Boomerang Pan

Water running downhill across bare ground carries soil with it. A boomerang pan is a low bank of earth shaped like a crescent, open side facing uphill, its two tips level with one another. Flow from above is caught along the whole arc, and anything wider than the tips passes round the ends. Water and the soil in it settle in the pocket behind, where a tree or shrub is planted in the deepest, dampest soil on that slope face. Working at one point rather than along a line, it suits broken or steep ground where a network of banks cannot be laid.

Size follows slope. Two documented designs exist, both for gentle ground: one of 6 m (20 ft) radius with tips 3 m (10 ft) apart and a bank 25 cm (10 in) high, at 70 to 75 per hectare, and one of 20 m radius at four per hectare. Both are written for slopes of 1 to 2 percent and 200 to 750 mm (8 to 30 in) of rain a year. On steeper ground the crescents must be smaller, lower and more numerous, since overflow becomes the normal event rather than a rare one. The untreated ground above supplies all the water, at roughly 1.4 to 3 times the planted area. The base has to sit level - off the contour it drains to one tip and scours it - and flow already cut into a rill hits one arm and cuts round it.

The tips are the weak point, since every overflow leaves by them: an unarmoured tip rounds off in one storm and the bank then discharges at a point instead of spreading water, the thing it was built to stop. Where overflow is expected the tips are faced with stone. The first season is the risky one, before the earth has settled and grown a cover, so a breach is repaired at once; in the long run stiff-stemmed perennial grass on and below the bank is what keeps it. Unlike a trap built to be dug out, this one is built to fill: the pocket rises into a small planted bench, though it holds less water each year. It is hand work that does not mechanise, which limits how much ground gets treated.