Ponds In Series

Ponds in series are two or more ponds built at different heights down a slope and connected so that water spilling from one runs into the next below it. The point is to keep water high in the landscape rather than in one basin at the bottom, because water held high can reach anything below it by gravity alone. The ponds are not copies of each other: the upper ones are working storage, drawn on whenever water is wanted, and the lowest is a reserve that catches the seepage and runoff of everything above it.

What makes the arrangement pay is vertical fall, the height difference between a pond and the ground it waters. Fall turns into pressure at a fixed rate: about 15 m (50 ft) of drop gives roughly 1.4 bar (20 psi), which is enough to run most sprinklers. Ground that far below a pond can be watered with no pump at all, so the fall a site offers decides whether a series is worth the earthmoving. Splitting one volume across several basins spreads more water surface to the sun, so a chain loses more to evaporation than one deep pond holding the same water, not less.

A chain carries risks a single pond does not. Where it is fed by water running back off irrigated land rather than by clean rain, each reuse removes volume and leaves the dissolved salts and nutrients behind, so every pond down the line is more concentrated than the one above, and the lowest becomes a disposal problem rather than a store and the first to run short of oxygen. The structures depend on each other as well: one pond's outflow is the next one's inflow, so every outlet must be sized for what arrives from above, and a blocked outlet or a burst bank empties into a basin never built to take it, carrying off the ones beneath. Every embankment and spillway needs inspecting yearly and after any storm big enough to run one, and the top pond silts up fastest, catching the sediment for the whole chain.