Trees Are More Awesome Than You Thought: Hydraulic Redistribution

Trees Are More Awesome Than You Thought:  Hydraulic Redistribution

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:


Graphics courtesy of FC Meinzer, USDA Forest Service.

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.


  1. Neal, love your work! I listened to your PC voices podcast and was inspired. This blog is really neat and I did a little more digging and found a lot of additional research on the subject matter.

    Keep up the great work, I aspire to do something like your Saudi project in the future!

    • Rob! It is a mutual feeling. I have lots of family in Alberta and I keep waiting for someone to show some interest so I can recommend you to them. If you find some related research please post links here!

  2. Are acacia easier to get established than other trees for some reason? They don’t have a deep root system when planted, so do they have some other advantage?

    • Hi Steve.

      The local acacias almost certainly have deep roots once they are established, and at least for Acacia Tortilis, its ability to perform hydraulic redistribution is one of the reasons it can survive so well in very arid areas.

      • Hi Neal,

        Yes, I understand that. My question was about before they are well established. How do they survive long enough to develop the deep root system, and why don’t other species?

        • Got it. I don’t know the answer to that though I have a few suspicions. First I suspish that they grow their taproots very quickly, and that their development is initially quite bottom heavy, which is why they appear to grow so slowly when left to their own devices. Second, I suspish that they can also go dormant during severe drought periods while they wait for more precipitation to allow further growth. However, those are stabs in the dark at this point and i could be completely wrong.

          • Thanks, Neal. Your guesses sound very plausible. I appreciate your honest explication. And being introduced to a new word: “suspish”. I think I’ll get some good use out of that one.

  3. Hey Neal, thanks for this article. I’m curious about hydraulic redistribution and came across this site as I was researching. I’ve found very little information about any specific trees which perform this function (I’m generally curious but primarily interested in temperate climate plants). I tracked down the broken forest service graph on and it mentions Pacific Northwest conifers which is the only lead I’ve found. Do you know of any other resources about hydraulic redistribution? Many thanks for any pointers.

    • Hi Adam. I don’t have any recommendations for you except for google-fu. I did fix the link though…I imagine that temperate climate plants wouldn’t have a need for this kind of function because rain is much more available, so I expect that it would be more rare in that climate.

      • Thanks! My Google-fu is failing, but I shall continue exploring. :-)



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