If you manage a farm dam, water storage, or reservoir in a semi-arid part of Australia, you already know that water is precious — and that it disappears faster than you’d like. But how much water are you actually losing to evaporation each year? And how do you measure it accurately enough to make informed decisions?
Knowing what evaporation is is one thing. Being able to quantify it at your specific site — with the data you actually have available — is another challenge entirely.
This article breaks down the four most effective methods to estimate evaporation from a dam, examines the science behind each, and explains how Australian dam operators can apply this knowledge in practice.
Why Accurate Evaporation Estimation Matters for Dam Management
Climate change is reshaping Australia’s water landscape. Rainfall periods are becoming shorter and more intense, particularly in arid and semi-arid regions, while temperatures continue to rise. The Bureau of Meteorology’s evaporation data shows that average annual pan evaporation in inland Queensland can reach 3,000 mm or more — far exceeding average annual rainfall in the same areas.
For the roughly 70% of Australia classified as arid or semi-arid, this imbalance is critical. Surface water storages — farm dams, irrigation reservoirs, mine pit lakes, and tailings dams — are essential buffers against dry periods. Yet evaporation and seepage are constantly eroding those reserves.
Before you can mitigate evaporation loss, you need to measure it. Underestimate your losses and you risk running out of water at the worst possible time. Overestimate and you may over-invest in mitigation infrastructure. Accurate estimation is the starting point for every smart water management decision.
Overview of Evaporation Estimation Models
Hydrologists and water engineers have developed a wide range of mathematical models to estimate evaporation from open water surfaces. These include well-known approaches such as:
- Penman-Monteith — widely regarded as the most physically complete model, but data-intensive
- Makkink — radiation-based, commonly used in temperate climates
- Hamon — temperature-based, simple but less accurate in arid conditions
- Jensen-Haise — solar radiation model originally developed for irrigation
- Morton’s CRAE (Complementary Relationship Areal Evapotranspiration) — popular in Australia for regional-scale estimation
- Brutsaert-Strickler and Granger-Grey — aerodynamic approaches suited to different stability conditions
Each model involves trade-offs between data requirements, accuracy, and suitability for different climates and water body types. For this introduction to evaporation and its estimation, we focus on the four methods most rigorously compared in peer-reviewed research for semi-arid surface water bodies — and the ones most directly applicable to Australian conditions.
The 4 Most Effective Methods to Estimate Evaporation
1. Bowen-Ratio Energy Balance (BREB)
The Bowen-Ratio Energy Balance method is one of the most physically rigorous approaches available. It is based on the principle that the total energy available at the water surface is partitioned between sensible heat (warming the air) and latent heat (evaporation). The ratio between these two — the Bowen ratio — determines how much energy goes toward evaporating water.
How it works: BREB requires measurements of net radiation, soil or sediment heat flux, energy advection, and atmospheric gradients of temperature and humidity at two heights above the water surface. The latent heat flux (and therefore the evaporation rate) is calculated from these inputs.
Accuracy and limitations: Research comparing estimation methods for semi-arid reservoirs found that BREB performed well under unstable atmospheric conditions (error approximately 11%), but was less reliable under stable conditions (error around 21%). This is a significant caveat for still, calm mornings that are common in many parts of Australia.
The main drawback of BREB is the cost and complexity of the instrumentation required. It is best suited to small water bodies where a single measurement station can adequately represent the whole surface.
Best for: Research-grade monitoring, small reservoirs, or situations where high accuracy justifies the investment in equipment.
2. Mass Transfer (MT) Method
The Mass Transfer method is grounded in the Dalton equation, which treats evaporation as a linear function of wind speed and the vapour pressure difference between the water surface and the overlying air.
How it works: The equation requires measurements of wind speed at a standard height and the vapour pressure deficit. A mass transfer coefficient — calibrated empirically — links these variables to evaporation rate.
Accuracy and limitations: The Mass Transfer method is sensitive to the accuracy of its empirical coefficient, which can vary significantly depending on the size of the water body, the fetch, and local atmospheric conditions. Research findings consistently show high relative variations in the method’s output, meaning results can be unreliable across different sites and conditions.
For this reason, the MT method is generally discouraged as a standalone estimation tool for reservoir management planning.
Best for: Preliminary or supplementary estimates only; not recommended as a primary method.
3. Priestley-Taylor (PT) Method
The Priestley-Taylor method is widely regarded as the most practical and reliable approach for estimating evaporation from open water surfaces in semi-arid regions — and it is the method that consistently performs best in comparative studies.
How it works: PT is a simplified version of the full Penman-Monteith equation. It removes the aerodynamic component and replaces it with an empirical coefficient — the Priestley-Taylor alpha (α) — set at 1.26 for freely evaporating water surfaces. This means PT relies primarily on net radiation and air temperature data, both of which are widely available from meteorological stations.
Accuracy: In comparative research, PT produced a coefficient of 1.31 against pan evaporation observations — very close to the expected range. It was found to be the most accurate and consistent method across varying atmospheric stability conditions, and critically, it requires the least amount of input data of any of the physics-based methods.
Australian relevance: For Australian farm dam and reservoir operators, PT’s low data requirements are a significant advantage. Net radiation and temperature data can be sourced from nearby Bureau of Meteorology stations or estimated from gridded BOM climate datasets. This makes PT practical even in remote locations where dedicated on-site instrumentation is not feasible.
Best for: Semi-arid reservoirs, farm dams, irrigation storages — particularly where data availability is limited.
