Pond Evaporation Loss & Water the Air Steals
Measures pond loss
Enter the evaporation rate and your pond's surface area to get water lost in litres and m³ per day and month, the share of stored volume lost, and the days to empty with no inflow.
Your pond
Next: cut losses by keeping ponds deeper and narrower (less surface per volume), using shade, floating covers or windbreaks, and drawing the pond down before the dry season.
Evaporation varies with temperature, humidity, wind and sun; use a local pan or weather estimate for your site.
Pond evaporation — key facts
- Loss/day
- rate (mm) × area (m²)
- 1 mm over 1 m²
- = 1 litre
- Open-water rate
- ≈ 4–10 mm/day
- Worst weather
- hot, dry, windy
- Days to empty
- depth (mm) ÷ rate (mm/day)
- Cut losses
- deeper/narrower, shade, windbreaks
- Excludes
- seepage, rain, inflow, use
- Privacy
- Runs in your browser; nothing uploaded
Open water leaks straight into the sky
A farm pond loses water you never see leave: sun, dry air and wind lift it off the surface day after day, and in a hot windy summer that can be 8–10 mm a day — over a wide pond, thousands of litres before any animal drinks or any crop is irrigated. Because evaporation acts only on the surface, a shallow, sprawling pond bleeds a far bigger share of its store each month than a deep, compact one. Knowing the number turns a vague worry about "drying out" into a plan.
This tool computes the water lost in litres and m³ per day and per month, the percentage of stored volume gone each month, and the days to empty with no inflow from your evaporation rate, surface area and depth. Use it to judge whether a pond will last the dry season and to weigh covers, windbreaks or a deeper design. Pair it with the Farm Pond, Rainwater Harvesting and Water Tank Capacity tools to size and protect your storage.
See the real loss
Litres and m³ off the surface every day and month.
Will it last?
Days to empty with no inflow, as a worst case.
Surface vs depth
See how much storage shallow ponds give away.
Plan defences
Weigh covers, windbreaks and a deeper design.
Frequently Asked Questions
How is pond evaporation loss calculated?+
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.
How much does an open pond lose to evaporation?+
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.
What drives the evaporation rate?+
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.
How do I find the share of stored volume lost?+
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.
What does 'days to empty' mean?+
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.
Does surface area or depth matter more for losses?+
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.
How can I cut evaporation losses?+
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.
Does this include seepage or rainfall?+
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.
Can I use it for a reservoir, dam or tank?+
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.
Are the figures exact?+
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.