Transcript of the tools for better irrigation scheduling webinar

Heather Field:

Webinar today, in our climate webinar series we will here from Rob O'Connor from on irrigation scheduling rules to achieve top yields and water use efficiency. Rob will provide information on evapotranspiration tools and services available to irrigators and will share results and learnings using soil moisture monitoring equipment for more informed irrigation decision-making.

Heather Field:

Rob O'Connor is a senior irrigation extension officer with Agriculture Victoria. Rob has a long history working with farmers on irrigation related issues. The past five years, Rob has specialized in the area of irrigation scheduling. I'll now pass over to Rob O'Connor from Agriculture Victoria.

Rob O'Connor:

Thanks Heather and thanks to everyone for joining in and I hope you are looking after yourselves in this current environment, and if one thing, I guess it's helping us to appreciate social and communication media like this Skype session.

Rob O'Connor:

My PowerPoint is not moving. There we go. Okay. What I plan to do in this session is to provide some background firstly in terms of irrigation settings, where irrigation scheduling fits in and what irrigators are currently doing in terms of irrigation scheduling. I'll focus on two methods of irrigation scheduling, that being evapotranspiration and soil moisture monitoring. I'll talk about what it is, how it is used and some of the tools available for both of those methods. I'll also finish with a summary, just summarizing the different methods and tools and the pros and cons of each of those. So the key messages that you'll hear in this session is that there are some great tools and services available to help to get irrigation scheduling right and both evapotranspiration and soil moisture monitoring provide objective information to help to eliminate the guesswork and when those tools are used in conjunction with the other scheduling tools already in your toolbox, it better enables you to achieve high yields and high water productivity.

Rob O'Connor:

In the last five to 10 years there's been increased interest in irrigation scheduling and our irrigation settings have changed at quite a rapid rate, particularly in the Southern Murray Darling basin, we've seen high water prices. Our irrigation supply system has been modernized and farm irrigation layouts have been upgraded meaning that farmers have more control over water on their farms and can better match irrigation applications with plant water requirements. There's also been new irrigation systems go in like the pressurized systems and irrigators are growing different crops for the first time which are responding quite well to improve irrigation scheduling.

Rob O'Connor:

Irrigators are seeing the results of improved soil moisture management and by keeping soil moisture in that zone of readily available water or optimum soil moisture, able to maximize plant growth rates. Also irrigators are quite busy, farms are getting bigger and they've got a lot on their plate so these irrigation scheduling tools can help reduce the labor involved in irrigation management. Irrigators currently use a number of different tools including their experience, the time of the year, a shovel just to dig up the soil and see what moisture is there or an electric fence post as a penetrometer to gauge how hard the soil is and what moisture is in there, constantly monitoring the weather conditions. So yeah, they're already using quite a lot of tools to determine when to irrigate. Most of these tools are subjective by someone on gut feel, so that's where soil moisture monitoring and evapotranspiration data can come in to provide some numbers and some accuracy around irrigation scheduling. So the first method that I'd like to talk about is evapotranspiration using evapotranspiration or ET data to make better irrigation scheduling decisions.

Rob O'Connor:

So evapotranspiration is just water loss from the plant and from the soil and its driven and calculated using four different weather variables, sunshine, temperature, wind and humidity measured in millimeters, usually you see it as millimeters per day or millimeters per week. And it tends to be relatively consistent across a flat landscape. So yeah, five to ten kilometers generally from the weather station, generally will be very consistent. Evapotranspiration data is usually provided as reference evapotranspiration or ETo which is the evapotranspiration from a good actively growing pasture stand. So, to work out when to irrigate or to work out irrigation requirements it's just a matter of adding up the daily ETO minus rainfall in millimeters so with the pressurized system it's just a matter of applying the volume in millimeters on our reference stand of pasture with surface irrigation, it's a matter of working out how much water that your particular bay or paddock can hold, the readily available water and using that as a refill point.

Rob O'Connor:

So, I mean, ballpark figures for that readily available water or refill point of 40-50 mLs for pasture, 60-70 mLs for maize and 80-100 millimeters for lucerne as a guide. ETO is multiplied by a crop coefficient to estimate crop water requirements, so for instance, a stand of mature actively growing lucerne or maize will use water at a higher rate than our reference stand of pasture. So in that case, we're using a crop coefficient of 1.2, so we're saying it's using about 120% of the water requirement for pasture. For the fresh early cut lucerne for example, it's only using 0.6 of what a standard good pasture would use, or a nearly emerging crop it might be as low as 0.3.

