Irrigation budgeting and seasonal planning tool for pear growers

Video transcript

Hi, I'm Lexie McClymont. I'm a research scientist with Agriculture Victoria, and today I'm explaining an irrigation budgeting tool that's available from the department for pear orchards in the Goulburn Valley. I'm going to explain the information inputs that are needed, and show you how it can be used to help plan irrigation management strategies for a season under different scenarios and how it can provide a guide to scheduling within a season.

The budgeting tool is an Excel workbook. There are two worksheets in it, one being this seasonal water budget and the second being the scheduling plan. The seasonal water budget calculates orchard water use, and from that determines what your irrigation requirement would be. Each of the gray columns shows you where data is either required to be input or can be altered so that you can investigate different scenarios. For example, what the impacts of a particularly dry year could be, or a particularly wet year, on your irrigation requirements. Likewise, with the scheduling plan, there's certain information that you put in that is specific to your orchard. Then you can run different scenarios to see what the impact of a different emitter output or run time would have on your scheduling within the season.

When you schedule irrigation, whether you're doing it purely by experience or with some form of decision support, you're taking into account a whole range of factors from the weather conditions to the crop type. You might consider the fruit growth stage and the size of the trees, the understory conditions and the soil type, your emitter type and the wetting pattern; all these things are going to influence when and how much you irrigate. The irrigation budgeting tool takes typical values for a pear orchard in the Goulburn Valley, along with some orchard specific information that you can enter and provides you with a prediction of orchard water use over a season.

The irrigation budget is a monthly budget and the two main outputs in this sheet are the orchard water use and the irrigation requirement. They are calculated in mm and ML/ha. The irrigation requirement is obviously less than the orchard water use because of rainfall during the season and soil reserves from winter rain.

Next, I'm going to step through each of these gray columns. Within gray cells you can enter your own data or alter the data that we've prefilled. The white columns automatically calculate from the information provided in the gray columns. The first column is reference crop evapotranspiration (ET_o), and the values that are already in the spreadsheet are those for long-term average ET_o at Tatura. Anywhere in the Goulburn Valley, your long-term average ET_o is going to be fairly similar to these values here.

Reference crop evapotranspiration is calculated from solar radiation, wind speed, humidity and temperature data. It is a calculation of the water use of a reference crop. The reference crop is full cover grass of a particular height. If you're interested during the season to see what the daily ET_O values are compared to the long-term averages, this website (www.bom.gov.au\watl\eto) is the place to go. Click on the ‘Victoria’ tab and look up your nearest weather station. Although for those in the Goulburn Valley, I would tend to look at the Tatura weather station rather than the Shepparton weather station simply because the Shepparton weather station is at the airport, whereas the Tatura weather station is at the Tatura SmartFarm and is a little more reflective of orchard conditions.

This slide shows the long-term reference crop evapotranspiration for the Goulburn Valley, the blue open symbols show, the long-term values. I've also plotted each of the last four seasons. Generally there's little difference for the monthly totals between long-term ET_o and the seasons. You might recall a few summers ago, we had heatwave conditions in January and you can see a higher ET_O response in those two seasons compared to the other couple. Generally, there's not a huge shift in ET_O between seasons but it is something that you might be interested in altering in a spreadsheet just to see what the effect is. Within a season you have will have periods of hotter, dry conditions and wetter, milder conditions compared to the long-term averages and you’ll adjust your irrigation scheduling accordingly. But, in terms of a seasonal budget, there's usually not a lot of difference between seasons.

Once we have the reference crop evapotranspiration, we then need to adjust that to convert from the water use of grass to orchard water use. The next two columns help us to do that. The effective area of shade column helps us to convert from grass water-use to tree water-use, and the understory coefficient calculates the transpiration of the understory and the soil evaporation.

To convert reference crop evapotranspiration (the water use of grass) to the water use of your trees, you need to take account of several factors including

- the size of the trees and the training system; for example, whether you have a vertical hedgerow training system or a Tatura trellis system,

- the planting arrangement (tree and row spacing and row orientation), and

-  the density of the canopy.

