4 Best Methods to Estimate Evaporation Levels in Semi-Arid Areas

If you manage a farm dam or water storage in a semi-arid part of Australia, you already know water disappears faster than you would like. The useful question is how much, and how to estimate evaporation from your dam well enough to make decisions about covers, design or supply.

Evaporation is the change of liquid water into vapour at the surface, driven by heat, dry air and wind. For the background, see what evaporation is and what drives it.

Why accurate evaporation estimates matter

Average annual pan evaporation exceeds 2,000 mm across much of inland Australia and passes 3,000 mm in parts of inland Queensland, as shown on the Bureau of Meteorology evaporation maps. In many of those areas, annual rainfall is under 500 mm.

Farm dams, irrigation storages, pit lakes and tailings dams are the buffer against dry periods, and evaporation and seepage wear them down. Underestimate your losses and you can run short at the worst time. Overestimate them and you may overspend on mitigation. A sound estimate is the starting point for both.

Which evaporation models exist?

Hydrologists use many models to estimate open water evaporation, including:

• Penman and Penman-Monteith: physically complete but data hungry.
• Makkink: radiation based and common in temperate climates.
• Hamon: temperature based, simple but weaker in arid conditions.
• Jensen-Haise: a solar radiation model first developed for irrigation.
• Morton’s CRAE: the Complementary Relationship Areal Evapotranspiration model, used for regional estimates in Australia.

A study of 18 methods on a semi-arid reservoir found that the Bowen ratio energy balance method is the reference standard but needs data that is often unavailable, and that simpler temperature and radiation methods tracked it closely, per Majidi and colleagues in Water Resources Management (2015). The four methods below cover the practical range for Australian dams.

The 4 most useful methods to estimate evaporation

1. Bowen ratio energy balance (BREB)

The Bowen ratio is the ratio of energy used to warm the air (sensible heat) to energy used to evaporate water (latent heat). BREB measures the energy arriving at the water surface and splits it between the two to calculate evaporation.

It needs net radiation, water heat storage, and temperature and humidity measured at two heights above the water. That makes it accurate but expensive, and it suits small water bodies where one station represents the whole surface.

Best for: research-grade monitoring and benchmarking other methods.

2. Mass transfer (MT)

Mass transfer is based on the Dalton equation, which treats evaporation as a function of wind speed and the vapour pressure difference between the water surface and the air above it.

Its weakness is the empirical coefficient linking those inputs to evaporation. That coefficient changes with dam size, fetch and local conditions, so results vary widely from site to site unless it is calibrated locally.

Best for: supplementary estimates only.

3. Priestley-Taylor (PT)

Priestley-Taylor simplifies the Penman equation by dropping the wind term and replacing it with a fixed coefficient, alpha, usually 1.26 for freely evaporating water. It relies mainly on net radiation and air temperature.

Because those inputs are available from Bureau of Meteorology stations and gridded climate data, PT works even at remote sites without instruments. It can underestimate on very windy, dry days, when the wind term it ignores matters most.

Best for: farm dams and storages with limited site data, cross-checked against pan data.

4. Pan evaporation (PE)

Pan evaporation is the most widely used method in Australian water management. A Class A pan is a standard circular steel pan, 1,207 mm across and 255 mm deep. The daily drop in its water level, adjusted for rain, gives the pan evaporation rate.

A pan heats up and catches wind more than a dam does, so the pan figure is multiplied by a pan coefficient, typically 0.6 to 0.8, to estimate dam evaporation. It is simple, cheap and backed by a long national record.

Best for: everyday planning on farm dams.

Comparing the 4 methods

Method	Data needed	Accuracy	Best for
Bowen ratio energy balance	High: net radiation, heat storage, two-height sensors	Reference standard	Research and benchmarking
Mass transfer	Medium: wind speed, vapour pressure	Variable between sites	Supplementary only
Priestley-Taylor	Low: net radiation, temperature	Good where wind is moderate	Remote dams, limited data
Pan evaporation	Low: BOM Class A pan data	Good for planning	Most farm dams

For most Australian dam owners: start with pan evaporation and cross-check it with Priestley-Taylor if you have the data.

How to estimate evaporation on your farm dam

1. Find your nearest BOM evaporation station, or read your location off the gridded evaporation maps.
2. Get the monthly average Class A pan evaporation for your location.
3. Multiply by a pan coefficient. Around 0.7 is a reasonable start for inland storages, adjusted for season and dam size.
4. Multiply by your dam’s surface area to get the monthly loss in volume. Our guide on measuring your dam’s surface area shows how.

For example, a dam with a 0.5 hectare surface, 2,000 mm of annual pan evaporation and a 0.7 coefficient loses about 7 megalitres a year (5,000 m² × 1.4 m). For a worked guide by region, see how much water your dam is losing to evaporation.

What is the CSIRO floating evaporation pan?

For mine pit lakes, town water storages and large irrigation dams, land-based pans are not always accurate enough. CSIRO developed a floating evaporation pan that sits on the water and measures daily evaporation directly, along with wind, temperature, humidity, pressure and rainfall.

CSIRO hydrologist Dr David McJannet’s team has run the floating pan since 2012, first for mine pit lakes and tailings dams and later for town water and agricultural storages. Deployments typically run 6 to 12 months to build a site model, according to CSIRO.

What to do once you know the loss

Once you have a figure, you can weigh the fixes against it. Depth matters: see why depth in dams is so important. For covers, see how floating covers and windbreaks stop evaporation and the shade ball cost calculator.

Lindsey Hughson and the Big Ditch team use the same pan and surface area method when sizing evaporation control for a client’s dam.

Frequently Asked Questions

What is the most accurate method for estimating dam evaporation?

Bowen ratio energy balance and direct on-water measurement such as the CSIRO floating pan are the most accurate, but they need instruments. For most farm dams, BOM pan data with a pan coefficient gives a practical planning figure.

Can I use Bureau of Meteorology data to estimate evaporation from my farm dam?

Yes. Take the Class A pan evaporation for your location, multiply by a pan coefficient of 0.6 to 0.8, then multiply by your dam’s surface area.

How much water can a farm dam lose to evaporation each year?

Inland pan evaporation commonly runs from about 1,500 to over 3,000 mm a year. After a 0.7 pan coefficient, a 1 hectare dam surface would lose roughly 10 to 21 megalitres a year.

Does the mass transfer method work well for farm dams?

Not on its own. Its results depend heavily on a site-specific coefficient, so without local calibration it can be well off. Use it only as a cross-check.

Want a real number for your dam’s evaporation loss? Book a site inspection with Big Ditch and we will measure it and price the fix.