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Irrigation Scheduling

Rob O'Connor, an experienced Irrigation Officer with DEDJTR works with Accelerating Change on all things irrigation, including scheduling. Rob shares with us, below, lessons learnt from the project about soil moisture monitoring and the use of moisture probes, and the value of using ET data to schedule irrigations.

Lessons learnt about soil moisture monitoring

Soil moisture probes have been used on the partner farms under different forages in 2015-16 and through the recent winter-spring period. The probes have provided useful information largely for irrigation scheduling, particularly when the moisture probe information was regularly revised and used in conjunction with other scheduling methods already used on farm.

Getting the most out of probes

It was evident from probe data that a shallower depth of moisture extraction was occurring under pasture compared to lucerne. Moisture uptake was typically occurring to a depth of 50-70cm under pasture and 100-120cm for lucerne. (Refer to figures 1 and 2 below.) Usually when the soil probe software was configured to only collect moisture data from soil depths where plant roots were active (rather than leaving all the probe sensors switched on for the different depths), more representative soil moisture information was obtained.

Figure 1. Soil moisture content at each sensor depth on Stewart-Matthew’s perennial pasture. Relatively more moisture is being extracted from the upper depths of the pasture root zone compared to lower depths.

Figure 2. Soil moisture content at each sensor depth on Stewart-Matthew’s lucerne. Moisture is being extracted more evenly and over a greater rooting depth with the lucerne compared to perennial pasture.

A means of using probe data to determine when to irrigate is to observe when the rate of soil moisture extraction begins to slow down and plant available moisture becomes limiting. This point is identified on the moisture graph where the slope of the line starts to flatten out and takes on a curve-like shape. (Refer to figure 3 below – labelled “classic moisture stress curve.”) The point at which this change occurs was more obvious over-all for probes installed under lucerne than for pasture, making optimum irrigation timing more obvious for lucerne using this approach.

In a pasture situation, plant roots are likely to be most active in the top 10-20cm with activity progressively declining down the soil profile to a depth of approximately 60-70cm. With some software packages this variation in root activity can be accounted for by changing the relative weightings of moisture probe sensors at different depths. More accurate estimates of the relative moisture uptake occurring at each depth down the rooting profile results in a better representation of changes in plant water availability. This enables improved irrigation scheduling.

One tool in the toolkit

For Stewart-Matthew’s lucerne there was a reasonably good match of the indicators for optimum irrigation timing sourced from the probe data and from ‘evapotranspiration minus rainfall’ (ET-R) data. Typically, the soil moisture depletion between irrigations shown in the moisture graph (or the size of the “dip”) corresponded with the magnitude of cumulative ET-R. (Refer to figure 3 below.) This match provided increased confidence in the use of the moisture probe in this situation and demonstrates the value of using more than one tool for irrigation scheduling. Kelvin (partner farmer) relied heavily on probe data to determine when to irrigate his lucerne for a significant part of the season.

It should be noted the correlation between probe readings and ET-R data is never going to be perfect in this situation because of other factors influencing the water requirement of lucerne including variation in canopy height and grazing.

Figure 3. Average soil moisture content for Stewart-Matthew’s lucerne, combined with ETo-R between irrigations. There was a reasonably good correlation between recorded soil moisture depletion and ETo-R.

Advantages of moisture probes

An advantage of moisture probes is they can provide a permanent record of soil moisture levels. In the words of one Coleambally agronomist “probes gather information day in day out, without forgetting, changing their mind or only remembering the good bits.” This information can then be used to review irrigation management practices through the season and help with better irrigation decision making in the future. Using cumulative ET-R between irrigations in conjunction with probe data provides a comparative benchmark to review probe data and irrigation timing. In figure 3, it can be seen irrigations usually occurred at approximately ET-R=100mm, which is well in the recommended ballpark for lucerne. Where there were any significant variations from 100mm, questions can be asked about reasons for this variation. In this case, farm management practicalities such as grazing management and trafficability provide a reason for varying irrigation timing - particularly extending the interval for a drought hardy plant like lucerne.

Partner farmer feedback

The partner farmers (Kelvin and Tim) both mentioned another advantage of using probes is an ability to see the impact of rainfall events on soil moisture at different depths and how quickly it is used up by plants. “It’s easy to think you won’t have to irrigate (pasture) for another 9 days after an inch of rain, but you might realistically only get another 5 days when you look at it” explained Tim. Tim added “I’ll also be watching the probes when I think it’s time to start-up irrigation (for the season), which is beneficial.” Kelvin summed up. “I’m happy with the probes. There is definitely a place for them. They give you more confidence with your decisions.”

