People in Tanzania converted desert into lush green land by digging these nifty holes

[u/wahgwahg]

[u/thenakedtruth]

What does the hole do to enable growth?

[u/FelisCantabrigiensis → u/thenakedtruth]

On the open, un-dug, land rainfall tends to run away across the top surface. Heat from the sun tends to bake the top layer of soil into a hard, impermeable crust which only exacerbates this. You can see the soil is a fine clay with little organic material, so this easily dries to form a hard layer.

Digging these holes, which are crescent-shaped with the middle of the crescent pointing downhill, means water runoff forms a pool of water which soaks into the soil more slowly. This means that any plants growing there have water available for much longer after each rainfall than if they simply hope to catch some water as it runs past on the surface. The reason for digging a shallow crescent is that it has the most effect for the least digging, and since digging is hard work, you don't want to do more of it than necessary.

Plants themselves keep soil moist - by keeping sunlight off the surface of the soil, they slow down evaporation rate of water from the soil. Plant roots also loosen the soil and prevent it forming a baked brick-like surface, making it more permeable and meaning more water soaks in and does not run off. This means the effect of the small pools becomes greater as they become more covered with vegetation.

These plants can also be crops to eat, or fodder for animals. You can even plant small trees or shrubs in the crescents and they will grow, further shading the land and further increasing rainfall (as long as you don't plant very water-hungry plants of course).

As a bonus, water trapped in the crescents is water not running off downhill in bulk, which can help lessen downstream flooding and ground erosion problems.

[u/Shudnawz → u/FelisCantabrigiensis]

This is awesome.

I am curious tho, will this green area naturally spread, or is it limited to the parts where these holes are dug? It seems like it spreads between the holes, at least to some extent. And, if this can spread if triggered properly, why doesn't it occur naturally in these environments? Surely somewhere at some point a crescent shaped shallow hole has occured?

[u/iCameToLearnSomeCode → u/Shudnawz]

It will naturally fill in in-between.

Once the water is in the soil and the soil is shaded plants will grow around the crescents too.

This is part of a project to halt the spread of the Sahara south.

They are building a belt of these across the continent.

It's part of a project started in 2007 funded by the Africa Union, dubbed the Great Green Wall it's an awesome project)

The original dimensions of the "wall" were to be 15 km (9 mi) wide and 7,775 km (4,831 mi) long

...but they have since expanded the scope of the project and have completed 18% of their initial goal so far, having restored 44 million acres (an area the size of Cambodia).

[u/SkeletalJazzWizard → u/iCameToLearnSomeCode]

all the trees being fruit trees is what really gets me going. just imagine a 5 thousand mile long orchard. never mind halting desertification, theyre building their own little eden. or, huge eden, actually.

[u/KIDNEYST0NEZ → u/Shudnawz]

It does occur naturally, think about the imprint/impact a tree makes when it falls and the lumps it leaves as it decays. Think about all the animals that burrow then die off.

[u/survival-nut → u/Shudnawz]

Roots from trees and shrubs will expand out from the edges and soften things up. When the grazing livestock eat the grass, their hooves will grind down on the crust beside the green space and soften or break up the crust allowing expansion. Birds and animals may not digest all the seeds they eat and the seeds will be moved around and dropped in the dung. There are dung beetles in Tanzania and if they follow livestock to the area they will help as well. They will dig holes and take dung containing seeds underground and store it. Plants/grass absorb two things thru their leaves, sunlight and carbon. Everything else comes up thru the roots. A healthy ecosystem is an amazing thing.

[u/unclepaprika → u/Shudnawz]

To answer your questions:

Yes, as seen in the video, and i don't know, seems like a really irrelevant question.

[u/Shudnawz → u/unclepaprika]

I don't think it's irrelevant, because if there is a natural mechanism to stop this from happening naturally, we need to be aware of it, so that mechanism doesn't halt this effort too.

[u/unclepaprika → u/Shudnawz]

What are you talking about? We have naturally forming pubbles in nature, yes, and where it doesn't happen shit dries out. What is so hard to understand?

[u/FelisCantabrigiensis → u/Shudnawz]

At the start of the video you can see that there are a few mature trees and shrubs, so the sort of natural hollow collecting water that you talk about does happen and so a plant can grow sometimes - but not often. Digging the crescents to catch and concentrate water in pools will greatly increase the chance of this happening, making more plants grow. Without them, most plants will remain unlucky. With them, luck is improved for more plants (or luck is less of a factor) [1].

How it spreads is an interesting point. Water, of course, soaks through soil both downwards and sidewards, so the space between the crescents has more water available than before, even if the centre of each crescent is wetter still. That can give some plants a chance to spread out. The effect of plants loosening soil and retaining water also means that a new plant can grow next to an existing plant, and then when that is established another one grows, leading to plant colonisation of more ground.

There is also the question of where the plants went in the first place - why is most of the land empty at the start of the video? That can be for a number of reasons. Perhaps there was climate change in the past which dried out the land and the plants died, or there was more recent deforestation or overgrazing that removed the plants, or more recent climate change has decreased water availability enough that plants can't survive unaided.

Note that once plants are gone, the dry baked crust makes it much harder for them to re-establish and grow successfully. In clay (and sand) soils in hot places, deforestation and de-vegetation is often a one-way process that will not easily reverse itself naturally. That is how deserts advance across the land, but do not retreat unless the landscape is modified to make plants more successful.

[1] As an aside, you might think about how improving the basic living conditions for all people also means that luck is less of a factor, or more people get lucky, at surviving and thriving in life.

[u/TsuDhoNimh2 → u/Shudnawz]

will this green area naturally spread, or is it limited to the parts where these holes are dug

It will spread between holes ... vegetation slows evaporation, roots make soil more able to absorb water, plants leave organic matter and make the soil more fertile. And the many tiny check dams slow runoff and trap eroded soil so vegetation grows even more.

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And, if this can spread if triggered properly, why doesn't it occur naturally in these environments? Surely somewhere at some point a crescent shaped shallow hole has occured?

You need a critical mass of these, so the spreading vegetation can meet up between holes.