By Yann Kerveno
Climate change and its procession of increasingly more frequent droughts emphasises the role of irrigation in agriculture in temperate countries. In the Mediterranean region, it has already become the sole solution for maintaining agricultural production. However, this is an area where there are still many developments to be implemented, with paradoxes and solutions that sometimes go against the general way of thinking.
After driving along never-ending perfectly straight roads, we finally arrived at the farm of Franck Minjat, a producer of maize and legumes in Blagon, not far from the Bay of Arcachon, hidden away there behind the pines. On the ground, the soils are characteristic of the Landes, dark grey and white, but they are not soils in the specific sense as we understand them, rather they consist of sands laid over an impermeable bed. And there is a very strange paradox. There is water everywhere, but without irrigation, we would not be able to grow anything…
The Landes Forest and its sandy soils together represent a truly iconic location for addressing the subject. The very short history of agriculture in the region, which extends from the Médoc to the edge of the Basque Country, is inextricably linked to the development of the irrigation techniques. “Originally, the Landes Forest was a vast swamp that Napoleon III wanted to drain, planting maritime pines and developing the agriculture”, explained Julie Campguilhem, irrigation consultant with GRCETA-SFA (A Group that carried out Research of the Crops and Agricultural Techniques used on the Forest Soils of Aquitaine).
By the XIXth Century, only the pines had become successfully established, while agriculture never really got off the ground. It took a major disaster for agriculture to return to the region, this taking the form of the deadly fire of 1949, which destroyed 50,000 hectares to the south of Bordeaux.
The fire devoured everything. Some pioneers who had come down from the north of France then took the opportunity to circumvent the obstacles provided by nature and reintroduced agriculture. So how did they do this? By adopting three main measures: drainage to remove the stagnant water; liming to correct the pH of the soil; and irrigation. J. Campguilhem continues:
“The soils here are composed of 85% sands, the depth not exceeding forty centimetres and resting on a hard subsoil base so called alios, a sandstone derived from a barrier layer of sand and agglomerated iron. The groundwater, which is replenished with the rains, in an amount of between 800 and 1,200 millimetres per year, lies just below this layer. This special feature means that the soils have a very low readily available soil water reserve, around thirty millimetres of water on average, even though this may vary within the same field.” Hence the need to irrigate the crops regularly. In the Landes, the water is withdrawn from the water table situated just below the sandstone layer, at a depth of eight to twenty metres. « This water is not potable because it has a high iron content”, explained F. Minjat, to remove any ambiguity while anticipating any possible critical issues that may arise.
The surface runoff theory
Even though the practice of irrigation is quite recent in the Landes region, it has nevertheless been a very important feature of agriculture for a very long time. «From the period of the Roman Empire until the Second World War, water was conveyed by gravity, using its own potential energy (gravity). Apart from a few exceptions, such as the gardens of Versailles, pressurised irrigation was not really developed until after the Second World War. Of the 320 million hectares irrigated in the world, 70% is gravity-fed and the other 30% uses pressurised irrigation systems. More than three-quarters of irrigation (80-85%) uses sprinklers and drip irrigation accounts for the remainder », explained Bruno Molle. Recently retired from INRAE, this researcher has spent his career dealing with these issues, in particular as part of the organisation UMR G-Eau de Montpellier. He continues with his explanation.: «China and India are the two leaders in terms of irrigated land area, each with fifty million hectares. These countries lead the ranking because the classification includes the paddy fields, where gravity irrigation is the only option. These figures may be a little dated but we can assume that irrigation accounts for 17% of the world’s cropped area and produces more than 40% of the world’s food. »
“Irrigation can also be stressful; it is a huge responsibility because of the risks involved.” Julie Campguilhem, GRCETA-SFA.
1.1 litres applied for 1 litre to be made available to the plant
Nowadays, with the climate change and pressure on the resource and costs, the key concept of irrigation is the efficiency, ensuring that the plant uses a percentage of the water applied as high possible. “On average, worldwide, irrigation has an efficiency of 45%. So, 2.2 litres have to be applied for the plant to be able to use one litre. In gravity-fed systems, where the majority are paddy fields, the figure is between 30 and 60%.
