The search for more precise water monitoring technologies
Australian macadamia growers, like many orchardists, have focused on indirect indications of plant water use by using soil moisture sensors to help schedule irrigation events. Irrespective of the soil moisture sensor technology, relying on very small (millimetres) samples of soil and expecting a meaningful or representative response across a large area (hectares) has always been problematic for precision agriculture.
Australia macadamia growers are now seeking more precise monitoring technologies that can directly measure the plant and its water use. The way of doing this is by using the plant itself as a sensor by measuring sap flow. By directly and continuously monitoring the whole plant, which uses its root system to sample a large volume (cubic metres) of soil, a very accurate and representative recording of water use and water status through the orchard can be made. Real-time information about a plant’s water requirement as it interacts with its ever-changing environment throughout the day and night helps growers better determine crop water requirements and hence to improve irrigation practices.
Listening to the tree with Sap Flow
In the past, managers have had access to pump and flow meter data to estimate volumes of applied water with each irrigation event, but the fate of the applied water was mostly unknown and difficult to determine.
The new approach in irrigation management begins from understanding and measuring the volume of water moving through tree stems in the orchard system. Daily water-use patterns measured by sap flow meters and water stress measured by stem psychrometers allow growers to see when their trees are active (day or night) and to more closely match the total applied irrigation water to tree water use at exactly the right time.
Identifying the seasonal differences in macadamia water use and linking these changing demands with key phenological stages of the tree’s yearly cycle will be the key to the foundation for sound and effective irrigation management. Below is water use using data for the cv816 tree over the observed 37-day period (27 August to 2 October 2018). This is the critical flowering period.
Full Article: Manson, D., & Downey, A. (2018). Sap flow monitoring a new frontier in irrigation management. AMS News Bulletin, Summer 2018. https://australianmacadamias.org/industry/resources/sap-flow-monitoring-a-new-frontier-in-irrigation-management
Tree Monitoring Proves Itself on Australian Macadamia Farm
A grower who manages more than 120 ha of mature macadamia orchards in the Bundaberg region achieved between 15-20% reduction in applied water during winter and early spring compared to the same period in the prior year by using sap flow monitoring (accounting for rainfall pattern difference between years). This grower is confident all the trees’ water requirements are being met with the improved irrigation schedule, which was developed by closely observing the constant feedback from sap flow sensors.
An added benefit is that maintaining reduced soil moisture levels has also reduced soil pathogen pressure and resulted in healthier trees, especially on the heavier soils. The lower operating soil moisture levels created by the improved scheduling have also increased the residual buffering capacity of the soil profile against over-saturation during heavy rainfall events, substantially aiding in erosion control and vigour management.
Timing the Irrigation Using Sap Flow Measurement – An Example
Macadamia flowering in Southern Queensland Australia, commences around September 1st lasting approx. one month. It is important to maintain high water use and low water stress during flowering to maximise potential yield.
Interpretation of the sap flow and water potential data during the last week of August indicated the commencement of water stress as the sap flow rate steadily dropped from approx. 20L to 12L/day.
Irrigation was applied on September 2nd and water use or sap flow increased from approx. 12 L/day to approx. 24 L/day.
Time series overlay of sap velocity (Inner & Outer) 7 days prior to irrigation showing a steady reduction in sap velocity.
Changes in Plant Water Use 7 days prior to Irrigation.