Next steps

As part of the Accelerating Change project, capacitance probe data will continue to be monitored under perennial pasture (seasonal conditions permitting) and lucerne on the partner farms through the 2016-17 season. The suitability of gypsum blocks to gauge soil moisture conditions on dairy farms is also likely to be examined. Like capacitance probes, gypsum blocks can be connected up to telemetry systems for screen viewing. The value provided by soil moisture monitoring under sorghum, fescue and annual pasture is also up for investigation.

Rob measuring lucerne canopy temperature at the Stewarts farm at Yarrawalla.

Cool tool back soon

The weekly evapotranspiration (ETo) email updates that were received by PIT members last irrigation season will start up again in the next couple of weeks (or when it stops raining,) for the 2016-17 irrigation season.

How irrigators have been using the ETo updates

It’s been great to receive feedback over the last 12 months about the ETo emails. Most of the feedback has been positive and there were also some questions about ETo and how it can be used to improve irrigation management.

Some PIT members have been getting good results from using the service as described in the following quotes, “With ETo, I’m improving efficiency heaps” and “I’m growing the best pastures on this place… (along with some other changes on farm) milk production is up…”

Irrigators have been using information in the ETo emails in different ways. Some say they simply use the recommended surface irrigation interval as a guide for irrigation timing in conjunction with other scheduling methods they already use on farm. “I use it (ETo) to see if I’m on the right track… now I check more often and do some ground-truthing (in the paddock).”

Other farmers said they used both the recommended irrigation interval and daily ETo figures shown in the email. “I look at the recommended irrigation interval first, then at the ETo figures.”Typically the ETo figures were used for checking other scheduling methods already being used by calculating cumulative ETo minus rainfall (ETo-R). For rye grass based pasture, irrigations are usually timed when ETo-R equals 40mm to 50mm, starting from when surface water has drained away following the last irrigation.

A number of irrigators commented they browse the forecast information to alert them of upcoming weather events so they can better plan for their next irrigation. “I look at what the ETo rates are and if there are any high days or low days, and I look at predicted rainfall.”

One farmer with pressurised irrigation was using the ETo data to plan irrigations to minimise pumping at the higher electricity tariff or the “day rate,” to reduce power costs.

Using ETo to schedule irrigation for crops other than pasture

“Irrigation boffins” regard good pasture as the “reference crop” and give it a ‘crop coefficient’ of 1. ET from a stand of good pasture is referred to as “reference ET” or “ETo.”

The theory is other crops that use more water than pasture such as a good mature stand of lucerne or a tall dense maize crop are given a crop coefficient closer to 1.2. Where-as recently grazed lucerne, which has a low canopy density will use less water and will typically have a crop coefficient of less than 1. The appropriate crop coefficient can be multiplied by cumulative ETo since the last irrigation (also taking rainfall in to account) to determine the irrigation requirement of the particular crop (“ETc”).

Deeper rooted crops will typically be able to access more available moisture in the soil profile. This explains why a crop with deep roots like lucerne is suited to a longer irrigation interval (than pasture) of ETo-R = 80mm to 100mm. Whist maize is usually suited to an irrigation interval of ETo-R = 60mm to 70mm.

That’s the theory, which can be very useful for irrigators to know. In practice on most farms, like with pasture, irrigation scheduling for crops is typically determined using a number of tools that have already been tried and proven. However, ETo can be an objective measure to add to the toolbox to help ‘keep on track’ and better plan irrigations for optimal production. As one learned cropping farmer explained, “I use the ETo figures in combination with my bike and shovel…. ETo is handy because sometimes you don’t realise how much the plants have used, 50mm can come up pretty quick.”

Other ETo Services

ETo is widely regarded by different agricultural industries as an objective and reliable tool for scheduling irrigations. As well as dairy farmers, irrigation croppers and horticulturalists in the region have subscribed and are using the (‘northern Victorian’) ETo email service. Currently there are approximately 300 irrigators and service providers on the email list. AgVic provides other ETo email services in North East Victoria, Macalister Irrigation District (East Gippsland) and in Sunraysia.

Keep your eyes out for a simplified weekly ETo feature in future editions of the Country News this irrigation season.

A graph from the weekly ETo email showing daily ETo and rainfall at Kyabram for the past week and the forecast week. This information is being used by farmers to improve irrigation practices.

Of interest is that cumulative daily ETo (17mm) only just exceeded cumulative rainfall (15mm) over the last seven days at Kyabram indicating practically no irrigation was required. Next week cumulative rainfall (40mm) is forecast to exceed cumulative ETo (20mm).

If you would like to subscribe to the weekly ETo email update, or find out more on irrigation scheduling, please email robert.oconnor@ecodev.vic.gov.au


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