Rob O'Connor:

This table's just showing how we can use ET data to work out the irrigation interval on surface irrigated mature actively growing maize. So, running from day zero to day nine in the rows and irrigated on day zero. In this particular case we're saying there's 60 millimeters of readily available water to the maize crop. So we've got our reference evapotranspiration daily in its first row here applying a crop coefficient of 1.2 for this particular maize crop and not applying the crop coefficient by the reference evapotranspiration to get crop water use. In this third row, an allowance for any rainfall. So just subtracting rainfall from the crop evapotranspiration and then accumulating the crop evapotranspiration minus rainfall in this row here and then when we near our 60 mills of readily available water we know it's time to irrigate on that day nine.

Rob O'Connor:

So that's just a... yeah. It gives you a bit of an idea of how to use ET and integrating in-crop coefficience in that readily available water. So you've done well if you could follow me through that and there won't be any more maths after this. The good news, there are tools available to help you to be able to use ET without doing the math. So in Victoria here, there is a weekly email, irrigation requirements service for the Goulburn-Murray, North East, North West of the state and also for Gippsland. I'll talk a bit more about those in a tick but they can be viewed on this extension Aus. Irrigating Ag. Website and you can subscribe through that website for your particular irrigation district. Another tool is IrriSAT, a web-based tool that uses satellite data and ET information and tailors that to your paddock actually, your farm situation.

Rob O'Connor:

I'll talk a bit more on that in a tick and there are other commercial tools, integrated tools that not only help you with irrigation scheduling but... so they would use ET data, soil moisture monitoring data, they might use irrigation flow and other weather variables to help you with not only irrigation scheduling but things like fertilizer and pest management on your farm and an example of those is the SWAN system and the yield which is listed down the bottom here, yeah. So there's certainly much more complex tools available.

Rob O'Connor:

Just in terms of sourcing ET data, it can be sourced through the bureau of meteorology and this picture in the top right corner is showing that and you can see the ETO and rainfall daily data for the past week. Currently ETO is sitting around about 4.5 millimeters per day or about 32 mills per week so it's certainly coming of the summer highs and will continue to reduce as we move through autumn. You can also source past ET data through a site called SILO which is a Queensland group and that is for any location around Australia, so a good source of data there. It can be sourced through SWAN, which is a Western Australian company and that is shown in this table here showing the forecast evapotranspiration for the coming week.

Rob O'Connor:

So that SWAN data is sent as a daily email and provides useful information about ET but also other weather information, chance of rain and amount. Also wind speed can be pretty handy, so it's just a useful little email to get each day. And the other source of ET data is through a company called TheYield and they provide past daily ET for the past week and forecast ET for the coming week. So ET data is readily accessible, you just need to know how to get it. It can also be provided through IrriSAT if you are prepared to set that up. Just in terms of the weekly irrigation requirements email, I'll just focus on the one for Goulburn-Murray Irrigation District which is the one I send out. You can see it is for specific locations but as I mentioned ET is consistent across a reasonably large distance, 10-20 kilometer type of a distance between these weather stations.

Rob O'Connor:

So we've got information provided for the previous week, reference evapotranspiration and that's converted into the required surface irrigation interval so this time of the year depending on the location but 10-12 day type irrigation interval based on a 40 ml readily available water for our reference crop of pasture and just showing how many mLs needs to be applied for a spray irrigation situation. There's a similar table that's also provided for the next week and daily ET provided for each location for the previous week and next week. There's also an interactive scheduling tool that is provided in this email, so that can be tailored to your unique circumstances. Just a matter of adding in your location the irrigation deficit and refill point and crop coefficient so there's plenty of support information available with that spreadsheet.

