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Irrigation 101: Why, what, and the Gap theory


This is part one in our Irrigation series, where we go through the basics around irrigation and how it is increasingly important to do it well in the sector.

What is the purpose of irrigation?

The purpose of irrigation is to fill in the gaps that nature doesn’t in terms of water. 

Over the summer, when it’s not raining, you can use water irrigation to strategically keep your pasture or crop growing. Just like rain, there is such a thing as too much. Like the flooding down in Southland , there was way too much rain, even though it may have been previously dry. It’s the same as irrigation, it’s completely possible to overdo it. 

To make sure you’re irrigating properly, you need to know  about your soil, your crop requirements and how you operate your system in order to get that balance right. There’s no point in leaving it too long and not applying the water you need when you have the capability - but conversely you don’t want to over-apply your water.

You also need to keep in focus of what your desired outcome is - and then plan around how to do that. 

The main thing you need to then consider is, that most irrigation systems now are designed to apply 5ml of water per hectare, per day, but across a whole season, that’s actually much more water than what’s needed to fill those gaps.

Filling Gaps

It’s good to think of irrigation as filling a gap, as opposed to a whole lot of water and the rain as a convenience. The main thing that determines what kind of gap you’ve got is the amount of evapotranspiration that occurs. Evapotranspiration can be simply explained as what occurs when a plant grows - as it’s growing, it’s drawing water out of the soil and that’s how the water leaves the soil. 

Over in early springtime, before the heat of the summer, there may only be a 3-3/1/2 ml loss from evapotranspiration every day. In that case irrigating 5ml a day would be over-irrigating - it’s not matching the gap that’s there. 

It’s only in the height of summer, when the evapotranspiration rates might be 6ml or even 7 mls a day, the amount you irrigate is less than the amount the evapotranspiration is taking out of the soil. 

So how do we actually know what the gap is? There are different methods to do that – whether that’s through directly measuring the soil moisture or running a soil water balance model (which Regen does) - both of those are valid ways to measure the gap and know how much irrigation you need to put on.

Irrigation over time

Measuring soil moisture with sensors is not new cutting edge technology. The Aquaflex sensor was developed by Lincoln University and has been available in NZ for at least 20 years and is used internationally, extensively. 

However, in NZ, soil moisture sensors  never went hand in hand with the start of irrigation. In NZ, many early irrigation schemes were what they called border dyke (https://teara.govt.nz/en/photograph/19637/border-dyke-irrigation) or flood irrigation, where they shaped up the paddocks or large areas of ground to be like little baths, fill them up with water, let it soak and let some of it run out. 

With this type of irrigation, the soil went from being saturated, then it had to wait a long time before drying out, before becoming saturated again. Most of these kinds of irrigation systems have been converted to what they call spray irrigation – which is with pivots or K lines, or it has to be changed out. 

What makes New Zealand irrigation different? 

Most of the pivot irrigation in New Zealand started 20-25 years ago. Over that time, although it was common in the US, there have been adaptations to irrigation that again have impacted our ability to do it well for our conditions.

In the US, pivot sizes are generally smaller and farmers have adapted to the circular pivots through planting crops in circles - whereas our paddocks are squares. 

Whereas in New Zealand, we have very large pivots, with arms that spin around. These arms can end up being 1km long - while in the US, 400m is generally the average length.  The takeaway of that for New Zealand farmers is that it therefore takes a long time for a pivot to get around to do a full circle. If you’re putting 5mls on a day but it takes 3 days to go around, then you’re actually putting on 15ml. You need to be thinking through how you’re filling a gap, you don’t want to put on more water than the soil can hold, you must wait when your soil has space for 15ml before you start – but then by the time you go around, and you had 15ml capacity in the beginning, and you had three days to use 5, that’s another 15ml capacity. These large pivots are quite inflexible in terms of how they can be used - they can’t whizz around too quickly as it would be  too bumpy and challenging.

As people are improving their irrigation and efficiency they’re realising our infrastructure has got constraints and we have to work around that.

Types of Irrigation Systems

Different irrigation systems apply water in different ways. Depending on your needs and the combination of what systems you might have is the starting point of what you have to work with for your irrigation set-up.

  1. Fixed-grid sprinklers are used to fill in corners with. They are similar to a household garden sprinkler (the ones that go tsk-tsk-tsk-tsk!) They can be timed to go off in a different pattern to smaller or larger amounts of water, and because they can be fine tuned operated individually, there are opportunities to be very tactical with their application in certain spaces.
  2. Pivots are further up the continuum. Generally if they are set up and operating well, they apply consistently an even amount of water across a larger area – they can be putting on three, four, five, six ml, and this can be controlled.
  3. K-lines are bigger sets of sprinklers joined together in a little train that gets moved around, and the amount of water they apply can be operated by the amount of water they’re on. If they’re left on for too long they might apply too much water - 15 or 20ml.

Roto-rainers – are moving further up to what irrigation used to be, and these might put out around 30-40 mls.

What’s the cost of over-irrigation?

It is no surprise that soil moisture and pasture growth travel hand in hand together through the season.  If you are in an area where water restrictions generally occur at some stage through the season it is tempting to make sure that you keep “ahead” until then.  But over-irrigation is wasted water – it causes leaching and reduces pasture growth.  To have confidence not to irrigate you need to know every day know where your soil moisture is compared to the optimal zone for pasture growth. Combined with the weather forecast get a recommendation on when to irrigate and how much to keep in the optimal zone – that is the only way to optimise the value of irrigation by growing more grass.

How much grass doesn’t grow when it is too wet?

Faced with water restrictions it is hard to hold off irrigating.  But if the soil already has enough water for grass growth any extra firstly makes the ground too wet and then even more water leaches away.

If the soil is too wet grass grows slows, maybe 20% less. Every 10 days this difference adds up.

Grow 68 kgDM/ha not 85 kgDM/ha in 10 days  = 93kg DM

93 kg DM per ha from 175ha =  16,275 kg DM

At 80% utilisation, 12:1 FCE & $4.50/kg MS = $5,000 every 10 days

Plus;

  • One less irrigation = $500 electricity
  • Less water irrigated will reduce leaching loss in Overseer

Bridgit Hawkins

CEO/Executive Director