These factors influence how much radiation your trees are intercepting, and that is a major driver of water use.

What is effective area of shade (EAS) and how do we measure it? It's the fraction of the orchard floor that is shaded throughout the day. A simple calculation is the width of the shade band divided by the row width.

The photo gives a simple example in a vineyard, we see a continuous band of shade down the row and the fraction of shade is the width of the band divided by the width of the row. In some orchards you'll have discreet trees and there will be light between shadows, or you might have light windows in the canopy. You need to adjust for that light to calculate the shaded fraction of the orchard floor. The fraction of shade will change throughout the day. Consequently, we measure it three times during the day to account for that diurnal pattern. We take the measurements at solar noon, which in the Goulburn Valley ranges from 1:15 to 1:30 PM Australian Eastern daylight saving time, and about 3.5 hours before and after that time. Solar noon is when the sun is directly overhead. Effective area of shade is the mean of these three measures. Typical values of effective area of shade in the Goulburn Valley range from 40 -60 %. For trees on Tatura trellis with a dense canopy, 80% EAS is possible, but 70% is generally considered quite high.

This figure shows the seasonal pattern of effective area of shade for a mature pear orchard. It starts at around 20% at full bloom. And, in this example, increased to a peak of 60% where it remained steady for the remainder of the season. That means that you can make a single estimate of effective area of shade once leaf-up is completed and you needn’t worry about that value changing for the remainder of the season.

Conversion of ET_O, reference crop of evapotranspiration, to tree water use isn't a straight multiplication of effective area of shade and ET_O. There is an adjustment factor and the horticulture team at Tatura through many measurements has shown that factor for mature pear trees is 1.1. In other words, tree water use equals 1.1 times effective area of shade times reference crop evapotranspiration. That calculation is embedded in the irrigation budget, so you don't need to worry about it, but adjusting the EAS values so that they're appropriate for your orchard will give you better predictions of orchard water.

A brief aside, in the photos on this slide, you can see some of the traditional tools for measuring effective area of shade for research purposes. Some of our current research is looking at the potential use commercial sensor-based platforms to take LIDAR measurements and make predictions of effective area of shade for orchards. If this is successful, it will enable spatial maps to be generated for orchards without growers having to actually go out and try and judge EAS for themselves; which would make life a lot easier in terms of fine tuning irrigation.

Moving on to the understory coefficient: this is providing an adjustment, a conversion from reference crop, ET_O to the water use of your understory. The main factor that influences the understory coefficient is the wetted area, and that will be determined by your emitter type. For example, whether you're using microjets or drip emitters. Other factors include:

• soil texture and soil water content, those two factors will affect the rate at which water evaporates from the soil surface,
• the understory components (grassed or bare),
• and shading from trees.

These photos show some very young pear trees that are not providing any shade over the wetted patterns, but you can imagine with a mature orchard, the shading provided by your trees will counteract, to some extent, the evaporation and transpiration from the understorey.

These photos aren't of a pear orchard, but they do contrast a couple of different scenarios. On the left, we have an orchard that's microjet irrigated. Within the season they have a grassy interrow that will be transpiring water, and underneath the row, you can see quite a bit of light still. The large wetting patterns from the microjets will not be as shaded as in the orchard on the right that is drip irrigated. The drip wetting patterns are heavily shaded through the day. And the interrow won't be transpiring because it's quite bare.

A useful guide for your understory coefficient is that it's the fraction of the understory that is well-watered and sunlight. This table gives some examples: early in the season, you would expect to have a large wetted area because of winter and spring rainfall and very little shading and an understory coefficient of 0.7 is reasonable. Mid-season, your wetted area is going to contract and as leaf up occurs you'll have increased shading, so for drip at that time an understory coefficient of 0.1 to 0.15 is suggested. Some publications suggest a coefficient even lower than that, but for a pear orchard in the Goulburn Valley, an understory coefficient of about 0.1 is typically used. If you've got microjets, the understorey coefficient is largely going to depend on the type of microjets you have and what the radius of their wetting pattern is. From the radius, you can work out the area of the wetted pattern and then, accounting for emitter spacing, you can work out what fraction of the orchard floor is being wet.