Rob O'Connor:

And just plug in when you last irrigated, you can change the rainfall data based on the rain received in your rain gauge and this is just showing the cumulative ET and we can see I've put in here it's irrigated last on the 17th of March and looking at the projectory here, we'll need to be irrigating probably in late March, probably the 28th or 29th by the time it hits this, I've put in a 50 mill refill point. So just a handy little tool to help with irrigation decisions. We have had good feedback on the email service. Just that it's a good reminder, handy tool particularly when used in combination with tools that have already been used and getting results in terms of improved production. The IrriSAT tool I mentioned, web-based tool using satellite and ET information, a matter of going to the site, do need to create a google account and mapping out your paddocks. So once you've done that it will give you a crop coefficient for your paddock. IrriSAT, it's a free program and it uses three different satellites so you can usually get a satellite image within the seven day period.

Rob O'Connor:

You can see there's a little bit of variation in the crop coefficient on these particular paddocks. Probably of more value in those terms in a pressurized system like central pivot, lateral move where you might get a bit more variation to help diagnose non-uniform sprinklers for example. IrriSAT also provides the past daily ET values and future ET values daily for the next week. Just a matter of entering in previous irrigation whether it's surface irrigation or pressurized and rainfall data and it will tell you when the next irrigation is due based on the refill point that you enter into IrriSAT. So a pretty handy tool, just takes a few hours to learn how to use it upfront and a little bit of time to enter in your irrigations for each paddock.

Rob O'Connor:

I'll move on to soil moisture monitoring. And just to note that the examples I'll use here are actually in pasture and irrigated cropping situations. These tools are equally as effective in horticultural situations and the results of draw on are from on farm trials run as part of a joint accelerating change Murray Dairy Project which was run between 2015 to 2018. So there are a range of soil moisture monitoring equipment including different types of senses, different types of telemetry and different software packages that show the soil moisture monitoring data on your monitoring device.

Rob O'Connor:

So the three types of sensors that I've selected here are gypsum blocks or a granular matrix sensor. In this particular picture it's shown here connected up to a manual read device called a G-Dot, I'll talk a bit more about that in a tick. Capacitance probe in this situation connected up to telemetry for remote monitoring and capacitance or TDR spikes. In this situation connected up to a smaller type of transmitter. So these come at different cost. For this type of a set up you're looking at close to $1000 with the three capacitance spikes that you can put it at different depths connected up to your transmitter. The capacitance probes usually quite a bit more expensive and with the telemetry to transmit the data to your monitoring device, usually looking at thousands of dollars for this type of set up and the gypsum block really depends on what type of telemetry you're using in terms of the overall cost and whether it's a manual read or a remote read type of a setup.

Rob O'Connor:

So this is just a closer look at this G-Dot device. Set up on a dairy farm under pasture, the particular dairy farmer was quite happy with this system in his words it was a winner. Set up next to the lane way here, he was whizzing past on his motorbike every day, so he could see this gauge from his motorbike. Just had to be, I suppose, a little bit careful with these gypsum block devices in that they can have a lag period in terms of the moisture content of the gypsum block equilibrating with the moisture content of the surrounding soil. So, it was fine in winter and spring, in summer when we were on a shorter seven day type irrigation interval. We noticed there was probably about a two day lag so what we were doing instead of irrigating when four of these dots had disappeared, we were irrigating when only two of the dots had disappeared to allow for that lag period.

Rob O'Connor:

This screenshot is showing the image from a capacitance probe showing the individual soil moisture at individual depths. So you've got the typical stepping pattern as moisture is drawn out at a faster rate during the daytime than at night time. You can see that moisture is drawn out firstly from the shallower depths so that the 10 centimetre depths here then deeper as things dry out and time progresses. So this particular capacitance probe went down to 80 centimeters I have turned off the top four of those sensors just to simplify this image but also because most of the active root zone is in the top 30 centimeters with pasture. You clearly see the impact of a rainfall event, here down clearly to the 20 centimeter depth. You can also see after an irrigation event that the soil is drying out quite quickly at the 10 centimeters, it only takes about eight hours for it to dry out, probably about 20 hours at the 20 centimeter depth, so just indicating that the drainage on this irrigation bay is quite good.

Rob O'Connor:

This is the same graph as the last one but I've just taken an average over that top 40 centimeter depth. So just noting that some software packages have the ability to do an average trace. Other software packages don't have that ability but it's a pretty neat way just to look at changes in soil moisture in that active root zone. I've just added this slide in to show the value of taking soil cores. One is a simple irrigation scheduling tool, you can also get a pretty good gauge of the rooting depth just by sorting through the soil core. We've got the depths of the core written on this half piece of PVC pipe, bit of equipment here that we use to take the cores and it can also be used just as a cross-check with the soil moisture readings at that particular point in time.

