Floating CSIRO pan measures evaporation

In a boon for water management efforts around Australia, CSIRO scientists have developed a new way to measure evaporation rates with expert precision.

The new technology has been in development since 2012, when CSIRO Hydrologist Dr David McJannet started work to understand evaporation losses from mine pit lakes, water storages and tailings dams for the mining industry.

Since then, the CSIRO team has expanded the technology to understand evaporation for town water supply and the agricultural industry, with the newly developed floating evaporation pan measuring wind speed, direction, temperature, humidity, atmospheric pressure and rainfall.

Two sensors provide non-contact measurement of water temperature in the pan and in the lake, which enables scientists to make necessary corrections for temperature differences.

“The evaporation pan uses state-of-the-art instruments, ultrasonic algae control systems, and fully automated filling and emptying routines,” McJannet told CSIRO Ecos journal.

The floating evaporation pan is supported by a 6x3m PVC pipe frame, which is anchored in the dam or lake.

Every night, a small automatic water pump refills the pan to a predetermined level. As water level drops due to evaporation, magnetic level sensors give a high resolution reading of the water level.

“The floating pan is then left in the water body to provide ongoing daily evaporation numbers. We typically run the system for six to 12 months,” McJannet said

“Over the six to 12 months, we develop a computer model that relates lake evaporation to the meteorological variables measured by the floating pan or nearby weather station. The code we develop enables ongoing estimates of daily evaporation.”

Compared to traditional land-based evaporation pans, the floating pan is immersed in the waterbody, enabling a more accurate representation of the unique wind and energy transfer processes, and the measurement processes and data reporting are automated.

The new technology also features an 'ultrasonic algae killer' which bursts algae cells growing in the pan, allowing the system to stay on site for months.

In February 2021, McJannet’s team set up a floating pan in the Darwin River Dam, which provides 85 per cent of Darwin’s water supply, but has recently been at record low levels.

“Dam owners like Power and Water Corporation are really interested to see how evaporation rates behave in tropical environments,” McJannet said.

“Evaporation rates in the tropics are very poorly understood; most of the work has been done in southern Australia and Murray Darling Basin.

“We needed the ability to measure evaporation in situ, on the lake’s surface, which is also simpler than other techniques out there. This is a nice, robust piece of equipment that can be deployed for long periods in remote locations.

“When we’re estimating volumes of dams or lakes, we currently use models from other parts of the world. But we need to know if they’re accurate for Australia. We’re keen to test these numbers in the field, and to see if they’re true.”