Literature values provide a valuable guide but what is really needed is a simple way all grower can find out accurate values, for their farm, by themselves.
Here we show how.
See latest video on anticipatory irrigation by adaptive scheduling
Irrigation scheduling based on site measurement of crop factor and water holding capacity |
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To irrigate correctly we need to know
the amount of water the plants are using and the water holding capacity
of the soil. Many such values are available in the literature,
however there is a major problem;- every farm is different the topography,
hills, facing north or south, the row spacing and the size of the plants
and the type of soil. Literature values provide a valuable guide but what is really needed is a simple way all grower can find out accurate values, for their farm, by themselves. Here we show how. |
MenuThere are four sections, The first is about how plants use water and nutrients and is called basic scheduling theoryThe second section shows how growers can measure the water consumption of their plants called measuring crop factorsThe third section is about shows how growers can measure the water holding capacity of their soil, called measuring water
holding capacity
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Plants use water as a way of transporting
the nutrients they need from the soil. They use the sun to provide
the energy, water evaporates from the leaves creating a water tension
(intermolecular forces) to lift water and dissolved nutrients. The nutrient
solution enters the root system by osmosis, e.g. water moves from the
weak solution in the soil into the stronger solution in the roots. If the nutrient solution is too strong the roots cannot extract the solution from the soil. The water evaporates leaving the nutrients for the plant. The amount of water used in the plant tissue is minuscule in comparison with the water evaporated. Water is not actually consumed by the plant, it is lost to the air by evaporation. |
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In a closed container like a terrarium
no water is used, it is simply recycled. The weaker the nutrient solution, the more water is used. The plant responds by increasing the leaf area. The stronger the nutrient solution in the soil the smaller the amount of water needed to provide the plants nutrient requirements, allowing more energy for fruit and carbohydrate production. The aim of irrigation is more than simply ensuring the plant has enough water, it is to manage the way the plant grows by controlling the amount of water available and the strength of the nutrient level in the soil. |
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Some plant such as lettuce have a
small root system, but many of the larger plant have two distinct root
systems, fine feeder roots in the nutrient rich upper zone and deeper
tap roots. They will extract nutrients and water, giving rapid
growth, using the fine upper feeder root system while moisture is available.
When this is exhausted they will survive, but have little growth,
using the deeper roots, Irrigation should be regularly applied to the upper zone, with less frequent deeper irrigation's ahead of hot spells, to ensure the plant survives, this is called dual cycle cycle or when coupled with weather forecasts anticipatory irrigation. The key aspect of irrigation scheduling is to control the irrigation depth, by knowing how much water to apply. |
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Measuring crop factors |
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The amount of water a plant uses depends
on the weather, so we cannot say that this plant uses 1.5 litres per day.
Instead we use a ratio called a crop factor which is the ratio
of the water used by the plant to evaporation. This picture shows a simple manual gauge which is as easy to read as a rain gauge. It works exactly like a plant. Water in the container (like water in the soil), travels up the wick (like the trunk) and evaporates from the disc (like the leaves). It is a cheap and easy way for growers to measure their local evaporation. Automatic versions and data from conventional evaporation pans are readily available. Evaporation data is very reliable and cheap (often free). |
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Crop factors are normally measured
by growing a plant in pot and weighting to find out how much water has
been used. The problem is that we measure the amount of water used
in litres, but we measure evaporation in mm so we have to convert litres
to mm by using an area. 1 litre per square metre is 1 mm. But what is the area, the area of the pot or the area of the canopy or what? We will get a different answer for the crop factor depending on how we choose our area. |
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What about in the field, when we have
row crops, what area do we use then? Closely spaced plant
will obviously use more water than widely separated plants. If
we choose the wrong area we may end up by applying totally the wrong
amount of water. Problem! Slopes facing or sheltered form the sun, exposure to the wind, these all affect crop the facto. |
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We would like to be able to measure
the real crop factor directly in the field. This may seem simple
with modern soil moisture sensors but there is a catch, called wetted
volume. If we irrigate into a water tank we could easily calculate out how full it would get by converting from litres to mm. (1 litre = 1mm/square metre) however when we irrigate into soil we do not know how far the water will penetrate into the ground, this depends on the pour spaces in the soil and how much water is there already. |
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In practice we cannot apply water
uniformly over an area, with a dripper systems in particular we
only wet a small volume, but we have the same problem with all irrigation
systems. There is no simple way of determining the wetted volume.
Plants to do take up water uniformly from the soil, they will extract water
from the surface layers where the roots are most powerful first. The fact is that soil moisture is just not uniform, and water does not readily move through the soil, variable water distribution is one of those things we have to learn to live with. |
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What can we do. Modern soil
moisture sensors give an accurate reading of soil moisture, in what
is called their sphere of influence. This varies but 300 mm would be
typical. In theory we could use a large number of sensors to measure
the entire soil volume but this is just not practical, we need
to think of a different way. |
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The answer is adaptive learning.