Rob O'Connor:

You can see we can only form balls with the soil below 30 centimeters indicating that the soil was drying at this particular point in time. Below 30 centimeters we could easily ribbon it indicating that it was quite moist at depth. So this particular core was taken 30 ml ETO minus rainfall of 30ml after the last irrigation. So still a bit of juice left in the system, I suppose in terms of plant water extraction and after we took those cores irrigator put the order in for watering and by the time we irrigated the cumulative ETO minus rainfall was at 41ml so pretty well timed.

Rob O'Connor:

This is showing soil moisture under irrigated lucerne so a screenshot and I've added in some extra details such as rainfall, the triangles are irrigation and those numbers accumulative ET minus rainfall since the previous irrigation. Of note is the impact of grazing on the lucerne. You can see there's a slower uptake of moisture after grazing which you might expect with a smaller lucerne plant. Also important to note the flattening of the curve where there hasn't been any grazing indicating that moisture is getting hard for the plant to extract and indicating that it's time to irrigate. Yeah. And a reasonable sort of relationship there with the size of the cumulative ET value and the size of the dip in the curve or the change in soil moisture allowing for grazing.

Rob O'Connor:

This is just another soil moisture screenshot using these capacitance spikes. So, this particular software system we weren't able to average soil moisture for the 20 and 30 centimeter depths but you can see here giving our refill point, irrigator's doing a pretty good job irrigating close to that refill point and again a good relationship with accumulative evapotranspiration and the change in soil moisture indicated by the soil moisture gear.

Rob O'Connor:

So the feedback we had on the soil moisture gear from these monitor farms was that the information from the soil moisture probes was very useful, helped with decision making and helped to improve irrigation management and production. I should mention just for those irrigators in the Shepp irrigation region, there is a financial incentive currently available for soil moisture monitoring gear for both pressurized and surface irrigation systems. It's on a dollar for dollar basis up to $5000 of incentive per farm, available for a limited time possibly till the end of this financial year but contact Rabi Maskey for more details.

Rob O'Connor:

In terms of soil moisture monitoring projects, the smarter irrigation for profit project is running a national project with on-farm optimization sites demonstrating this irrigation technology. In the Murray dairy region there's two sites, one that's surface irrigated, one with a center pivot and both of those are being double cropped. So it does involve soil moisture monitoring equipment, weather based scheduling and crop yield measurement and if you want to get involved, you can do so through field days, through monthly irrigation reports and by hoping on the Daily Australia or Sustainable Irrigation for Profit websites.

Rob O'Connor:

This as a way of summary. Been going for about 35 minutes now. Just the pros and cons of the different irrigation scheduling methods and tools. So like the ET method is free, you have reasonably high confidence in it as a scheduling tool, very tried and proven. It applies to a large area certainly what applies at one end of the farm or will apply to the other end in terms of evapotranspiration rates. And it also provides forecast information to help you plan out irrigation events that are upcoming. I guess the negative is the use of the crop coefficient and interruption from grazing or hay cutting, not an issue in pasture. The crop coefficient stays pretty constant with grazing on good pasture and there are tools to work through that for you . Not as useful for the spring irrigation start-up. Just the first irrigation in spring due to the number of error and the number of daily readings required since our last irrigation back in autumn. And there is some data source variability for instance, the SILO data, it tends to be a little bit lower than the ET data that you would source from the BoM.

Rob O'Connor:

But really it's I guess, a little bit of a crooked stick and if you can use the same crooked stick, the same data source for your ETO that can help. The ET is readily accessible. You don't have to install or maintain equipment as you do with soil moisture monitoring gear. Pretty intuitive I think most of us tend to monitor the weather anyway and you'll find that ET probably aligns pretty well there and like all of these tools it's advised to use them in conjunction with other tools. Soil moisture monitoring, an advantage is that you can see what's happening at different depths down the soil profile in terms of wetting up and drying out with plant growth. It does provide valuable data for spring irrigation start-up, grazing and hay cutting is automatically adjusted and can provide a very useful record of your irrigations and particularly if you want to review your irrigation practices or your remote from the farm and someone else is irrigating you can still monitor what's happening.