I'll just recap quickly. We have the long-term ET_o. In other words, the water-use of grass, the effective area of shade and the understory coefficient with these two columns converting the long-term ET_O to your orchard water use. In this example, I've said I have an orchard that reaches an effective area of shade of 55%. You’ll notice in the last couple of entries we decrease that value as we reach the end of the season and leaf function starts to decline and leaf fall begins. And for the understory example, you'll see the understorey coefficient started at 0.7 at the beginning of the season. Evidently the example uses drip emitters because the understorey coefficient decreases to 0.1 later in the season.

We'll move on now to the next few columns that adjust orchard water use back to irrigation requirement. The rainfall column is the long-term average values for Tatura. Not all rainfall is considered effective. In other words, it's not all going to contribute to your water balance. A general rule is that an event has to be greater than 10 mm to be considered effective. And even then we only consider 75% of the volume of those falls to be effective. The balance sheet is calculating 75% of the value entered in the rainfall column. You don't have to worry about calculating the effective rainfall so much, but if you're entering your own data, for example, you'd really only be including the rainfall events greater than 10 mm.

The soil water reserve: this is the water held by your soil available for plants to uptake at the start of each month. This value depends on your soil type and the depth of your soil profile. It's calculated from the rainfall and water use in the previous month. The initial value of 54 mm assumes that you have the clay loam soil type that's commonly used for pear orchards in the Goulburn Valley. It also assumes that you have a rooting depth of 80 cm; in other words, the bulk of your roots are in the top 80 cm of soil. And lastly, it assumes that you've had a wet winter and spring. 54 mm is at capacity for the soil water reserve. If you've had a particularly dry spring, you might want to adjust that value down. Likewise, if you have a different soil type, you'd need to adjust for a different water holding capacity.

We don't always want to provide irrigation to meet the potential water use; either because we don't want excessive vegetative growth or perhaps because we don't have the water available to us. We know with pears that it is safe to apply deficit irrigation at certain stages of the season without impacting yield. The stress coefficient column lets us investigate the impact on irrigation requirement of applying different levels of deficit irrigation during the season. This figure illustrates the main phases of shoot and fruit growth for pears throughout a season, the green line being shoot growth and the red line being fruit growth. In the first phase, at the beginning of the season, you have some shoot growth and the fruit are in a cell division phase. Next you move on to a period of rapid shoot growth; at that same time, the fruit are going through a slow growth phase. In late November/early December, that shoot growth terminates and the fruit enters a period of rapid fruit growth.

In the first phase, the irrigation requirements and where you'd set your stress coefficient really depends on the winter/spring conditions. At this time, you're wanting to allow the soil to dry out, to about -200 Kpa. If you've had a wet winter and spring period, you may not need to apply irrigation at all during this time. You may just want to let the soil begin to dry out so that when you enter the second phase of rapid shoot growth, you've got some capacity to control the vegetative vigor through regulated deficit irrigation.

When you enter the final phase, where you have rapid fruit growth, you want to be applying irrigation at the full amount. At that time you'd use it a stress coefficient of one - that means you're not applying deficit irrigation in that later phase.

In September/October, your stress coefficient may be 0.3, if you want him to dry out the soil profile; or, if you've had a dry winter, you might use a higher stress coefficient.

In November, a stress coefficient of 0.3 could be a good starting point. Monitor shoot growth and the soil conditions to get an idea of whether that factor is adequate to control your vegetative growth or if the soil is drying out too much.

In the rapid fruit growth stage you don't want to be stressing the trees. You want to apply water so that the potential fruit growth rate can occur.

After harvest, you can cut back the water quite severely, but again, you'd be taking into consideration the seasonal conditions. For example, how dry is the soil at that time of year and what your mite pressure is. You don't want to overstress the trees if you have a level of mite pressure that could cause damage.

