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Tanzanian Half-Moons

Short answer: Half-moons are semi-circular bunds, called demi-lunes in Francophone Africa. Each is a shallow basin dug in a crescent, with the dug-out soil piled into a low curved bank on the downslope side and the open side facing uphill.

They're laid out in staggered rows along the contour so rain running off hard, crusted ground pools in each crescent and soaks in instead of washing away. FAO's water-harvesting manual describes them as a modern technique, not a traditional one, and gives an example from Niger's Tahoua region in the mid-1980s. Sizes vary from about 2 m in radius for crops and trees to 6-20 m for rangeland. In northern Tanzania, the Dutch nonprofit Justdiggit works with LEAD Foundation, Trias and Maasai communities in the Arusha and Manyara regions. It reports more than 50,000 'water bunds' dug there, along with grass seed banks and tree regrowth from old stumps. Results depend on rain, seeding and grazing control. A 2024 remote-sensing study of Kenyan and Tanzanian sites found mixed vegetation changes, and Justdiggit's own evaluation flags overgrazing as a risk.

VideoWrite-up4 sources
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What should you know about Tanzanian Half-Moons?

Quick facts. Each one stands alone and has a stable ID you can cite.

  1. Half-moons are open to the uphill side, so they catch runoff.

    DD-664-F01 · Write-up · Exhibit A (What a half-moon is)

  2. More than 50,000 water bunds have been reported dug in Arusha and Manyara.

    DD-664-F02 · Write-up · Exhibit D (In Tanzania)

Video transcript: Tanzanian Half-Moons: Crescents That Catch the Rain

DID YOU KNOW? In Tanzania, particularly in regions like Arusha, communities use half-moon pits, shallow crescent-shaped depressions dug along land contours to combat desertification and restore degraded soil. These pits, typically a few meters wide, capture rainwater otherwise runoff, allowing it to seep into the soil and provide moisture for plants and seeds. Over time, this simple technique transforms barren land into productive grasslands.

Burned-in captions, read frame by frame (see transcript.md) · 25.1s Reel ·

Video coming soonTanzanian Half-Moons · Instagram Reel
Tanzanian Half-Moons: Crescents That Catch the Rain Instagram Reel · · 25.1s

The write-up

Crescents that catch the rain. Semi-circular bunds slow runoff on degraded land.

  1. Exhibit A

    What a half-moon is

    A half-moon is a shallow basin with a curved earth bank on the downslope side, open uphill, laid out in staggered rows along the contour.

  2. Exhibit B

    How it works

    Runoff that would sheet off crusted ground pools in each crescent and soaks in, which helps buried seeds sprout.

  3. Exhibit C

    Not ancient

    FAO calls the technique modern, not traditional, and gives an example from Niger's Tahoua region in the mid-1980s.

  4. Exhibit D

    In Tanzania

    Justdiggit and its partners report more than 50,000 water bunds dug with Maasai communities in Arusha and Manyara.

  5. Exhibit E

    Results vary

    Grass returns where rain, seeding and grazing control line up. A remote-sensing study found mixed vegetation changes.

Questions people ask about Tanzanian Half-Moons

Which side of a half-moon is left open?

It's open uphill so it catches runoff.

Source: Write-up · Exhibit A (What a half-moon is) · FAO (Critchley and Siegert, 1991), Water harvesting manual, ch. 5: Semi-circular bunds

What's the French term for half-moons?

Demi-lunes.

Source: Write-up · Exhibit C (Not ancient) · FAO (Critchley and Siegert, 1991), Water harvesting manual, ch. 5: Semi-circular bunds

Which Tanzanian regions does the Reel's footage relate to?

Arusha and Manyara.

Source: Write-up · Exhibit D (In Tanzania) · Justdiggit: Regreening work in Northern Tanzania (Arusha and Manyara); Justdiggit: On-going regreening activities in Arusha

About how many water bunds have been reported in that project?

More than 50,000.

Source: Write-up · Exhibit D (In Tanzania) · Justdiggit: Regreening work in Northern Tanzania (Arusha and Manyara); Justdiggit: On-going regreening activities in Arusha

What can undo the gains from half-moons?

Grazing control is key.

Source: Write-up · Exhibit E (Results vary) · Justdiggit: Regreening work in Northern Tanzania (Arusha and Manyara); CEUR Workshop Proceedings vol. 3795: Evaluating reforestation techniques in arid regions of Kenya and Tanzania through remote sensing

Test Your Knowledge

All quizzes →

5 questions, all from this case file. Answers are revealed instantly.

  1. Which side of a half-moon is left open?

    1. The uphill side
    2. The downhill side
    3. Neither side
    4. Both sides
    Show the answer

    Answer: A. The uphill side It's open uphill so it catches runoff. Source: Write-up · Exhibit A (What a half-moon is) · FAO (Critchley and Siegert, 1991), Water harvesting manual, ch. 5: Semi-circular bunds

  2. What's the French term for half-moons?

    1. Demi-lunes
    2. Zaï
    3. Fanya juu
    4. Négarim
    Show the answer

    Answer: A. Demi-lunes Demi-lunes. Source: Write-up · Exhibit C (Not ancient) · FAO (Critchley and Siegert, 1991), Water harvesting manual, ch. 5: Semi-circular bunds

  3. Which Tanzanian regions does the Reel's footage relate to?

    1. Arusha and Manyara
    2. Zanzibar and Pemba
    3. Mbeya and Iringa
    4. Mwanza and Kagera
    Show the answer

    Answer: A. Arusha and Manyara Arusha and Manyara. Source: Write-up · Exhibit D (In Tanzania) · Justdiggit: Regreening work in Northern Tanzania (Arusha and Manyara); Justdiggit: On-going regreening activities in Arusha

  4. About how many water bunds have been reported in that project?

    1. More than 50,000
    2. About 500
    3. About 5 million
    4. About 50
    Show the answer

    Answer: A. More than 50,000 More than 50,000. Source: Write-up · Exhibit D (In Tanzania) · Justdiggit: Regreening work in Northern Tanzania (Arusha and Manyara); Justdiggit: On-going regreening activities in Arusha

  5. What can undo the gains from half-moons?

    1. Overgrazing
    2. Too much shade
    3. Cold winters
    4. Snow
    Show the answer

    Answer: A. Overgrazing Grazing control is key. Source: Write-up · Exhibit E (Results vary) · Justdiggit: Regreening work in Northern Tanzania (Arusha and Manyara); CEUR Workshop Proceedings vol. 3795: Evaluating reforestation techniques in arid regions of Kenya and Tanzania through remote sensing

Cite this case file

Easterling, Marcus. "Tanzanian Half-Moons." The Detail Department (DetailEnjoyer), case file DD-664, last updated Oct 20, 2025. https://detailenjoyer.com/case/tanzanian-half-moons/

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