Choosing a sensor for irrigation: capacitive, tensiometric or pyranometer probes?

Irrigation in agriculture is a subject with many challenges. Climate change and rising temperatures mean that crops need more and more water. However, the pressure on water resources is also leading to restrictions on their use. So it's becoming all the more necessary to be able to optimise irrigation management, thanks to very precise monitoring of the soil's water requirements. This ensures that plants develop properly, while limiting water use and costs.

To help you, many companies have developed their own sensors for irrigation management. Today, there are three main types: capacitive sensors, tensiometric sensors and weather stations with irradiance sensors (also called pyranometers).

Soil or above-ground sensors: choosing the best option

Whether soil probes or other sensors, these measuring tools will allow you to evaluate the water availability of your soil. 2 types of instruments are mainly used in the framework of irrigation management in agriculture

• Soil probes (capacitive and tensiometric probes) are the most commonly used instruments. They enable the temperature and humidity of the soil to be monitored directly. Each uses a specific measurement method, which is described below.
• Above-ground weather stations are made up of several essential sensors, including the pyranometer. These sensors measure the solar irradiance needed to calculate the ETP and the water balance.

Whether you want to determine the water needs of your soil according to your crop, trigger automatic watering at the right time, avoid a water deficit, push your crops to the limit of water stress (in viticulture) or launch the last watering of the season, these devices will be of great help.

Capacitive probes

Capacitive probe principle

Capacitive probes are devices that measure soil moisture based on the reaction of the soil when an electric field is emitted. It is therefore possible to measure the amount of water supplied by rainfall, which actually penetrates to the roots, but also the actual evapotranspiration.

How the probe works

There are 2 types of capacitive probes:

• Fixed probes, which continuously measure soil moisture in a given place in the plot.
• Mobile probes, which can be moved to several locations in one or more plots, where tubes are already installed.

Capacitive probes have sensors (electrodes) arranged one below the other, usually every 10 cm. For each 10 cm soil horizon, an algorithm converts the electronic measurement into a percentage of moisture, depending on the soil composition. These probes are therefore very finely tuned, so that the soil moisture at the various probe depths can be calculated precisely.

Reading the results

The tool is connected to a remote transmission system that will produce a graph. It will show the evolution of soil moisture between field capacity (soil fully saturated with water) and the bottom of the soil's readily usable reserve (URS), which identifies when the plant is under water stress.

Installation conditions

Installation requires an auger (manual or electric). You need to ensure that no air pockets form in the soil to ensure reliable measurements. Information on the composition of the soil is also needed to set the parameters for the calculations.

The variability of a heterogeneous soil does not make it easy to understand its water behaviour. An initial assessment of the specific conditions of the site (soil survey) is essential in order to know the composition of the soil at the location where the probe is installed. The quality of the data transmitted depends on this.

If your soil is very heterogeneous, it is recommended to install several probes in your plot or to install it in a place representative of this heterogeneity. The depth of the probe will depend on the type of crop and its rooting.

⚠️ If they are installed not far from the line of passage of the tractor, they will have to be removed at each passage of the agricultural machinery before being reinstalled. There is a loss of data during these phases.

Tensiometer probes

Measurement method

Tensiometric probes allow you to quantify the force that your plants' roots must exert to extract water from the soil (measurement expressed in centibars). These devices measure the matrix water potential (kPa).

How the probe works

The water will flow through a porous candle and become an electrical contact. The sensor at the end of the probe reacts to the different voltages of the water in the soil. These electrical variations are then transmitted to a reader box which translates this electrical influx into voltage.

Reading the results

The data can be read manually (reading box) or automatically with remote transmission. The recording of these data will allow you to draw up moisture evolution curves. However, the volume of soil measured remains quite small (a few centimetres around the sensor).

The higher the tensiometric value, the more force the roots need to extract water and the higher the risk of water stress. Thus, many grids exist to find out the values at which it is advisable to irrigate, published by official organisations.

Installation conditions

Installation is similar to that of the capacitive probe. Probes need to be positioned in various places on your plot of land, depending on the rooting depth of your crops. The settings need to be very fine, but will give you a quick measurement. It is therefore advisable to be accompanied in the installation and monitoring of these probes, to avoid inaccuracy in the measurement.

⚠️ It is important to note that, as with the capacitive probe, if they were to be installed on the line of passage of the tractor, they would have to be removed at each passage of the agricultural machinery before being reinstalled. This results in a loss of data during these phases.

The pyranometer, a key component of the weather station

New innovative tools have emerged in recent years to help you manage your irrigation: pyranometers (or irradiance sensors). The main difference? Their ease of installation and their quality/price ratio.

Measuring method

This is a tool that converts the solar radiation (or irradiance) it receives into an electrical signal. This solar radiation is the basis for calculating the ETP and the water balance: two essential tools for optimal watering of your crops. This measurement must therefore be absolutely accurate for the calculation to be reliable.

How the sensor works

The tool measures solar irradiance very accurately. It therefore lacks the wind, temperature and humidity measurements to obtain the actual evapotranspiration and establish the water balance.

Reading the results

From the application linked to the tool, follow the amount of water lost (in millimetres) through evapotranspiration, and the water entering through rainfall and irrigation (also in millimetres). By adding the variables of your soil type and crop, you get a simplified visualisation of the ETP and water balance.

Installation conditions

This solution requires only one installation in the heart of your plots. In order to properly measure the solar radiation, it is necessary to take care that no shadows interfere. This is why it is necessary to orient it to the south.

In conclusion

Whichever solution you choose, the installation will be of paramount importance to ensure an accurate reading. The choice of sensor type depends on the degree of accuracy and reliability the farmer is looking for in relation to the issues at stake: difficulty of installation, number of crops you need to monitor, budget, method of reading/transmitting the data, or the measurement method you trust most (all of which have been proven to work).

The key is to master the interpretation of the data provided by the sensor you choose in order to adapt the right amount of water to be applied at the right time, to avoid wasting water and to avoid unnecessary expenses.