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SEEDS & agricultural MACHINERY - with expertise and services for successful crop management


 

Soil & Water

 

WHAT WE SELL

Irrigation equipment

Expertise for improvement
of irrigation management
systems
 

ADDITIONAL INFO

Importance of soil
structure management

Method for irrigation
scheduling
 
 
 
 
 


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 Choose an appropriate irrigation scheduling method

With water resources getting low in arid and semi-arid regions, many techniques and methods have been developed over the years for the management of irrigation water. They are based on information from the soil, the climate or the plant, either in an exclusive or complementary way. Some methods help decision making for scheduling (when should I irrigate?); others go as far as calculating the amount of irrigation water needed.

The choice of a method relies on many parameters such as the species (i.e. crop cycle & management practices), type of application (surface, sprinkler, drip...), monitoring equipment costs, available time to collect data etc. Personal knowledge can also be a choice factor because data must be properly interpreted to be useful. Because weather-based scheduling can be difficult to implement (accurate data might not be available), and expensive or difficult-to-use, devices are required for plant-based scheduling, we recommend soil-based irrigation management. Our purpose here is to give basic information on the sensors and method used.

 Soil-based scheduling - water potential vs moisture content

Soil-based measurements are practical and sufficiently reliable for effective irrigation management. Depending on the device, they can complete or even replace the water balance method based on meteorological data (quick description on right hand side). Measured parameters are usually soil moisture content (% of water in a volume of soil) or soil water potential (also known as soil water suction or tension).

Soil water potential - when to irrigate
This is a measurement of the pressure required to extract water from the soil (expressed in centibars or millibars). The drier the soil, the higher the suction level, because the water is held more strongly on soil particles. Of course, one would think that it is more interesting to measure soil water content directly and calculate the amount of water in the root extracting area. However, soil water suction is a very powerful measurement when properly interpreted.

Below is an example of a soil-suction plot in a ray-grass field. The sensors are installed at three depths and soil water potential is monitored every week. The movements of the curves indicate the variations in soil moisture content.

We can clearly observe the quick dehydration of the top layer where root volume is important. After a few days, the water flow is oriented towards the soil surface: the crop should be irrigated.

 

From soil water potential to moisture content - water profile
Both variables are related by a curve which is specific for a particular soil type. Usually, the relationship is realized in a laboratory for each soil horizon. It is also possible to estimate this relation with soil texture data and a reference table (Hanks, 1992). Although not as precise as the laboratory method, this global estimation is often sufficient for irrigation management. This means that when water potential is measured at a certain depth, soil moisture, and thus the amount of water around the sensor, can be calculated. With 3 or 4 sensors, you can built a water profile without using more complicated soil moisture devices such as TDR, capacitive or neutron probes.

 

      

Useful links

Crop evapotranspiration - guidelines for computing crop water requirements - FAO

Weather-based scheduling - a reminderl of basic definitions

In this method, irrigation management is based on the calculation of water balance terms

I = R + S - ET - D

where: I = irrigation; R = rain; S= capillary rise; ET = evapotranspiration; D = drainage.

The calculation of evapotranspiration from climate and crop data is quite difficult. Meteorological data (reference evapotranspiration) must be collected as close as possible to the field to be accurate which isn't always possible. Real evapotranspiration must be estimated in the field from reference data or directly measured which is also difficult.
Moreover, capillary rise and drainage are difficult to estimate and thus often neglected which can falsify the water balance result.

 

The Watermark® sensor

Among the sensors available on the market for measuring soil water potential , we recommend the use of Watermark® sensors. Unlike tensiometers and ceramic cups, they don't use water but are made of gypsum and measure the electrical resistance between two electrodes. They are then much more appropriate for arid regions where soil moisture fluctuates greatly during the crop growth period.
A reading device integrates the calibration relationship which converts the electrical resistance into water potential. A data logger can be connected to the different sensors for automatic monitoring. The interest here doesn't come from the sensor but from the way the collected data is interpreted (see left).

 

 

© ICS 2002 - 2009
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Document created : june 2002 - updated : juillet 7, 2009

Publication director : Jean-François MARTIN - ICS - 12 rue du Soleil, ZA Croix-Fort - 17220 La Jarrie - France

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