Farm dam evaporation: how much water are you actually losing?
I have built over 341 dams across NSW, and the one thing that surprises new dam owners is how fast a dam can drop without a leak. In most cases it is farm dam evaporation. It is silent, it is constant, and in a hot, dry Australian summer it will chew through your stored water before you know it.
In this post I will show you how to estimate what you are losing, what changes those losses, and what actually works if you want to keep more water in the dam.
What is farm dam evaporation?
Evaporation is water turning into vapour and leaving the dam surface. It is driven by sun, heat, wind and dry air. The key point is simple: evaporation comes off the surface area, not the volume. A wide, shallow dam loses water faster than a deeper dam holding the same amount of water.
In practical terms, you see it as a steady drop in level over days and weeks, even when the wall and the back of the dam look sound.
How much water can a farm dam lose to evaporation?
You can get very technical with this, but you do not need to. For farm decisions, you need a rough, defensible number.
Here is the back-of-the-ute method I use on site:
- Work out the surface area of your dam (in square metres). If the dam is roughly rectangular, length × width gets you close. If it is irregular, pace it out and sketch it, then estimate.
- Estimate an average daily evaporation rate for the season. In many inland parts of NSW and QLD in summer, you can easily see several millimetres per day. Coastal areas are often lower but wind can push it up.
- Convert millimetres to litres. Every 1 mm over 1 square metre equals 1 litre. So if your dam is 10,000 m² and you lose 5 mm in a day, that is 50,000 litres gone that day.
That number shocks people because it adds up. Over a month, even a modest daily loss turns into a serious chunk of water.
Example: A 1 hectare surface (10,000 m²) dropping 4 mm per day is losing about 40,000 litres per day. Over 30 days that is about 1.2 million litres. If you rely on that dam for stock water or irrigation, that matters.
Why your dam level drops faster some weeks
Evaporation is not a flat line. It swings with the weather. These are the big drivers I see:
1) Wind
Wind strips the humid air sitting on the water surface and replaces it with drier air. That speeds evaporation. A dam on an exposed ridge will lose more than a dam tucked behind a tree line or a hill, all else equal.
2) Low humidity
Dry air can hold more moisture, so it pulls more water off the surface. Those hot, dry westerlies are brutal for evaporation.
3) High temperature and strong sun
Heat and solar radiation supply the energy for evaporation. Clear, still, hot days can still burn water off, even without much wind.
4) Surface area
This is the one you can control with design. When a dam is shallow and spreads out, it exposes more water to the air. As the level falls, some dams keep a big surface area for a long time, so losses stay high.
Evaporation or leak: how to tell the difference
Before you spend money, you need to know what you are dealing with.
I use a simple check:
- Look at the pattern. Evaporation is steady and weather driven. Leaks often show as a faster drop that does not match the conditions.
- Walk the outside of the wall. If you see wet patches, lush green strips, boggy ground, or a trickle, treat it as a leak until proven otherwise.
- Check the overflow and outlet. Sometimes the problem is not the wall. It is a valve, a pipe, or a siphon leak.
- Do the bucket test. Put a bucket of water in the dam, weighted so it does not float away. Mark the water level inside the bucket and the dam level outside. If the dam drops more than the bucket, you likely have leakage as well as evaporation.
Many properties have both. A dam can evaporate and leak at the same time. That is why guessing costs money.
What actually reduces farm dam evaporation
People hear about tricks and products all the time. Some help. Many do nothing. Here is what I recommend, in order.
1) Build or reshape for depth, not spread
If you are planning a new dam, this is where you win. A deeper dam with less surface area for the same volume will lose less to evaporation. It also tends to stay cleaner and cooler.
If you already have a wide, shallow dam, you can sometimes reshape it during a clean-out. It depends on your soil, your available catchment, access for machinery, and whether the wall and spillway can be modified safely.
2) Wind protection, done properly
A windbreak can reduce the wind speed across the water surface. The key is placement. A few trees on the wrong side do not help. A shelterbelt in the right position can.
You also need to think about leaf litter, roots near the wall, and access for maintenance. I have seen well-meaning planting create more problems than it solves.
3) Reduce dead water and keep the dam functional
When a dam silts up, it becomes shallower. That increases surface area and increases evaporation. A clean-out that restores depth can reduce losses and give you back storage you already paid for.
4) Floating covers and shade systems
These can work, but you need to be realistic about cost, durability, and maintenance. They suit high value water, small storages, or critical supply points. They rarely make sense for a big farm dam unless the numbers stack up.
5) Products and additives
You will hear about additives that form a film on the surface. I do not like relying on them. Wind breaks the film, stock disturb the surface, and you end up with an ongoing cost and mixed results. If you are considering a product, ask for independent testing in conditions like yours.
Design choices that help you keep more water
If you are building new or upgrading, these are the design choices I focus on:
- Shape and depth: aim for practical depth and avoid sprawling shallow shelves.
- Catchment match: bigger is not always better. If the catchment cannot fill it, you just built a large evaporating pan.
- Spillway and freeboard: protect the wall so you do not lose the dam in one storm and start again.
- Access and maintenance: you will need to desilt and inspect. Plan for it.
I have seen dams that hold water beautifully because the design suits the site. I have also seen dams that fight the climate because they were built too wide and too shallow.
Quick checklist: what I look at during a site visit
- Surface area versus depth at current water level
- Exposure to prevailing winds
- Signs of seepage or leakage around the wall and abutments
- Silt levels and whether the dam has lost capacity
- Catchment yield and how often it should refill
- Stock access points and erosion risk
When you measure those things, you stop guessing. You can decide whether to live with evaporation, reduce it, or rebuild for better storage.
FAQ
How many millimetres per day is normal for farm dam evaporation?
It depends on season and location. Hot, dry, windy weeks will push it higher. If the drop spikes well beyond what the weather explains, check for leakage as well.
Does a deeper dam really lose less water?
Yes, because evaporation comes off surface area. For the same storage volume, a deeper dam usually has less surface area exposed to the air, so total losses are lower.
Do shade balls or floating covers work on farm dams?
They can reduce evaporation, but they are expensive and need maintenance. They suit smaller storages or high value water. Most farms get a better return by improving dam design, depth and wind exposure.
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