Claims checked › Claim
A farm pond stores water for the summer
“Dig a pond in winter and you will have water through the summer.”
In a Mediterranean summer an open pond can lose almost half its depth to evaporation before you use it. Measured at Alqueva in Portugal: 450 mm between June and September.
Where it comes from
Temperate-climate intuition, where ponds do hold water year round, exported to climates where they do not.
Who benefits
Excavation contractors and pond-liner suppliers, and programmes whose output is measured in structures built.
What the evidence says
Eddy-covariance measurement at Alqueva Reservoir, Portugal, recorded 450.1 mm of evaporation between June and September 2014, peaking at 4.5 mm a day in August (HESS 24:5973, 2020). For a one-metre pond that is roughly 45% of its depth in a single summer, our arithmetic from the measured rates. Across 76 reservoirs in Andalusia, evaporation averaged 8.0% of inflows, and the losses were driven chiefly by the ratio of flooded area to stored volume (Journal of Hydrology: Regional Studies). Water stored in the soil has no equivalent loss.
What to say instead
"Store water in the soil first. If you build open storage in a hot climate, build it deep and compact, or shade it." In wet temperate Europe, winter-fill storage does work, because the evaporation penalty is small. The water retention page sets out the ordering, and the permits page what you need before you dig.