In the light of the very controversial Silvens dam project (on the River Tarn) and the ‘infringement of regulations’ proceedings initiated by the European Commission, the French government has decided to place limits on the amounts of water withdrawn by the farmers. More specifically, no reservoir can be built for irrigation purposes unless proof can be provided beforehand that the local use by farmers has been reduced. It is the responsibilty of the Prefect (official government representative for the region) to monitor usage and keep an account of the amounts of water withdrawn from the reservoirs. As soon as the farmers have provided proof of their efforts to reduce consumption, they will be able to benefit from the financial assistance offered by the water agencies, hence the interest in the drip irrigation system that a farmer from the Tarn region is using on hismmaize (corn) crops.
Francis Bourges, a farmer from Bioules, in the Taru region, produces seeds on 150 ha of irrigated land, half of which is maize. He uses three traditional irrigation systems: pivot, hose reel and solid set. This year he has been using a drip irrigation system on 20 ha. “The aim is to replace the solid set system entirely, as it is very efficient but costly and extremely time-consuming, particularly as far as maintenance is concerned. If I am satisfied with the results, I will do the same with the other 25 ha”. The first reason discussed is entirely unrelated to the technical issues involved. “I am looking to develop my methods according to the prevailing public opinion. The image of the maize crop has to be restored. If the hose reels disappear from the fields, then it will be less common to see restrictions on irrigation. I cannot imagine a future on my farm without maize.”
Subsurface or above ground?
The installation of subsurface micro-irrigation is more difficult. The tubes are placed at a depth of 35 cm. This system is suitable for those who have adopted simplified tillage/ cultivation methods or direct drilling. The fact that irrigation cannot be seen then becomes a great advantage as far as the image is concerned, not to mention the time saved in the different handling operations. Its life span is estimated to be 20 years. The return on investment is completed within seven years.
Drip irrigation has been shown to be easier to use. This relates to wide spaced crops. The installation can be moved each year from one field to the other, in order to adapt to the crop rotation. The return on investment is achieved in three years.
The saving on time is also very significant.
“It takes half an hour to move a hose reel. I have seven of them. It is quite different with drip irrigation, which also requires monitoring but all that is needed is a quick drive around the field to check that the rows have been wetted and that there is no leakage.”
The constraints of flow rate and pressure have to be taken into consideration. “I pump water from a communal network, so several farmers could be drawing water at the same time…” Drip irrigation only requires a pressure of 3 bar at the intake.
Small amounts applied on a regular basis
Micro irrigation consists of applying the water in small quantities on a more regular basis. “I irrigate every day for 3-4h, sometimes even more often”, said Francis Bourges. The capacitance sensors evaluate the quantity of water present in the soil and provide information about the immediate soil and plant water requirements. “Every day,” advises Fréderic Honoré, a sales technician from the installation company Modern Irrigation 82, an installer, “you need to apply an amount of water equivalent to the Etp, taking into account the available soil water reserve if it is known.” Francis Bourges adds: “And I am quite sure that the 1,000 m3 will be used on the maize, but I need a tensiometer. I’ll see what I can do later.”
It is possible to combine a fertigation system. “Such an installation ensures that, when irrigation is applied, there is a uniform distribution of water and fertilisers and this can be continued even during the stages when tractor work is normally not feasible.”
Frédéric Honoré refers to the economy of scale involving the installation of a drip irrigation system. “It reduces weed growth because only the row is irrigated, thus reducing the amounts spent on herbicides. It resolves the problem of soil compaction by preserving the structure, with a positive effect on root propagation and the distribution of water and fertilisers. Finally, and above all, it improves the efficiency of these two elements. This represents real progress compared with sprinkler irrigation, for example, where 30 to 40% of the water is lost”.
A developing market
The cultivation of trees and market gardening are the core activities of Netafim, a specialist in micro-irrigation. “Arable or field crops,” explains Louis-Georges Lafout, head of the south-west sector, “represent a developing market. Today we have a thousand acres irrigated with a subsurface drip system and 50 with an above ground method. This system is installed on a hundred or so extra hectares each year. The first installations go back five years or so, while the market gardeners have been using the system for twenty years!”
All the different types of installation are still in demand today, except the permanent sprinkler or solid set system, even though the restrictions on the resource have increased. “However, it would appear that these measures are not really that severe and the price of water is still at an acceptable level. There is no financial incentive whatsoever to be gained in having a micro-irrigation system since all the grants and subsidies are directed towards pivots and hose reels.”
A ten-hectare field will be divided into four blocks, which can be managed separately.
The installation of the pre-punched tubes takes place during the 4-5-leaf stage of the maize plant, after a hoeing. The spacing between the two lines of drippers depends on the crop.
For maize, there should be one line of drippers for every two rows of crop. The movement of the machinery across the field during detasseling was a concern for the farmer but the tubes were resistant.
“This was already planned for”, Louis-George Lafont assured us. “This is another advantage of the technique of being able to irrigate while the teams are working in the field.”
The filter, the key component
Frédéric Honoré explains the importance of the filter. There are a number of systems available: screen, sand or disk filters, which can be chosen according to the water quality and the end objective. “I have heard about the clogging of the drip system but, of course, that occurs when there is no filtration component”, explained Frédéric Honoré.
“It is essential to have a filter in the system! It is the core component of the installation, the most important item of expenditure.
The success and quality of the irrigation depends on this item. And I recommend that an automated filter should be used, capable of operating the back-flushing of the screen, if necessary.”The installation cost depends on the general shape of the field and the number of connections. “For 20 hectares, you have to allow for 1,200 to 1,500 € /ha. And each year allow for 205 to 400 €/ha for renewing the tubing. For anything above 5 ha it will be worthwhile, but the larger the surface area, the greater will be the reduction on the relative cost of filtering.” Installation requires about 5 to 6 h/ha, but a few days have to be put aside each year for removal and relaying. Francis Bourges expressed “a concern about this post-harvest stage. The manufacturer should think up a product that can be injected into the tubes to destroy them. That would be the ideal solution.”
• Polyethylene pipes, with a wall thickness of 200 microns.
• Recovery and recycling of 100% of all the materials after their removal.
• The flow rate of the installation is restricted to 7 m3 of water per hectare. The drippers are positioned every 30 to 50 cm and have a flow rate of 0.7 to 1.1 l/h (of water). A number of pressure regulators placed at different points ensure that the water arrives in the field at a pressure of 1 bar.