One of the major obstacles to reforesting the desert through floods is getting foliage established, and one of the reasons foliage does not establish more readily is because when it does rain, water does not penetrate the thin layer of clay that forms before a flood flows. You can see that clay layer starting to form in the video posted below, only 2 minutes after the start of a 5 minute rainfall:
This rainfall did not cause a flood–but it did show what happens in a wadi before a flood occurs–clay layers form, and as clay is hydrophilic, they become saturated, after which more water cannot sink into the landscape unless there are strategies and structures put in place to make it do so.
One of the greatest tools to disrupt this clay plan & establish foliage at the same time is to plant trees that can perform Hydraulic Redistribution. Hydraulic redistribution is the ability of some trees to use their shallow roots and taproots as pumps. When the shallow soil is saturated or wet, these trees can pump that water through the tap root deep into the soil, and keep that water in reserve until a drier period. In that drier period, it can pump the water in the lower levels of the soil up to the shallow roots, thus cooling the soil temperature, making water available to other plants in the tree’s vicinity, and allowing the tree to continue to respirate even in very hot and very try times. Here is a graphic of how it works:
This is a truly awesome function that you can stack with trees. The known desert species that perform hydraulic redistribution are acacia tortilas and prosopis, whose taproots can reach up to 120 feet (about 40 meters) below the surface.
Thus in only one function of some trees, we can penetrate the clay layer of floods, and literally pump flood waters into the ground when it is wet. Then when times get dry, the trees themselves will bring that water back to the surface to nurture other plants growing in their root zone.
To do this without the assistance of a tree would require digging holes, laying pipe and filters, and installing cisterns–just to get the water into the ground. Then to bring it back up we would have to install pumps and irrigation systems. That is what some would recommend, and that is actually the function of some standard dams in Saudi Arabia. Why do that when we can do this passively by understanding how nature works, then tailoring our designs to facilitate her wonders, and then simply cooperating with her? That is what we can do with hydraulic redistribution.