Irrigation control through the use of information

Agriculture accounts for almost 70% of the annual water supply.

In 2050, it is expected that it will be necessary to increase the demand for this resource by 55% to maintain the food needs of our growing world population.

European agriculture uses 73,000 hm3 of water each year, of which it could reduce up to 70% by avoiding transport losses, applying precision irrigation techniques, and reducing food waste, in a circular economy scenario.

In the digitalization of agriculture in general, and in the management of irrigation water in particular, more and more technology is available to the Smart Farmer.

To prevent agriculture from being left out of technology, we must advise the farmer in the best way knowing which technology is closest to the information you want to obtain, and taking into account the investment/maintenance cost, and the necessary technological level.

There are different ways of measuring or estimating soil moisture to make irrigation decisions, among them we can mention capacitive probes that measure/estimate soil moisture, indirect measurement by estimating evapotranspiration, and direct measurement using tensiometers.
 

In this article we will focus on how to use tensiometers to carry out efficient irrigation control.

To begin with, the tensiometer measures the effort that the roots must make to extract moisture from the soil so that as the soil dries, the water tension in the soil increases, or, in other words, the tension is a measure which determines the force with which soil particles hold water molecules: the greater the moisture retention, the higher the tension. The tensiometer is not affected by salinity and does not require site calibration.

Generally, a superficial tensiometer is installed in the active root zone and a second tensiometer deeper, at the end of the root bulb to control irrigation drainage. 
 There are farmers who install tensiometers at various depths, something that we can avoid knowing the water retention capacity of the soil.

The information provided by the tensiometersmonitors is displayed in the following graph:

Armed with this information, we can make the following decisions:

• We water when the surface tensiometer (blue line) approaches the maximum tension limit and,

• We water the necessary time so that the surface tensiometer does not drop below the minimum tension limit.

To proceed with this decision model, it is necessary to know the moisture retention curve of the controlled soil and its retention capacity, which we see in the following graph:

The moisture retention curve represents how the tension increases (horizontal axis - tension in mB) as the soil loses moisture (vertical axis - volumetric soil water content).

Through this physical analysis of the soil, we obtain the following information:

• The soil is at field capacity when it reaches 50 mB, which is when it stops draining and from this tension up to 150 mB, the water found in the soil is available to the plant.

In terms of volumetric soil water content, we can see that it is between 8-15% moisture at field capacity.

• The water retention capacity of the soil in question is 7%

(=15-8%). In other words, if the active root bulb of the tree occupies a volume of 100 liters, with a retention capacity of 7%, it means that each irrigation does not have to exceed a volume of 7 liters, and if each tree has 6 drippers of 2 L/H the approximate irrigation time would be 35 minutes plus the network filling time. (35=7*60/12).

In summary, the measurement of the tensiometer indicates the water situation of the soil, and the moisture retention curve does not facilitate the interpretation of said measurement, thus obtaining "information" to make the appropriate irrigation decisions.