Turkey Nest Dam

On flat ground there is no valley to dam and no slope to catch runoff from, so a turkey nest dam catches nothing. It is a ring of earth raised above the surrounding ground, its wall built from soil dug outside the ring rather than inside, and water is lifted into it from a bore, a well or a store elsewhere. What it gives back is height: water held above the ground around it runs out to float-valved troughs by gravity. It holds nothing that was not pumped in first, so it covers the hours or days a pump is down rather than carrying a wet season into a dry one.

Two things have to be right. The basin has to be watertight, which takes about a fifth clay by weight in the fill, well compacted; even then a new pan often leaks at first and seals only as fine particles settle into the pores over several months or more, so an early leak is not proof of bad work. The second is the lifting device, engine-driven, wind-driven or worked by hand, because the store exists to cover the spells when it is not delivering and is also the thing most likely to stop. Size follows demand rather than rainfall: a beef animal drinks roughly 15 to 40 litres (4 to 10 gallons) a day in cool weather and 65 to 85 litres (17 to 22 gallons) in heat, so the store is built for the hot figure.

Its worst enemy is the sun. A shallow pan raised clear of the ground and open to sun and wind loses more water than any other store, over 2 m (6 ft) a year where the air is hot and dry, which is deeper than many of these dams are dug. Blown dust and washed-in soil fill it slowly, so it is cleaned out every four or five years. Because the water is pumped rather than caught, nothing stops the filling by itself, so an overflow is needed or the wall is topped. The part that fails most often is the fence: rings have lost their fencing within weeks of coming into use, after which stock walk on the wall and foul the water.