Soil moisture sensor installation
INSTALLATION
The science team have been flat out over the last few months installing soil moisture sensors across the Mulloon catchment as an important part of the Mulloon Rehydration Initiative’s integrated monitoring program. While installation had initially been held up by Melbourne’s COVID lockdown which prevented HydroTerra team members from bringing sensors and installation kits to Mulloon. Work finally got underway when Dominic Dillon and Ben Butson were able to travel to Mulloon and work with Tony Bernardi, Joe Skuse and Luke Peel for the first week of installation. Since then Tony, Joe and Luke have continued the installation and now nearly all of the 31 Sentek sensors have been installed.
Installing these sensors involves using a purpose built auger to drill a hole into the ground from within a PVC access tube. When the auger has extracted approximately 40cm of soil, the access tube is driven into the subsequent precision hole with a sledge hammer and dolly. This process continues until reaching the preferred depth of 2m, and a ‘plug’ or bung is set into the bottom of the pipe to seal it and prevent water from entering the tube. Then the sensor is lowered into its new home and connected to the data logger and telemetry system to send data back to base.
Read more about the soils and aquifers that were encountered during installation, in ‘What Lies Beneath?’.
How it measures
For each site there are six sensors attached to a central spline at the required depths, so that for a standard hole (2m) the sensors are arranged at 10, 30, 50, 80, 100, 150cm but can be adjusted depending on final depth of the access tube. The depth taken by the bung and internal computing board also have to be taken into account as they take up approximately 40cm at the bottom of the access tube.
There are six of these dual bands of copper along the sensor. Each band produces an electric field that arcs from one ring to the other and in doing so arcs through the adjacent soil layer, measuring soil moisture. Measurements are taken every five minutes.
Each sensor sits flush with the access tube which is directly in contact with the surrounding soil (due to precision augering) and measures soil moisture by emitting an electric field out into the soil, to between 5 and 10cm. The frequency of this electric field changes in response to the soil’s dielectric properties (conductivity), providing measurements that are calibrated into volumetric water content (numerical measure of soil moisture). This is why the tube needs to be firmly plugged and desiccant attached to the sensor centre spline to keep everything inside dry.
Measurements from the sensors are sent to a logger box nearby via cable and a modem transmits the data to Sentek and also into the integrated monitoring system designed and run by HydroTerra.
Soil moisture sensor resting on fence before installation. Behind Tony is the logger box with small booster antenna, which is powered by battery and solar and will transmit collected data via the mobile network.
Why measure soil moisture?
At the surface level, while a rainfall gauge can help distinguish between rain and irrigation applications, only a soil moisture probe can show you how much of that rain has moved through the soil to the roots of the crop.
Increases in soil moisture can be attributed to irrigation, rainfall, overland flow from higher to lower lying areas, rising water tables, or if groundwater is able to rise up into the soil profile, due to capillary action and or plant roots. While decreases in soil moisture content can be attributed to water draining (gravity), evaporation from soil surface, plants transpiring water, and how much travels through to groundwater recharge.
Our scientists will be using the soil moisture data to measure and evaluate how landscape rehydration works in the MRI impact soil moisture levels. Tracking water movement through a combination of soil moisture sensors, climate stations, and piezometers measuring groundwater, scientists aim to identify how water moves through the landscape (surface and sub-surface).
When combined with other data such as satellite monitoring products by Cibo Labs and fauna monitoring (frogs, fish, aquatic invertebrates, birds etc), the data will be used to evaluate the co-benefits for productivity and environmental outcomes. This is of significant value to landholders taking part in the MRI who will be able to access the data and make more informed land use and management decisions, while also gaining a better understanding of how their properties operate, and the significant improvements in productivity, water quantity and quality and other environmental benefits.
The Mulloon Rehydration Initiative is jointly funded through the Mulloon Institute and the Australian Government’s National Landcare Program. The initiative is also assisted by the NSW Government through its Environmental Trust.