To recap again, we have our orchard water use calculated by the budget and we're subtracting the rainfall and the soil reserve and adjusting for any deficit irrigation that we might want to apply. The spreadsheet then calculates the irrigation requirement. I forgot in the previous slide to mention one more thing that you might consider with the stress coefficient and that is your rootstock. For example, if you have BP1 rootstock, that we know seems to be less tolerant to extremes in soil water content, you might want to take that into consideration when you're setting the stress coefficients.

I'll give an example now of changing some of the input information to look at different scenarios. You'll notice at the moment we have an irrigation requirement of just under 4 ML/ha, but let's say we miss out on summer rainfall and you'll see that irrigation requirement increased to 4.6 ML/ha. Likewise, you might want to consider the effect of using microjets with a large wetting pattern. You can see that by changing that understory coefficient, the irrigation requirement has now increased to 7.5 ML/ha. Conversely if you have a different training system that intercepts less radiation, which will also affect your production potential, you can see the effect on irrigation requirement there as well.

You can alter any of the gray cells, the input data, and see what effect that would have on your monthly and seasonal irrigation requirements.

Now we're moving on to the second part of the workbook to the irrigation scheduling plan worksheet. If you're doing multiple of these for different blocks, you could enter your block description in the top left corner. The information in the upper right is the important information to enter correctly. The calculations in this spreadsheet are picking up on these values:

• tree and row spacing
• emitter output and emitter spacing.

When your irrigation systems were designed originally, you will have taken into account a whole range of things, including your pump capacity, the area of orchard that you have to irrigate, the time that you have to be able to irrigate your entire area orchard in, as well as the environmental conditions. For example, the infiltration rate of your soil and the expected wetting pattern of your emitters. This information will have informed decisions about appropriate run times.

You can see in this example I'm starting with a run time of two hours early in the season, but I know a two hour run time won't meet my requirements in summer. So I've increased my runtime in summer to 6 hrs.

The remaining columns in this spreadsheet pick up on the information that you've entered in the upper right corner and the information in the previous spreadsheet - the evaporative demand, ET_O, effective area of shade, and understory coefficient - feeds into this one to perform these calculations. The stress coefficient column is carried over. It's useful to have that as a reminder of what you've set those stress coefficients to. If you want to change them, you change them in the previous worksheet and it brings them automatically into here.

The output of interest is this spreadsheet is the irrigation interval. It's telling you how frequently you’ll irrigate, i.e. the number of days between irrigation events, based on the typical information that you've entered in this worksheet and the previous one. An interval value of one, for example, means that you're irrigating every day. A value less than one means that you need to be applying irrigation more frequently than once a day and greater than one means that you're leaving a number of days between irrigation events. For example, in October where that value is 1.4, that means that you're irrigating twice every three days, on average. So

The scheduling budget, the first worksheet, is a decision support for planning and investigating different scenarios over the entire season. This second spreadsheet then is a decision support for within the season. It's giving you an idea of what your typical irrigation frequency would be during the season. And obviously, in practice it's going to change because you'll have rainfall events where you delay irrigation for a few days in response to that, or you'll have heatwaves where you increase your run time or your irrigation frequency.

I'll give a demonstration now of setting different emitter rates, spacings and runtimes, and what effect that has on your irrigation interval. I'm going to change from using drip to microjets. I have already changed, in the previous spreadsheet, the understory coefficient, increasing it from 0.1 to 0.3. And now I'll change the runtime, the emitter rate (to 30 L/hr), and the emitter spacing (to 2 m). And you can see the effect that those changes have had on the interval between irrigation events.

I haven't covered decision support from sensors within the orchard at all in this presentation, but we are doing a video in future covering that topic. So watch this space.

That concludes this presentation. If you have any questions please contact me (lexie.mcclymont@agriculture.vic.gov.au). I’m more than happy to talk over the phone but email is the easiest way to trade numbers. Finally, I would like to thank Agriculture Victoria for its ongoing support of pear research. I would also like to acknowledge that this video is an output from the PIPS3 program and that the project has been funded by Hort Innovation via the apple and pear growers levy and the Australian government, so thank you to those organisations as well.