Net And Pan

On hard, worn-out ground in a dry climate, the rain landing on one planting hole is not enough to carry a tree. When rain falls faster than soil can absorb it, the excess slides across the surface in a thin sheet. A net and pan leaves the ground between plantings bare so that it sheds, then catches that sheet behind low earth banks and steers it into shallow pits where the trees are planted. Each tree then drinks several times the rain that fell on its own ground, concentrated rather than stored, so a season without heavy storms delivers nothing.

Each cell is a small catchment feeding its own pit. The banks run either along the contour - the level line across a slope - tied into cells by short cross banks, or as a grid of diamond cells. Banks stand at least 25 cm (10 in) high, and the pit at the low point of each cell is about 80 cm (32 in) square and 40 cm (16 in) deep. Both forms suit slopes under about 5 percent, soil at least 1.5 m (5 ft) deep, and 150 to 750 mm (6 to 30 in) of rain a year. The ground also has to shed: on sand that soaks up rain as fast as it falls, nothing runs off and the structure holds nothing while looking finished.

Because the cells are linked, they fail together: a bank that overtops hands its water to the cell below, which gives way in turn, and the net unzips into an erosion channel. The site is walked after every heavy rain and any breach repaired at once, most urgently in the first season before the bank has settled and grown a grass cover. Silt off the bare ground settles in the pits and has to be dug out near the inflow, and ponding is kept shallow and brief, since standing water drowns the tree the pit was built for. It is all hand labour, roughly a person-day per two pits, and is given up on cost more often than it fails. Young trees in a bare landscape are the most tempting food for a long way around, so fencing out stock and wildlife belongs in the plan.