Rob O'Connor:

A disadvantage is the upfront cost for the soil moisture monitoring gear and ongoing annual subscription or data costs for that gear. The soil volume that's monitored is only small with soil moisture monitoring gear. Most of those systems the volume of soil is probably only about as big as your hand so you need to make sure it's installed in a representative soil type and area of your farm. Installation and maintenance required and some of the soil moisture monitoring sensors are temperature sensitive meaning that as the soil temperature changes you can get a slightly different soil moisture reading. So, for instance, moving from the cooler soil temperatures in spring into summer, you might see a small upward drift in your soil moisture monitoring readings and refill point. Other advantages is that you can monitor your soil moisture remotely from wherever you happen to be with your monitoring device. There are a choice of software packages available, some are quite simple which suite some people and some are more complicated meaning you can get a lot more detail which suites others. You need to be careful to protect the gear from stock and machinery and training and back-up services quite important with soil moisture monitoring gear just so you know how to use that data and are quite confident doing it with irrigation scheduling.

Rob O'Connor:

So, a consideration there in terms of your purchase to make sure the training and back-up is provided. In terms of the individual soil moisture monitoring devices, the capacitance probes tend to have a long life, 10 years plus they'll monitor soil moisture multiple depths. Some of these probes can be 1.2 meters and you can get readings from each 10 centimeter increment. At the volumetric soil moisture measurement, which means it needs to be calibrated for soil types or alternatively you can go through a number of irrigation cycles to help to work out where that refill point needs to be and higher upfront cost with capacitance probe. So depending on the probe but somewhere about around $1000 per probe. They need to be installed by someone with experience, obviously installation is quite important in terms of the data you get from them and as a bonus you can get soil temperature measurements from the capacitance probe.

Rob O'Connor:

Gypsum blocks measure soil tension which is an advantage so that applies across different soil types for example with pasture you're looking to irrigate it at 35 kilopascals to optimize growth rates and that's uniform across soil types. It's a cheapest sensor generally around $100 a sensor. Does have a limited lifetime, you're looking at three to five years per sensor but there is a lag period that we talked about. And again you need someone experienced to help with installation. These capacitance spikes, a cheaper type of sensor and there is cheaper telemetry available for these particular sensors. It's again a volumetric measurement so calibration or irrigation cycles required to get it right. Just be aware of the annual subscription costs, if you're using one transmitter for each set of sensors that are connected to the transmitter. They're pretty easy to install and there's some simple software available that people like for those capacitance sensors.

Rob O'Connor:

Just overall summary, so irrigators already using a range of scheduling tools but if you can use ET data and or soil moisture monitoring information with those tools you can be better enabled to make better irrigation scheduling decisions and to get top yields and high water productivity. If you want more information try the weekly irrigation requirement summaries and there are fact sheets available both for evapotranspiration and soil moisture monitoring. I'll pull it up there Heather.

Heather Field:

Okay great. Thanks Rob and thanks for your informative presentation today. We've had some good interests and we've got about 45 people online at the moment, so I'm sure they'll be a few questions coming in. I've had a couple questions already but if anyone wants to ask a question please type that into the Q&A or the chat box which you'll see located if you hover down the bottom of your screen you'll see the icons for that and I can ask those questions of Rob. While we wait for some more questions I'll just read out what we've got at the moment. And apologies if I pronounce incorrectly but Yah Chan has asked just for some clarification between evapotranspiration and your ET and ETO.

Rob O'Connor:

Okay. So, ETO is a reference evapotranspiration and that is the evapotranspiration or plant water requirement for an actively growing stand of good healthy pasture and the specification here is for pasture that's 120 millimeters in height. There are different types of ET, I suppose you can have your ET crop which takes into account that crop coefficient. There's also other measures of ET such as short crop ET and tall crop ET but I'll not go into that right now. I hope that answers the question adequately.

Heather Field:

Thanks Rob. We've got a question from Nick. He wants to know what is a reasonable crop factor for irrigated eucalyptus trees ages four to five.

