Clay Catchment

A clay catchment turns a piece of ground into a roof. The surface is shaped and sealed so rain runs off instead of soaking in, and a low bank around the edge gathers that runoff and leads it to a tank or pond below. It collects water rather than keeping it: nothing arrives unless a vessel waits downhill.

Everything depends on whether the soil will seal. Compacting the ground crushes the crumbs clay naturally forms into, leaving a denser mass water struggles to cross. Where there is too little clay, more is carted in and laid in layers of at least 15 cm (6 in), worked at the moisture that lets a squeezed handful hold as a firm ball; or bentonite, a clay that swells when wet, is mixed in at roughly 5 kg per square metre (1 lb per square foot) on already clayey ground, up to 15 kg (3 lb) on sand or gravel. The apron is graded smooth with just enough camber, between about one in four and one in eight, that water leaves as a thin sheet rather than in channels.

Size it against a dependable poor year, not the annual average: about two-thirds of the rain that lands can be harvested, and halving the rainfall assumed roughly doubles the area needed. The failure that defines it is drying. A pond seal stays wet because it lies under water; an apron is dry by design between storms and open to sun and frost most of its life, so it cracks unless protected and patched. Whether a compacted seal survives repeated wetting and freezing at all is disputed. A surface that starts channelling erodes and silts the store behind it. Everything that walks across the apron ends up in that water, so it suits stock behind a fence with troughs piped outside, and people only with treatment after the store. Cover the storage: open water can lose more than 2 m (6.5 ft) a year to evaporation where this practice suits. Keeping the grade true, the drains clear and the seal sound is a standing job, and the surface runs about twenty years before it needs renewal.