Well-planned and executed irrigation events can significantly boost productivity and profitability, whilst achieving water-use efficiencies and a reduction in input costs.
By using soil moisture sensors or a weather-based water balance calculation, irrigation can be more accurately scheduled. It can also help determine how much water during irrigation is required to keep soil moisture in the optimal 'Readily available water' (RAW) zone.
In addition to using soil moisture monitors, adopting a soil water deficit approach rather than a fixed frequency and amount approach to irrigation scheduling, can assist in improving productivity and water use efficiency. A soil water deficit approach is simply based on a water balance calculation, with the amount of irrigation required being equal to the amount of soil water deficit.
A simple water balance calculation is a good starting point for irrigators currently not using a scheduling tool to plan their irrigation requirements. It gives an estimate of the irrigation water required, assuming the area to be irrigated was at field capacity (no deficit) on the previous day: Current soil water deficit (or irrigation required in mm/day) = reference evapotranspiration (ETo mm/day) – rainfall (mm).
Where the rainfall amount is greater than the RAW for the soil type and plant species, rainfall is lost as run-off or infiltration below the root zone. Using the soil water deficit method, the total number of days before irrigation is required again is Readily Available Water divided by Evapotranspiration (RAW/ETo). At this point, the soil moisture will be at the refill point.
For example, if rainfall is 30mm, but RAW for the soil type and crop type is 24mm, that means only 24mm will be held in the soil and easily accessed by the plant for growth. If Evapotranspiration for the pasture or crop type is 6mm per day, if there is no further rainfall over the next four days, irrigation will be required.
Irrigation scheduling tools
There are several free sources of information to help you establish and then maintain an irrigation schedule which sits above the refill point in the RAW zone. This includes:
- Weatherwise - is a free seven-day weather forecast notification email service with data supplied by the Bureau of Meteorology.
- IrriPasture - is an online weather-based water balance scheduling and recording tool from the University of Southern Queensland’s Centre for Engineering in Agriculture.
- IrriSAT - is an app used in grain and cotton industries that uses Landsat and Sentinel satellite data to provide water management insights and the crop coefficient (Kc) of an actively growing plant.
- Agriculture Victoria Weekly Irrigation Requirement Summary – is an interactive irrigation scheduling tool that services the Murray Dairy region.
- DairySA has an irrigation requirement spreadsheet that is used locally and a weekly Irrigation Requirement Report for the Mt Gambier and Mt Compass regions.