Pond Evaporation Loss & Water the Air Steals

Loss volume = evaporation depth (mm/day) × surface area (m²), using the fact that 1 mm of water over 1 m² is exactly 1 litre. So a 5 mm/day loss over a 2000 m² pond is 5 × 2000 = 10,000 litres = 10 m³ a day. The tool converts your rate and area into litres and m³ per day and per month automatically.

Open-water evaporation typically runs about 4–10 mm/day, and it climbs in hot, dry, windy weather. Cool, humid, calm conditions are at the low end; a scorching windy summer day in an arid region can exceed 10 mm. Use a local pan or weather figure where you can; the tool lets you enter the rate that fits your climate and season.

Heat, dry air and wind. Strong sun supplies the energy, low humidity gives the air room to take up moisture, and wind sweeps humid air away from the surface so evaporation keeps going. That's why ponds lose most in hot, dry, windy spells — exactly when you can least afford to lose irrigation water.

Enter the average water depth as well as the surface area. The tool works out the stored volume (area × depth) and expresses the monthly evaporation as a percentage of it. A shallow, wide pond loses a far bigger share of its water each month than a deep, narrow one of the same surface area, because evaporation only acts on the surface.

With no inflow, days to empty = water depth (mm) ÷ evaporation rate (mm/day), because evaporation lowers the surface by the daily rate regardless of how wide the pond is. A pond 1 metre (1000 mm) deep losing 8 mm/day would, ignoring seepage and use, drop to nothing in about 125 days. It's a worst-case planning figure, not a forecast.

Surface area sets how many litres you lose per day; depth (relative to area) sets how long the pond lasts. Two ponds losing the same mm/day lose water at the same surface rate, but the deeper one holds out far longer. To stretch storage, favour depth over spread — deeper, narrower ponds expose less surface per unit volume.

Build deeper, narrower ponds so there's less surface per volume; add shade or floating covers (shade balls, floating panels or aquatic cover) to block sun and wind; plant or build windbreaks to slow wind across the surface; and draw the pond down before the hot dry season so less surface is exposed when evaporation peaks.

No — it isolates evaporation, the loss straight off the surface to the air. Real ponds also lose water to seepage through the bed and banks and gain it from rain and inflow. Treat the evaporation figure as one term in your water balance and add seepage, rainfall, inflow and irrigation draw to get the full picture.

Yes — any open body of water loses by the same rule: rate times surface area. Enter the surface area of your reservoir, farm dam or open tank and the local evaporation rate to get the daily and monthly loss. Covered or roofed tanks lose far less, so this applies mainly to open surfaces.

The arithmetic is exact for the rate and area you enter; the uncertainty is the evaporation rate itself, which changes daily with weather and season. Use a representative local rate, run the numbers for hot-month and cool-month rates to bracket the loss, and remember days-to-empty ignores seepage, rain, inflow and your own use.