Recently both PITs have had technical sessions on the value of moisture probes. Rob O’Connor and Dale Board, both from DEDJTR, have been talking to PIT farmers about the benefits and challenges of using soil moisture monitoring tools. A summary of Dale’s previous research into these tools in the region can be found here: http://irec.org.au/farmer_f/pdf_186/Soil-moisture-monitoring.pdf
Soil moisture monitoring provides a tool that enables irrigators to make more informed decisions regarding water requirements of various crops, which can result in on-farm water efficiency gains and increased productivity.
Some key lessons discussed for moisture probe sessions:
Moisture probes allows you to refine your irrigation schedule to increase water efficiency and productivity
You get the best value from moisture probes when you aim maximise production rather than to minimise absolute water use
Moisture probes allow you to maximise return on investment in farm layout and delivery upgrades as you can improve the timing of irrigation. This compliments greater precision and flexibility offered with improved infrastructure to apply water.
There are a number of different types of soil moisture monitoring devices that can be installed in a range of price ranges. To assist you choose the right device DEDJTR have developed this note: http://agriculture.vic.gov.au/agriculture/farm-management/soil-and-water/soils/choosing-the-right-soil-moisture-monitoring-device
Probes with telemetry systems (wireless systems that use the phone network to remotely send data from the paddock back to your phone or computer) are rising in popularity and can be used in conjunction with automation to irrigate your farm remotely.
Benefits of Moisture Probes:
Number 1: You can clearly identify periods when your plant is not using moisture and therefore not growing at full potential. This could be from waterlogging or moisture stress.
In the above graph the flat line period after this indicates some water-logging (blue oval). The dry autumn had the soil profile dry out from Lucerne growth to wilting point (red oval) and moisture stress occurring in mid-April. The coloured lines show the moisture level at different depths in the soil profile while the vertical bars show rainfall.
Number 2: You can see the impact of rainfall events. This will allow you to reschedule irrigation events to maximise the impact of rainfall and reduce water applied without the risk of loss of production. The target is to apply water to the root zone and not beyond.
Number 3: You can use moisture probes to assist plan the first irrigation for the season.
The above graph shows the accumulation of soil moisture over winter and depletion in spring until the first irrigation was required in October.
A number of PIT farmers that use soil moisture probes have indicated that it can be difficult to work out the refill and saturation point for their pastures. Probe data is displayed with graphing software allowing the ability to apply indicative wilting and saturation point usually highlighted in background colours like red or blue on these graphs. It is important to ground truth what your particular points are specific to your farm by validating soil moisture conditions with a physical examination of the soil at the depths where the sensors are located in the soil profile. Further steps can be taken to dry these soil samples in ovens to determine the mm of water in the soil by using the wet weight and dry weights in a volumetric soil equation. Alternatively ground truthing can occur by using cumulative evapotranspiration data or other scheduling methods that have been established on the farm.
The graphs with one line show the summed value of soil water content over the total depth of the probe. This could be 80-120cm or more. It is also important to look at the individual stacked sensor graphs that show soil moisture change trend lines over different depths. This will show you where moisture reaches with irrigation events and the impact of rain. This graph will also display the rootzone of the plant and where the moisture consumption is occurring. This will , indicate if the plant is nearing moisture stress so irrigation scheduling can be planned in conjunction with forecast weather, plant growth and future management activities on the paddock.
Evapotranspriation and rainfall: one way of validating soil moisture conditions
Cumulative evapotranspiration data can help you validate the soil moisture conditions you are viewing on your soil moisture probe. You can use this data to estimate how much and when you soil moisture content is likely to have decreased and the impact of rain on your soil moisture. For example if you require 40mm of readily available water in the root zone:
For the past week at Kyabram, Evapotranspiration –Rainfall=37mm. This equates to an average daily ETo of 5.3mm /day
To work out the irrigation interval, divide the desired mm of readily available water in the root zone by average daily ETo and add 2 days to allow for excess water to drain away, which is typical following a surface irrigation event:
40mm/5.3mm+2=rounded up to 10 days.
ETo for next week is forecast to be 36mm. The good news (unless you have hay down) is that 22 mm of rainfall is forecast, which will allow the irrigation interval to be extended.
Daily historical (eg. past week) ET figures for select locations are available on the BOM 'Water and the Land' site http://www.bom.gov.au/watl/eto/
The forecast ET figures provided are from a 'fee for service' web site.
Rob O’Connor from DEDJTR also provides a weekly ET email update "Irrigation Requirements Summary".
Contact Rob at email@example.com to sign up.
Example Evapotranspiration and rainfall data at Kyabram: