What they are
A microcatchment is a small, deliberately-shaped piece of ground that collects rainfall runoff from a bare "catchment" area and concentrates it into a smaller "cultivated" area where a plant grows. Instead of trying to store water, it simply moves the rain a few metres — from where it would be wasted to where a tree or crop can use it — so that in a place receiving too little rain to grow a tree, the tree gets the rain from several times its own area and thrives.
The principle is the same as the zaï pit but at a slightly larger scale: each structure serves one plant or a small group, and the ratio of catchment to cultivated area (often 3:1 up to 10:1) is tuned to the rainfall and the crop.
The common forms
Half-moons (demi-lunes). Crescent-shaped earth bunds, typically 4–6 m across, opening uphill to catch runoff, with the tree or crop planted at the low centre behind the bund. They are cheap, quick to make with hand tools, and the standard structure for establishing trees on gentle dryland slopes. Rows of half-moons are staggered so that water missed by one is caught by the next downslope.
Negarim. A grid of small diamond-shaped basins, each a square catchment tilted so its lowest corner holds an infiltration pit with a tree in it. Developed in the Negev desert (hence the name), negarim are the classic microcatchment for planting orchards in true desert, each diamond harvesting its whole area into one corner.
Contour ridges and tied ridges. Small earth ridges along the contour, sometimes "tied" with cross-walls into a chain of rectangular basins, so runoff is held in place across a field rather than allowed to run along it.
Basins and saucers. Simple shallow depressions or ring-basins scooped around individual trees to pond water at the roots — the simplest microcatchment of all, and standard practice when planting any tree in a dry climate.
Why they work
Microcatchments turn the enemy — runoff — into the resource. On crusted dryland soils much of the rain runs off rather than soaking in; a microcatchment intercepts that runoff and forces it to infiltrate where the plant needs it. The concentration means a tree effectively receives several years' worth of its "fair share" of rain, enough to establish deep roots and survive the dry season.
They are a cornerstone of dryland restoration and reforestation, used to establish drought-tolerant species — acacias, argan, fruit and fodder trees — on land too dry to plant them conventionally. They pair naturally with mulch in the basin to cut evaporation, and with contour bunds and check dams at the larger, slope- and gully-scale.
Setting them out
The key decisions are the catchment-to-cultivated ratio (bigger ratio for drier climates), the spacing (close enough that runoff is captured, not lost downslope), and setting the bunds accurately on the contour so water spreads rather than concentrates and breaches. On steeper ground the structures must be smaller and stronger. Overflow points should be planned so that a heavy storm spills safely from one structure to the next rather than bursting the bunds — the same overflow thinking that governs any earthwork.
Like all earthworks, microcatchments are best laid out by eye and simple levels, refined over a season or two as you watch where the water actually goes.
See also
- Zaï Pits the same principle at pit scale
- Contour Bunds the slope-wide runoff-slowing partner
- Finding Contour laying the bunds out level
- Rainwater Harvesting Basics the wider family of techniques
- Reforestation Techniques establishing trees on dry ground
- Drought-Tolerant Species what to plant in the catchments