4. Pan Evaporation (PE) Method
The Pan Evaporation method is the most widely used approach in operational hydrology and water management in Australia. It directly measures the evaporation rate from a standardised open pan of water, then converts that measurement to an estimate of evaporation from a larger water body using a pan coefficient.
How it works: A standard Class A evaporation pan — a circular steel pan 1,207 mm in diameter and 255 mm deep — is installed above ground level at a weather station. The daily change in water level (adjusted for rainfall) gives the pan evaporation rate. To estimate evaporation from a lake or dam, this figure is multiplied by a pan-to-lake coefficient (typically 0.6 to 0.8 for Australian conditions).
Accuracy: The PE method produced a coefficient of 0.65 (KP) in semi-arid reservoir research, and performed comparably to PT and BREB methods in terms of overall accuracy. It is simple, low-cost, and uses a measurement infrastructure that already exists at hundreds of Bureau of Meteorology stations across Australia.
Best for: Operational water management, standard planning applications, and situations where BOM pan data is already available nearby.
Comparing the 4 Methods: A Quick Reference
| Method | Data Requirements | Accuracy | Best For |
|---|---|---|---|
| Bowen-Ratio Energy Balance (BREB) | High — net radiation, heat flux, dual-height sensors | High under unstable conditions (~11% error); lower under stable conditions (~21% error) | Small water bodies, research-grade monitoring |
| Mass Transfer (MT) | Medium — wind speed, vapour pressure | Variable — high relative variation across sites | Not recommended as a primary method |
| Priestley-Taylor (PT) | Low — net radiation, temperature | High — most consistent across stability conditions | Semi-arid reservoirs, farm dams, remote sites |
| Pan Evaporation (PE) | Low — existing BOM Class A pan network | Good — practical accuracy for operational use | Standard planning, general water management |
Recommended approach for most Australian operators: Use the Priestley-Taylor method where possible, supplemented by nearby BOM pan evaporation data for cross-validation.
Estimating Evaporation on Australian Farm Dams
Australia presents some unique challenges — and advantages — for evaporation estimation.
The Scale of the Problem
Evaporation is the dominant water loss pathway from surface storages across most of the Australian continent. In arid and semi-arid inland areas, annual pan evaporation consistently exceeds 2,000 mm per year, with values of 3,000 mm or more recorded at sites like Longreach and Mount Isa. By contrast, average annual rainfall in many of these areas is below 500 mm. A farm dam in these regions can lose a significant fraction of its stored volume to evaporation in a single summer.
Using BOM Data
The Bureau of Meteorology operates a national network of approximately 270 Class A evaporation pan stations across Australia. Their average annual and monthly evaporation maps provide long-term gridded data that can be used to estimate potential evaporation at any location.
To use this data for dam management:
- Locate the nearest BOM evaporation station or use the gridded dataset
- Obtain the monthly average Class A pan evaporation values for your location
- Apply a pan-to-lake coefficient (typically 0.7 for Australian inland storages as a general starting point, adjusted for season and dam size)
- Multiply by your dam’s surface area to estimate total monthly volumetric loss
The CSIRO Floating Evaporation Pan
For higher-stakes applications — mine pit lakes, town water supply dams, or large irrigation storages — CSIRO has developed a floating evaporation pan system that provides significantly more accurate site-specific measurements than land-based pans.
The system consists of a 6×3 metre PVC pipe frame anchored in the centre of the dam, with a free-floating circular metal pan inside it. Instruments measure wind speed, direction, air temperature, humidity, atmospheric pressure, rainfall, and surface temperatures. An automated pump refills the pan nightly, and magnetic level sensors measure the daily evaporation drop.
Developed by CSIRO hydrologist Dr David McJannet and his team since 2012, the technology was originally designed for mine pit lake monitoring but has since been applied to agricultural water storages and town water supply dams across Australia.
Frequently Asked Questions
What is the most accurate method for estimating dam evaporation?
For most practical applications, the Priestley-Taylor method provides the best balance of accuracy and data requirements. For the highest possible accuracy at a specific site, the CSIRO floating evaporation pan provides direct, automated measurement from the water surface itself.
Can I use Bureau of Meteorology data to estimate evaporation from my farm dam?
Yes. BOM Class A pan evaporation data is available for stations across Australia. Apply a pan-to-lake coefficient (typically 0.6–0.8) to convert pan evaporation to estimated lake evaporation for your site.
How much water can a farm dam lose to evaporation each year?
In semi-arid and arid inland Australia, evaporation losses from open water surfaces commonly range from 1,500 mm to over 3,000 mm per year. A one-hectare farm dam (10,000 m² surface area) could lose 15–30 megalitres of water per year to evaporation alone.
Does the Mass Transfer method work well for farm dams?
No. Research consistently shows that the Mass Transfer method produces high relative variation in results across different sites and conditions. It is generally discouraged as a primary estimation method for operational water management.
Ready to Reduce What You’re Losing?
Estimating your evaporation losses is the essential first step — but the next step is doing something about them.
At Evap.co, we provide engineered evaporation reduction solutions for Australian farms, mining operations, and infrastructure projects. From floating covers to shade structures designed specifically for Australian conditions, we help dam operators retain the water they’ve worked to capture.
Contact the Evap.co team to discuss evaporation measurement and mitigation options for your site, or explore our guide to 5 methods of mitigating evaporation loss in dams to see what’s possible.