Across the Fenceline: Soil Moisture Probes in Harden
A time-tested way that farmers gather information to better manage their land is to look over the fence and see what their neighbours are doing. The Across the Fenceline initiative began in 1999 as a collaboration between HMLG, CSIRO and the Grains Research & Development Corporation.
Since 2001, this project has provided local growers with real-time plant-available water measurements from soil moisture probes installed across the Harden district.
The aim of the original study was to investigate the possibility that leakage of excess water from beneath crops was contributing to groundwater recharge, elevated water tables and consequently, the extensive dryland salinity observed in the Jugiong Creek Catchment.
It all began with leaky soils...
Drainage meters developed by CSIRO were used to monitor deep drainage at selected trial sites; the first being the Harden Tillage Trial site. The ability to measure drainage would enable growers to understand the impacts of their land management practices on deep drainage and dryland salinity; and make adjustments accordingly.
Using funds from a National Heritage Grant (2001), five on-farm paddock comparison sites were established to compare soil moisture and deep drainage in two paddocks under different management regimes. However, with the onset of drought coinciding with the start of the trial, the project soon evolved into one monitoring plant available soil water alone.
Soil measurements are collected at each farm site in two adjacent paddocks. The soil moisture probes have been installed 20 m on either side of the fenceline to ensure they are unaffected by firebreaks and headlands.
Soil water content (i.e. volume) is measured at five depths at each location at depths of 0.2, 0.5, 0.8, 1.2 and 1.5 m below the soil surface. The shallowest depth was chosen to ensure that the sensor is below the depth of cultivation.
The sensors have been buried at least 30 cm deep. These are connected by underground cable to a solar-powered data logger located at the fenceline. The Sentek EnviroSMART® sensors take measurements every 30 minutes. A rain gauge is also located at each site.
Our soil moisture probes are located at various farm sites across Harden Shire
What does the data mean?
To obtain an estimate of the water stored in that soil layer, the soil water content or volume (mm) recorded at each depth is multiplied by each depth interval (mm) This calculation, referred to as the measured soil water content, includes all water in the soil, including that which plants cannot extract because it is too tightly held by soil particles.
The graphs show us how much water is stored in the soil
In a typical Australian duplex soil, such as those around Harden, the amount of water that plants can extract decreases with increasing depth. Stored water in deeper soil layers tend to show less seasonal variation as less water infiltrates to these depths, and few crop roots extend to these depths to extract water.
Not all water in soil can be taken up by plants. We can calculate this.
Plant available water refers to the volume of water available for uptake by plants (mm/mm). Calculation of plant available water requires an understanding of the soil’s lower limit for water storage. The lower limit refers to the water content of the soil when it is so dry that plants cannot extract any more water from it. Soil upper and lower limits can be obtained as a field measurement or in the laboratory. This usually requires multiple measurements made over time to minimise uncertainty due to within site/paddock soil variability. Based on our current data, we only have an estimate of the lower limit at one site, Traralgon.
Plant available water can be calculated for each soil layer by subtracting the soil lower limit (field- or laboratory-measured) from the estimated total stored water (as measured by the soil moisture probes). This value is our best estimate of the amount of water available to plants. Plant available water varies with depth and soil type/texture. Soils with high clay content typically have higher proportions of bound water that cannot be extracted by plants.
HMLG measure the amount of plant available water relative to how much water was stored in soil at the end of the 2013 growing season
As the soil lower limits are not known with sufficient certainty for all measurement sites, the data presented here is not plant available water as traditionally defined. Instead, we present the amount of water in each layer relative to how much water was stored at the end of the 2013 growing season.
This project was funded by Riverina Local Land Services Strategic Partnerships and undertaken in collaboration with Young District and Eastern Riverina Landcare Groups, Murrumbidgee Landcare Inc, Cross Property Planning Groups, Riverina Highland Landcare Network and Temora Agricultural Bureau.