In France, 75 to 90% efficiency is achieved with drip irrigation. With a new and well-designed system, this can increase over 90%; 1.1 litres have to be applied for the plant to be able to use one litre”, B. Molle further explained. However, even though the sprinkler or drip systems are more efficient, there is no question of throwing the flood irrigation technique where the water flows through the fields by gravity. “We must not only think about efficiency of the irrigation of the specific field. This is because the flood irrigation has secondary effects on the environment as it not only benefits the irrigated crop. It also benefits the crops found downstream. In Morocco, for example, we have seen areas situated downstream of sectors equipped with drip systems that have replaced gravity-fed systems, experiencing very serious water problems due to a lack of replenishment of aquifers”, explained the researcher. Closer to home, in the Pyrénées-Orientales region, the irrigation canals serve indirectly, through seepages and infiltration, to replenish the groundwater in the Pliocene aquifer, from which the potable water is withdrawn. “When we change a system, we want the improvements to be fair and equitable, i.e., that the users upstream do not receive the majority of the water, thus quotas have to be imposed so as not to jeopardise everything downstream.”
The wind effect
Efficiency is achieved from a combination of different factors, such as the equipment, expertise and farmer-training, irrigation advice and scheduling. “As far as the equipment is concerned, for some twenty years now we haven’t really paid it enough attention. We have worked hard on energy-saving and sprinkler sizing while the advice and extension side has evolved in its own way. But nowadays, we must remember complete optimisation can only be achieved by adopting both precise scheduling and using correctly calibrated equipment.
One does not work very well without the other”, continued François Torres, water and energy consultant at GRCETA-SFA. Typically, this involves the issues of pressure or nozzle type. «If you don’t know the exact flow rate of your pivot you could suffer huge losses. Here, with an evapotranspiration rate of between five and six millimetres per day, that only leaves enough in reserve for three days, if we consider that the readily available soil water reserve is twenty millimetres. Let’s assume that your installation is not very well calibrated; without being aware you could be losing one or two millimetres at each watering cycle, which lasts up to three days. At the end of the season that could lead to a massive effect on yields »
The droplet sizes
Technology is changing. “We are now able to make the low-pressure sprinklers less energy-consuming than the traditional sprinkler nozzles that have been used for several decades.” We are at a stage where we can take into account the droplet size. « Even though the sprinkler and low -pressure systems are fairly insensitive to evaporation,” explained B. Molle, “the challenge is to produce droplets that are neither too small, which would be blown away by the wind, nor too heavy, to the extent that there would be a risk of compacting the soil – something we call crusting. And, contrary to what is generally assumed, it is not the heat, when irrigating during the middle of the day, which causes most losses, rather it is the wind. We now know, from the studies that we have carried out, that evaporation linked to the heat, as measured in 2003 and 2005 (during heavy drough periods in South of France), for example, does not exceed 5% in the course of one day. Whereas this can exceed 15% when there is wind. »
In tubes
To this technical dimension must be added the know-how that has been developed over several decades. Firstly, we have the sensors or probes that provide us with information about the status of the fields. The first sensors made their appearance in the nineteen-eighties in the Landes. In Agen, Jean-François Berthoumieu, who founded the consultancy company Agralis, has spent his whole career developing these technologies that allow for irrigation to be scheduled as close as possible to the plant´s actual needs … “The soil moisture balance method is not sufficiently precise and it is easy to make a mistake and commit errors”, he explained. Therefore, when the sensors arrived, considerable progress was made. “With the tensiometric probes having shown their limitations in certain soils, it was the capacitive sensors that were real game-changers for the farmers”.
It must be understood that the tensiometer measures the suction pressure that must be exercised by the rooting system to extract water from the soil; the capacitance sensor directly measures the soil water content and monitors the actual efficiency of the rains and irrigation. But it gets even better! We now have remote sensing. “With the satellites, you can see what is happening in the visible wavelengths and near-infrared, which provides information about the leaf gas exchange area and the water applied to the plant. We can also see the temperature of the plants, because this is a good indicator.
