Ditch And Bank Pans

When rain falls faster than the ground can take it in, the surplus runs away across the surface, taking soil with it. Ditch and bank pans stop that on sloping land. Low banks of earth are thrown up along the level, short ties built between them, and a pit dug where each tie meets its bank. The slope is no longer one long run for water but a grid of small catchments, each holding its share until it soaks in.

What decides whether it works is the ground it is laid on. The ties sit at a fixed interval, so the land between them must behave alike all across the hill, which only even ground does. It suits slopes up to roughly 5 to 7 percent; a larger hand-dug trench-and-pit version reaches steeper ground. It wants rain in useful but unreliable amounts, 350 to 1000 mm (14 to 39 in) a year, on any cropping soil. Banks stand 15 to 20 cm (6 to 8 in) high on 1.5 to 2 m (5 to 7 ft) spacing, with a tie every 5 m (16 ft). The crop goes in the furrow at about 65 percent of its ordinary population, because the caught water supports fewer plants well rather than many poorly.

Built and kept up by hand, it moves 270 to 480 cubic metres of earth per hectare (140 to 250 cubic yards per acre) in a season of labour, and no machine can cultivate the planted furrow. Nothing stays built: banks are rebuilt each season, pits cleared of silt before the rains, and the network shifted downslope every second or third season because the cropped furrow depletes rather than builds up. The failure that matters is overtopping, usually in a storm before the earth has settled: a network that fills and spills sends water downhill with more energy than before, so one weak bank can scour the hill it was built to protect. On heavy clay the reverse happens: held water saturates the ground and can set the slope moving. Banks alone hold back the soil while most of the water still leaves; only the fully tied and pitted form keeps both.