Irrigation scheduling

News story from 'Climate Change, A NSW Department of Primary Industries special feature', in The Land, October 2008

Andrew Parkes, Moree

Irrigation scheduling involves applying the right amount of water in the right place at the right time in order to maximise production and improve water use efficiency.

Soil moisture monitoring tools are commonly used in the irrigation industry to assist growers like Andrew Parkes of Keytah, Moree, and Von Warner, the manager of Bullamon Plains, Thallon, with their scheduling decisions.

To have confidence in any soil moisture monitoring tool you need to ensure it is located in the most representative part of the field in which it is used to schedule irrigations.

A moisture probe placed in the wrong spot can result in over- or under-irrigating the main soil type in that field or management unit.

Traditionally growers like Von and Andrew would site their probes visually, from experience or gut feel, but today these growers believe they can do it better.

A visual only offers an inspection of the surface and until recently this has been good enough but today we have the technology to look below the surface, to build a more defined and accurate picture of the majority soil types.

Electromagnetic Induction (EM or EMI) surveying, used in conjunction with soil sampling, can be used to map soil variations across fields and farms. It does this by measuring the soil's apparent electrical conductivity (ECa), which is related to factors such as soil texture, soil moisture and salinity. Ground-truthing is essential to calibrate the instrument. This involves the collection and analysis of soil samples from known positions and relating the results to the EM readings.

An EM survey can give an indication of texture changes over the field and an analysis of the data provides maps of similar soil types. These maps can be used to locate the majority soil type within a field.

Andrew and Von are convinced about the benefits of EM soil surveys on their farms. Both growers have used calibrated EM maps to examine soil variability across their fields in order to position moisture probes in sites that are representative of the field, ensuring that their probes are located within the majority soil type, year in and year out.

"Using EM survey to assist siting moisture probes has given me more confidence with my scheduling decisions", Von said. "It gives me the ability to draw down water and stretch irrigations if necessary."

Von did point out that moisture probes are just one tool he uses to schedule irrigations. Keeping a close eye on weather forecasts and visual inspection of the crop is still vital.

For Andrew, the change in practice for siting moisture probes occurred when capacitance probes first came to the fore. The use of telemetry meant these probes could be placed anywhere in the field. Previously he would position the probe tubes in a section of paddock that looked representative, but was also easily accessed. Back in 2001-02 he was sitting down with Andrew Smart from Precision Cropping Technologies, Narrabri, looking at yield maps.

"I asked him how he knew the probe was placed in the right area in terms of soil water-holding capacity," PCT's Andrew Smart said. An initial EM survey using an EM38 showed that the EM data on Keytah was heavily influenced by clay content and therefore data from the EM survey could be used to provide a detailed map of potential water-holding capacity to around 1.2-1.5 metres.

(Farmer) Andrew Parkes took a GPS reference of the probe site and found that, as luck should have it, he had placed the probe in a site that was close to the fields majority soil type (hence majority water-holding capacity), but it also pointed out the variability of soil in this field. In fact, close to the probe site was a section of field that was much lighter in texture, and he could have just as easily placed the probe there by mistake and then irrigated the field by that area.

To further enhance probe placement, an EM soil variability map can be overlayed with a slope map to analyse variations from perfect plane (to make sure the probe is not placed in a hollow or a ridge) and also a cut and fill map if the field was laser levelled in the last 2-3 years.

These layers of data can then be combined to produce a map which best represents majority soil type, closest to majority slope and in some cases removal of areas of high previous cuts and is then used to site the location of the probe in the field. In conjunction with this type of map, Andrew (Parkes) reminds us that ground truthing is still critical.

"You need to check your probe is placed in an average plant stand which is also representative to the rest of the field."

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