Rob O'Connor:

For eucalyptus trees, wow. I really don't know to be honest Heather. Presumably it would be above the crop coefficient that we use for maize and lucerne of 1.2 but I think I'm not being in the role of growing eucalyptus trees. I would have to google that one.

Heather Field:

Yep. All right. We have got a question from Janet. Sorry I missed how we work out the RAW, would you please explain this again.

Rob O'Connor:

Yeah, okay. So, RAW is the readily available water, so that's water that your crop can extract from the soil before there's any yield penalty. So I'll put up some ballpark figures here for the readily available water for the different types of crops. So the RAW can be determined using lab analysis or even just from using textbook information for particular soil types just keeping in mind that the soil type will change as you move down the soil profile. Yeah. Generally... yeah. Okay. Yep, yep. I'll just pull it up here I think Heather, yep.

Heather Field:

Yeah, sure. Thanks for that. We've got a question from Paul. Are you aware of any councils who are using these tools for parks?

Rob O'Connor:

These tools. Okay. Again, I'm not in the business of working with councils. I do know that some of them manage irrigation quite precisely given the shortages of water that towns are faced with and they use different tools including soil moisture monitoring tools and evapotranspiration.

Heather Field:

Thank you. Graeme would like to know if you can run us through what's on the extension Aus website.

Rob O'Connor:

The extensionaus website. So, in the presentation I referred people to the extension Aus irrigating Ag website to see those weekly irrigation requirements updates for the different irrigation regions in Victoria so they're there. But there's also a swag of information on the Extension Aus Irrigation Ag website in terms of different irrigation and technical information whether that is popular articles about technical topics such as managing pressurized irrigation systems and other irrigation related information. AgVic and NSW DPI use it as a site to post their irrigation information and also for seasonal response or seasonal type information.

Heather Field:

Thanks Rob. We have a response to the crop coefficient for eucalyptus from Natalie Mason, so she says standard crop coefficient for eucalyptus ranges from 0.98 to 1.20 depending on time of the year.

Rob O'Connor:

Okay. Thanks Natalie.

Heather Field:

Thank you. I don't think we've got any other questions. I'll just double-check. Yes we've got... no, no, just a comment. Thank you Rob and Heather what about also ETOV and ETOR, what are the difference?

Rob O'Connor:

I'm not sure exactly what that is referring to unfortunately Heather.

Heather Field:

Okay. It was just ETOV and ETOR so let's see if we can get a little bit information on that. And will your presentation slides be available to us? Are you happy for me to circulate your slides Rob?

Rob O'Connor:

Definitely.

Heather Field:

Yep. I'll make sure I do that. That's all the questions we have at the moment. While we are just waiting to see if we get any more. I'll just remind everyone there'll be a survey following today's webinar so please take a minute to complete that. That would be greatly appreciated. And we do have another webinar scheduled for the 16th of April and at that webinar we'll hear from Jeff Steedman from the department of land, water and planning. We'll be talking about the Victorian water and climate initiative. So if you are a subscriber to our webinars you will get some more information about when you'll be able to register. We have another question that's come in. How seasonal climate forecast can be applied useful for irrigation scheduling? How can seasonal climate forecast aid irrigation scheduling?

Rob O'Connor:

I guess that's a difficult one Heather. I guess that's coming down to longer term water planing and more of the budgeting rather than immediate irrigation scheduling. So if the forecast is dry for example over the next three months particularly coming into spring, irrigators would be looking at management strategies to deal with that whether if they've got stock, potentially sourcing... how they source their feed, whether its brought in feed or through irrigation water. For example, it might influence what they're prepared to pay for water but mainly I think like the longer term forecasts are about the seasonal management strategies rather than immediate irrigation scheduling.

Heather Field:

Thanks Rob. I think that's all our questions. We've got a lot of thank yous for the presentation and for wanting copies of the slides so that's great. I think there're no other questions. I don't think there is. Pull it up there and definitely be sending out a recording of Rob's presentation and I'll also send out an email with an attached PowerPoint as well. I'd just like to thank Rob for his presentation today and hoping the technology worked for you all as well as it has today. So I wish you all a good afternoon and everyone stay safe over the coming month.

Rob O'Connor:

Thanks Heather and thanks to people for joining in.

Page last updated: 04 Oct 2021