Half the water that a plant evaporates is for its cooling, so if everything goes well the temperature of the plant should be less than that of the atmosphere. If this proves not to be the case, then the plant does not have enough water. And, furthermore, there are also sensors that measure the air in the soil so as to avoid the phenomenon of asphyxiation…” Other techniques are in the pipes, particularly with the support of drones or other new approaches. “In the case of the trees, we use other types of markers rather than the water they lose by evapotranspiration: we are looking at ways of adapting tools that measure the sap flow and thus we will be able to be more precise about their needs.
Is there a control tool?
All these data capture tools have to contribute towards the farmers´ deliberations, particularly as they can be complemented by modelling. This is the domain of Bruno Cheviron, from the UMR G-Eau Group in Montpellier, where he has developed the Optirrig model (generation and optimisation of Irrigation scenarios) : « In these models we are able to incorporate a large amount of information about the climate, the characteristics of the varieties, sowing dates, focal points regarding the flowering and grain filling stages, etc. but we can also recreate the farmer´s usual decision-making rules, i.e. the way in which he or she normally proceeds and, from these scenarios, re-structure his or her way of thinking. »
This may involve finding the optimum solution, for example, by playing around with the cursors, when the volumes of water are restricted in a dry year. There is no question of entrusting the scheduling fully to the model, explained the researcher: “We are in a situation where the model is simply a tool for deliberation and reflection. It is the farmers who have the know-how; they know their crops very well and especially their soils, but they can use modelling to determine the most efficient strategy from the water they have at their disposal.” With this idea in its embryonic stage: it is thus possible to change the way farmer looks at his or her system and decisions, so that it will lead the farmers changing their perspectives encouraging them to adapt their management practices. (To read more about this, see the section below “provoking a slight water deficit).
Using charcoal?
Even though we can consider that we have achieved an optimum balance as far as the equipment and scheduling is concerned, there is still leeway for making further changes? Making progress with irrigation also involves taking everything into account”, considers Sophie Gendre, the person in charge of R&D irrigation for Arvalis, an organisation that developed Irré-LIS, an on-line irrigation scheduling tool. “Irrigation is managed on the basis of the crop requirements but also according to the needs of the whole farm, not to mention crop rotation, for increasing system resilience. All possible aspects have to be incorporated, in particular the work carried out on the soils to allow water to infiltrate and be retained. Subsequently, it will be necessary to have a concerted and coordinated land-use management system for the resource.”
J-F. Berthoumieu puts forward a different approach: artificially increase the soil´s capacity to store water by working on its microporosity. “A dry soil absorbs ten millimetres out of every thirty or forty millimetres of rain that falls, the rest runs off along the surface.” So, what can be done about it? Add charcoal, because “this form of carbon buried in the soil can store, as a result of the microcavities that it contains, between five and six millimetres more water.” The problem at the moment is the price: 500 euros per tonne, bearing in mind that five to fifteen tonnes per hectare are required
We are currently looking for a way of reducing the cost per tonne to 100 euros. He also mentioned irrigating during the night which avoids photosynthesis being blocked for an hour or two, which leads to the asphyxiation of the roots when irrigating during the day. This leads to even greater efficiency.
Under pressure
There are many possibilities, but how many farmers now actually use decision-support tools? According to J. Campguilhem and F. Torres, the main obstacle is that some risk-taking is involved. Irrigation is not necessarily up to the challenge it represents, except perhaps where this particular component is concerned, they basically explain. « It is not only a serious commitment but it also has a stressful side.
It is a great responsibility. With only three days of soil water reserve, one single error could cost us dear. And this not only applies to the farmer, as the case may be, but also to the employees of the larger farms, whether they be crop managers or irrigation supervisors”, explains J. Campguilhem. « What is very important, in my opinion, is to have access to sound information, at the right moment”, considers her colleague. « Gone are the days of “Irrigate it and the crop will grow”.
The situation is becoming tense, the farmers also have to worry about their image and, basically, they all agree that less water must be used. As they are using less of the resource, then it also results in less work and less stress”, reiterated J. Campguilhem. According to J-F. Berthoumieu, who expects progress to be made in the area of artificial intelligence, this is a crisis that will change the rules of the game. “There is a certain reluctance to automate irrigation and there are very few who have decided to make the move. However, the question will soon be asked quite rapidly, about the cost of an employee versus that of going automatic.”