This is me with my grand daughter Kate. Now she knows all
about adaptive control, its is how we all learn to stand up and walk,
and later drive cars or fly airplanes. We do not learn to walk by understanding the physiology of our bodies and the complex engineering which enables us to balance and move on two legs. |
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We just give it a go, bump into things,
fall over, have a little cry and gradually get it right. We can do exactly the same with irrigation and learn the crop factor. We can call this process error correction, (or mathematically predictor corrector schemes). Make an estimate, measure the error, correct the error and retry. Kate knows all about this and she is only 2 that's how simple it is. (She is very cute too). |
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There are two methods. The
first uses soil moisture sensors. One sensor is placed at the
target irrigation depth (or if dual cycle irrigation is used
you will need one probe for each depth). You simply irrigate using your current best estimate of the crop factor. Simply multiplying the accumulated evaporation since your last irrigation by the crop factor, and converting this to litres based on the anticipated wetted area and then to run time. You would normally use software to do this for you. You need to measure and record the soil moisture reading before and a little while after irrigating. |
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You simply enter the two soil moisture
values, together with the evaporation and irrigation amounts into a
software program (called IIS-Planner) which automatically calculates
out the error function, correct the crop factor which, with your approval
is entered into the data base. Every time you irrigate (or when you want) the program wilt automatically adjust the crop factor. As the plants need changes, with the season or growth, the crop factor is automatically adjusted. It simply does not matter what your row spacing or the wetted area is, the program will automatically adjust to give you the correct crop factor. |
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The second method is based
on measuring irrigation depth. Some time after irrigation you
simply measure how far the water has soaked into the soil, using a simple
auger or soil sampler as shown, and enter this depth into the program.
Applying a known amount of water and seeing how far it soaks into
the soil is one of the simplest and most reliable way of measuring soil
moisture. |
Measuring water holding capacity |
| Water holding capacity is the amount
of water in the soil between field capacity and when the plant just starts
to go into stress. It depends on the type of soil, how effective the plant is at extracting water from the soil and the wetted volume. |
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Different species of plants have
widely varying ability to extract water from the soil. The tomato
plant shown was clearly suffering from water stress yet a soil sample
showed that it was still quite moist. Look for signs of stress such as
like leaf wilt, or preferably use a plant moisture sensor. |
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The grape vine was showing no
signs of stress yet a soil sample showed it was dry and crumbly. The grape vine is far more capable of extracting water from the drier soil (e.g. the onset of wilt occurs at a lot lower soil moisture level). Vines also have a much larger root system e.g. they are using a large volume of soil so the effective water holding capacity of the soil for a vine is much larger than for a tomato. |
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Dripper systems
have a relatively small wetted area giving small water holding capacity
while sprinklers and flood have much larger wetted area and hence higher
water holding capacity. As the plants grow they can access a larger volume of soil so the water holding capacity goes up. |
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To measure water holding capacity
apply an irrigation until the water reaches the target depth, checking
using a soil moisture or depth sensor after allowing time for the water
to soak. The soil is now at field capacity. You may not know the actual amount of water in the soil at this moment, (this depends on the field capacity of the soil and the wetted volume). This does not matter, simply think of the soil as full and use this as your reference point. |
Measure evaporation |
Measure your local evaporation. calculate
the accumulated evaporation since your last irrigation by simply adding
up each days reading, or if using the manual gauge just read the accumulated
evaporation directly. |
Monitor plant for stress |
This is best done using a plant moisture
sensor. Look for a reduction in moisture plant levels since the
soil was at field capacity. Alternatively you can simply look a the plant for the normal signs of stress, e.g. leaf droop etc. This only occur after the plant is well into stress an has lost production. |
estimate water usage since last irrigation |
multiply accumulated evaporation
by ‘crop factor’. You must use an accurate crop for your
site. Measuring crop factors |
Water holding capacity = deficit when plant just starts to stress |
Practical scheduling |
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The first step is to decide the irrigation
depth. Digging a hole and having a look is a good place to start,
but the currents roots may be more a reflection on you past irrigation practices
rather than where they should be. The top soil horizon, typically 300 mm deep is the most critical, this is where the nutrients are and where you should be regularly irrigating. You may want to irrigate the deeper zone when water is available to build up a reserve for hot weather. Use the dual cycle approach. |
Graph of mm of water below full. This is far more useful than a simple soil moisture graph. |
Now we need to settle on a starting
crop factor. If you do not have an estimate then simply keep
on irrigating as you normally would but enter the evaporation's and irrigation's
into IIS-Planner. This will tell you what crop factor and water holding
capacity you are currently using. The software reads evaportion, water applied and soil moisture data, and displays water content in mm below full (when wetted volume is at field capacity). The grower knows exactly how much water to apply to refill the profile at all times. |
Logging soil and plant moisture. This plant moisture sensor measures change in moisture since soil was at field capacity |
Using a soil moisture or depth sensor
see how deep the water is penetrating after irrigating enter
these values into IIS-Planner and see what crop factor is recommended.
You may decide to wait for values from several irrigation's
before adopting this value. IIS-Planner will tell you the amount of water you have to apply to reach the target depth so you will not waste water if you irrigate more frequently than strictly needed. When you have a stable crop factor you can increase the water holding capacity setting in IIS-Planner to increase the time between irrigation's. You can irrigate at any time up until the maximum deficit has been reached and you should apply the amount of water equal to the deficit . You do not need to be regularly inspecting soil moisture reading to see if it time to irrigate, only before and after each irrigation. With the correct crop factor the evaporation data will tell you how much available water is in the soil. |