half I can ask for more specific resources about it.