Thursday, 19 January 2017

A Guide to Making Sense of Soil Moisture Data

With an increasing amount of soil moisture monitoring sensors on offer in the market today there is growing importance on not only having sensors installed but actually understanding the information they provide. This blog is written to give some insight into the data they you might receive from one of these devices. The following traces are the output of an AquaCheck soil moisture sensor with 3G telemetry. The sensors measure soil moisture and temperature every 30 minutes. The data is available to Agri Optics clients from the AquaCheckWeb platform. For more info see our website

The key to getting the most out of your soil moisture sensor is to have an accurate field capacity (FC) and refill point for the probe site calculated. The most convenient way of identifying Field Capacity is to have the probe installed prior to the winter period. Typically there will be enough precipitation to allow the profile to recharge to FC. FC can also be identified by saturating the profile manually with a large quantity of water. The key points we are looking for when identifying FC is a repeated filling to saturation then drainage of the profile. The point where drainage ceases can be identified as FC. Night time events are more accurate as ET is not a factor.
Fig 1. Identifying field capacity
The next key feature to identify is drainage. Drainage is classified as the loss of soil water past the effective rooting zone. The effective rooting zone varies dependent on the crop. Once the depth of plant roots has been identified we can identify any drainage. For the graph below if the crop has a rooting depth of 600mm. The bottom pink line represents the sensor at 600mm. We can see the lift and subsequent drainage of soil water past the effective rooting zone of 600mm.

Fig 2. Drainage events
The third key piece of information that the AquaCheck package provides is the ability to set variable management allowable deficit (MAD) lines. These lines create the target “Green Zone” typically between 85% and 15% of readily available water (RAW). Using MAD lines leaves room for any rain so that any free rain water is not wasted as drainage. It also gives an indication when soil moisture is approaching stress point. The MAD is able to be adjusted to give a desired target soil moisture zone for crop and pasture growth stages e.g. establishment or harvest.
Fig 3. MAD Lines
The final bit of information that becomes available once the crop starts growing is the daily soil water usage. The staircase like moisture trace is showing us evapotranspiration and it allows us to see the impact that increasing crop biomass and increasing temperatures are having on crop or pasture water usage. Crop rooting depth can be identified by seeing how far down the water usage is occurring. In fig 2. above below the roots are drawing moisture down to 600mm vs a later spring sown wheat in fig 4. which is only drawing water to 400. Note the size of the usages. This relates to the root mass at the given depths.
Fig 4. Crop water usage
I hope these tips are useful when interpreting your soil moisture data and that it results in more efficient scheduling of your irrigation this summer. Irrigation New Zealand also has some more tips and info on their website

Post By Nick