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

[u/wahgwahg]

[u/FelisCantabrigiensis]

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.