In an exciting joint venture between the research infrastructure projects of AuScope, TERN and OZCZO and under the direction of Dr Graeme Beardsmore from the University of Melbourne, Heat Needle sensors are being rolled out at Australia’s Critical Zone Observatory (CZO) sites. The ability to introduce Heat Needle instruments into CZOs in Australia is thanks to funding from NCRIS-enabled AuScope.
What are Heat Needle measurements?
A Heat Needle continuously records the temperature profile in the top metre of the ground with high accuracy and precision at 15-minute intervals. For some readers, it will be enough to know that we are talking about what is effectively a very accurate ground thermometer. For the tech-savvy reader, we can tell you that the results are processed in the frequency domain to reveal the in situ thermal diffusivity and long-period trends in thermal gradient. The temperature response of the ground to an active heat pulse reveals in situ thermal conductivity. Gradient and thermal conductivity together give the conductive heat flow signal.
The objective
The Heat Needle initiative was set in motion in 2022 by AuScope with the objective of determining soil thermal diffusivity and thermal conductivity. Heat Needles are essentially ground thermometers detecting subsurface heat flow.
Calperum Station, about 279 km north-east of South Australia’s capital, Adelaide, was chosen as the first site because NCRIS-enabled TERN has been taking monthly recordings of soil moisture using a neutron probe to 2m for 12 years, thus providing accurate baseline data. There is now the tantalising prospect of correlating the soil moisture with thermal conductivity so that eventually a measurement of one will be enough to infer the other. Regular monthly/6-weekly measurements are taken at Calperum so the plan put into effect has been to measure the thermal conductivity at the same time as monitoring is carried out by TERN.
From a TERN-OZCZO perspective there has been great enthusiasm because of the prospect of energy-budget ‘closure’ for an ecosystem. Eddy covariance flux researchers have long wrestled with this problem because, up until now, it has not been possible to accurately determine the energy budget using eddy covariance flux tower instruments, the likely reason being that soil heat flux plates are poorly suited to capture the heterogeneity of the surface heat flux across a flux tower’s footprint. The expectation is that the Heat Needles will better characterise the heterogeneity of the system.
Left: the site of 1 of 3 Heat Needles at the Calperum CZO in South Australia showing its proximity to the TERN eddy covariance flux tower. Right: Dr Graeme Beardsmore logging a Heat Needle.
Progress
In February 2024, teams from the Universities of Melbourne and Adelaide met at the TERN Calperum Mallee SuperSite to install 3 Heat Needles to a depth of 1m adjacent to 3 soil moisture measurement sites. Thermal conductivity measurements were then made on each Needle. To date 3 campaigns have been conducted and measurements will continue regularly until about May 2025.
Results
The data below are the first measurement of heat flow for 1 of the needles at the Calperum CZO.
The ground, on average, conductively absorbed heat at a rate ~3.6 W/m2 at Site_A over a 24-hour period. The team will continue checking inward and outward radiation measurements on future campaigns to investigate if there is a similar imbalance as measured here.
The negative value of inferred conductive heat flow indicates that the profile is currently in a warming state. The shallow soil horizons are heating up so net heat flow is downwards. In cooler months the inferred conductive heat flow will likely have a positive value as the soil releases heat into the atmosphere.
The value of conductive heat flow will be influenced by the thermal properties of the soil as well as the water content because the physico-chemical and biological properties of the soil will affect the rate that heat conducts downwards/upwards. More conductive soil will stay cooler in summer at the surface, but the heating signal will penetrate deeper. Vice versa for less conductive soil. Water content and thermal properties of the soil are interrelated. Note that evaporation and condensation also carry heat into and out of the shallow soil
First measurement of heat flow at the Calperum CZO in South Australia from a Heat Needle
The future
Following a successful outcome, the intent is to install Heat Needles at the other 4 Critical Zone Observatories in Australia.
Even more into the future
There are hopes by the OZCZO team for the Heat Needles to be paired with infra-red cameras, all within the footprint of the eddy covariance tower that is already installed at each of Australia’s 5 CZO sites. This would enable scaling the soil heat flux to the flux tower footprint, and then comparing how energy budget closure compares with and without such scaling.
Feeling the heat: the AuScope-TERN-OZCZO field team at Calperum Station in the Australian summer. Graeme Beardsmore is seated logging the Heat Needles
Story and photos contributed by Professor David Chittleborough and Graeme Beardsmore: TERN-AuScope-OZCZO