Inducing a slight deficit: However, to achieve even greater efficiency, Bruno Cheviron proposes a complete change of approach. “In some situations, irrigating more is not necessarily synonymous with greater yields. It is, therefore, a matter of thinking differently and taking into account the variable costs (water and electricity), which means that maximum income is not necessarily achieved by maximising the yield.
This is understandable since the cost of an additional cycle of water will not necessarily be covered by an increase in yield. “The idea is to look at the relationship between the yield and farmers’ income and introduce premiums that encourage slightly less irrigation, making less than optimal yields economically viable.
If the Government could make a package available to the irrigation managers, then this could be a way forward and the whole community would benefit. This is one of the significant lines of thinking being adopted today. Up to now, the payments for environmental services has above all favoured quality rather than quantity. Therefore, this is a significant inducement that, without penalising those who irrigate, would have a threshold effect on the flow rates or groundwater that is under pressure.” Read also: “Irrigating with a slight deficit”.
In Blagon, Franck Minjat has managed to cover all his cropped areas with the irrigation pivots
Just what is required. Created in the seventies, the Franck and Patrick Minjats’ farm in the Landes now has 210 hectares laid down to a number of crops, reflecting the changes in the cropping patterns of this region. Maize (or corn), which was omnipresent in the nineties, has lost ground a little. It now accounts for only 40% of the regions surface area. The farm is split in two, one part being organic and the other conventional. “Organically, I produce soya, sweet corn for human consumption, potatoes, seed radishes, seed rye and peas, all under contract. Conventionally, I basically grow seed maize (corn) and carrots.”
Since setting up in 1978, Franck has witnessed all the new irrigation developments.
“At the beginning, as with all the other farms, the main pivots covered 90% of the surface area and the remainder was irrigated with the hose reels, but today the pivots are used over the whole area. The diversification in crops has significantly changed the schedule. “When there was only maize (corn), we irrigated from early June to early October. Nowadays, with the vegetables and winter crops, we begin in March and finish in mid-October.
However, since then, with all the scheduling tools available, a lot of progress has been made. We are better able to understand the status of the soil, crop requirements, peak periods… Furthermore, we have remote sensing, which is of great comfort. Instead of making four trips to the field to ensure that everything is working properly, I now only need to make two or three. That represents a saving of six hours per day. And the progress made is colossal: “Prior to the seventies, we didn´t apply enough water. In the nineties we applied too much.
Nowadays, we endeavour to apply just what is needed.” After factoring in the genetic improvements, the yields followed. From thirty-five quintals in the seventies, the yield has now reached 150 quintals today. “And if we can manage to deal with the lack of uniformity, we could achieve 200 quintals and I have seen this on certain key plots.” And this without adding any more water.
Wasteful or greedy? The group BRL, a concession-holder for the water-supply network, has produced a series of datasheets adapted to the Mediterranean context and has listed the presumed consumption figures for the different plants. We have learned, for example, that the asparagus, like the aubergine, requires 548 mm in the course of its cycle, being among the thirstiest of the vegetable crops, but they fall far short of the strawberry, which, in spite of its short growing cycle, requires 688 mm.
As far as the fruit trees are concerned, the kiwi consumes the most water, more than 1,000 mm, ahead of the peach trees, apple trees and pear trees, which require between 800 and 900 mm, while the olive tree, which is so well adapted to dry periods, can achieve its full potential with 738 mm and the almond tree will get by with 471 mm. For the field crops, alfalfa and pasture grass top the rankings as being the most water-consuming (more than 1,300 and 1,100 mm respectively). Grain maize requires 781 mm, soya 530 mm, sorghum 603 mm and sunflower 554 mm. As far as the disputes are concerned, they relate more to the periods when irrigation is applied rather than the amount of water used.
However, as Bruno Molle pointed out, the sharp controversy surrounding irrigation is a specifically French phenomenon; it does not exist in countries where there is even more pressure on water, such as Italy or Spain, for example. We have yet to understand the reasons for this.
By Yann Kerveno (Revue Sésame : https://revue-sesame-inrae.fr/)