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Agronomy Page 1
Agronomy Page 2
Examples of watering and water use.

Why do we need to monitor water??

Technically speaking, crops "use" very little water, but they "lose" a lot of water through the stomata for gas exchange for photosynthesis. If there is not enough water, the stomata close to conserve water and photosynthesis and growth slow down. This reduces yield, but there may be no visible symptoms.

Monitoring Water, using a continuous monitoring system.
With continuous monitoring you can clearly 'see' water use by the presence of 'stepping' (see below). This shows that during the night there is little water use and the water content stays the same and during the day the crop dries out in proportion to ET. Big steps show water use on a hot windy day and small steps show water use on a cold dull day.
By looking at a graph with data from the different layers it can be seen the relative amounts of water being used at each layer. In the example below it can be 'seen' that water use at 10, 20 and 30cm is very similar, while the water use at 50cm is comparatively small.
Also, you can 'see' the depth to which the water goes during irrigation. You would conclude that the soil is not back to 'full' in the top 50cm, as the irrigation has not refilled down to 50cm. Compare this graph with the following one.
The irrigation on the night of the 11th wets the soil to 50cm, but it is still quite dry there (only 25% compared to 38% in top layers). The irrigation on the 16th does not wet below 30cm. Note how small the steps are at 50cm - too dry.
This graph shows the 50cm layer by itself. You can 'see' when the soil is 'full' and when the crop runs out. Note that as soon as water is replaced to this depth on the 20th and the 30th the water use increases dramatically. This is the key to maximising water use.

For maximum growth rates each layer of soil needs to have an adequate supply of water.


When monitoring a crop you have to understand the market for that crop. For what is it being grown?

  • Potatoes - chips, crisps, bakers, baby scrapers, salads, starch, pre-packs etc
  • Lettuce - hard hearted, small size for supermarkets -larger 'fluffy' etc
  • Strawberries -fresh, IQF, pulp.

 

Identifying the limiting deficit has always been the way the grower looks at the needs of his crop. He then irrigates to keep above it. But, with modern monitoring equipment it is seen that each layer of soil has a 'full' point and a 'refill' point. Some layers may have plenty of water and others may have run out. In some crops this does not seem to matter as the crop just takes water from a different layer to compensate. But it is doubtful if this is the case in most crops that we irrigate in the UK.

If the water demand is high then it is probably important that all the roots are able to contribute to the needs of the plant. If only some are working this will also have an effect on the nutrient uptake.

For example, if a crop of potatoes is growing well and using water to 50cm, the water use in the top 30cm is very similar in each layer - maybe 2mm per day from each, while the 50cm layer will be providing much less. (Depends on soil type) When the top 30cm runs out, usually each layer runs out at about the same time, the 50cm layer rarely increases uptake to compensate -hence the crop comes under stress.

Assuming the grower has been monitoirng, 'correctly' he will see this. If not he may be watering too soon or too late - either way this will be affecting yield and quality. The 'standard' irrigation in potatoes would be to apply 25mm. Depending on the soil type this may be correct or may be miles out.
*30mm irrigation still not enough to refill the soil - see next graph.


During the period 23rd to the 30th above the water use was 23mm less than it could have been with more water in the soil. For every mm of water throughput you lose in potatoes you can loose .24 ton per hectrare. Therefore in the example above the 23mm 'lost' represents almost 6 tons per hectare in on week. Can you or your customers afford to loose this?


To water 'correctly' you need to measure at multiple depths, with an instrument that is agronomically sound.

The next graph shows why the above occurred. The first irrigation not only wetted the soil well at 10 and 20cm, but also at 30cm. However, the next irrigation did not refill the 30cm layer, which had already run out of water. Consequently, there was no water used at 30cm during this week and water use slowed overall, resulting in a considerable loss in yield and probably quality.

Conclusions;

  • Irrigation scheduling will become increasingly necessary for growers' survival.
  • More and more pieces of equipment will become available. See SOWACS report.
  • These will need independent verification. Irrigation World2000.com will continue to publish independent information from around the world.

The most important calibration of all this equipment is 'Profit'.
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