Optimising Irrigation with Soil Moisture Sensors

The key objective of a soil moisture measurement system is to provide the grower with continuous, accurate and representative data that helps to optimise yield and reduce operating costs.

Continuous soil moisture data is key – this will enable you to observe the effects of irrigation and rainfall events, hot dry days, etc. and to proactively manage your irrigation.

In order to achieve that ‘accurate and representative data’ above there are a few key factors to consider:

  • Sensor selection — what type of soil moisture sensor is most suitable for the application?

  • Site selection — selection of a site representative of the area to be irrigated is vital

  • Number of sensors — closely related to site selection, how many soil types, what area does each account for, how many irrigators etc.?

  • Installation — how should the sensors be installed?

When it comes to soil moisture sensors it is definitely not a case of one size fits all. All sensors are different and have different attributes, strengths, and weaknesses. Crop type, rooting depth, soil type, and many other parameters determine which sensor is the best fit for the application. Sometimes regularly updating a simple soil water balance may be the most appropriate option but it does involve working with more data. Be wary of a supplier offering only one sensor solution.

Selection of a representative site is crucial as soil moisture can vary significantly over very small distances. This requires some homework (to determine topographical information, soil type and characteristics etc.) and a site visit with the farmer to ensure representative and appropriate sites are selected. I am very reluctant to select soil moisture sites without the farmer present — they know where the old tracks, tree lines, water races (and in the case of one farm I was involved with, a WWII air strip) are.

More is not necessarily better when it comes to soil moisture sensors. A few sensors in representative locations that provide the ‘representative data’ mentioned above is the ideal. Preferably there should be at least one sensor in each soil type (provided it makes up a significant percentage of the total area) and under every irrigator. Wherever possible soil moisture sensors should be installed in undisturbed soil to reduce or eliminate any settling time.

Many farmers with soil moisture sensors are provided with a nice graph, but without the correct setup and advice this is like driving a car with a faulty speedo — you don’t really know what you’re doing.

An irrigation strategy based on robust soil moisture measurements can provide significant operational and environmental benefits, especially around the reduction of nutrient leaching. Partnering with a soil moisture sensor supplier with the necessary Irrigation Management experience and expertise will enable you to tap into these benefits.

The future

As above, one of the key issues with soil moisture measurement is the variability of the soil moisture and the fact you are using data from an extremely small area to manage irrigation of many tens of hectares.

In the future I envisage remote sensing in conjunction with sparsely deployed sensor networks to ‘ground-truth’ the remote sensing data being used for on-farm irrigation management. I was recently involved with installing a network of soil moisture sensors as part of a remote sensing project involving NASA, MIT, University of California, and the University of Auckland (amongst others) — watch this space!

For further advice or information about irrigation and soil moisture management

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