The irrigation of crops during the day causes the same misunderstanding every summer. Irrigation cannons turn under a blazing sun, and the common reflex is to see it as massive waste due to evaporation. The physical and regulatory data tell a more nuanced story, where the actual loss of water is marginal and where technical constraints dictate the time slots much more than apparent common sense.
Actual evaporation during spraying: measured data
The most frequent criticism is based on a figure often cited by local residents: 30 to 40% of irrigation water would evaporate in the heat. The work of Jean-Louis Durand, research director at Inra and bioclimatologist, directly contradicts this estimate.
According to his measurements, 97% of the water spread during hot hours benefits the plant. The remaining 2 to 3% evaporate at the level of the water jet, between the cannon’s outlet and the ground, without ever reaching the foliage or the soil. Laurent Huber (Inra) and F. Dubois de la Sablonière (Loire-Atlantique water agency) confirm this analysis in the journal Agricultural Perspectives, where they directly question the validity of the hypothesis of significant losses during hot hours.
To understand why farmers irrigate in full sunlight, one must look at what happens at ground level. Once the water touches the soil or the plant, the vegetation cover significantly limits direct evaporation. Bare soil, on the other hand, loses more, but irrigated plots are rarely bare during the peak growing season.
| Common belief | Measured data (Inra) |
|---|---|
| 30 to 40% evaporation in full sunlight | 2 to 3% loss on the water jet only |
| The water does not reach the plant | 97% of the water directly benefits the crop |
| Irrigating at night eliminates all loss | The difference in loss between day and night remains marginal on the jet |
Drought orders and irrigation: rules that vary by technique
Seeing a field being irrigated at 2 PM does not mean that the operator is breaking the law. Prefectural drought orders clearly distinguish between irrigation techniques and do not subject them to the same time restrictions.
Spraying with cannons or ramps may be prohibited during specific time slots (often between 11 AM and 6 PM or between 2 PM and 5 PM depending on the departments). In contrast, localized irrigation (drip, micro-spraying) is generally allowed even during the day, even in times of heightened alert.
The orders also provide for specific exemptions:
- Rainwater stored in reservoirs disconnected from the hydrographic network often escapes restrictions, as it does not draw from the resource during low water periods.
- Some high-value crops (vegetables, seeds, nurseries) benefit from extended time slots to avoid irreversible harvest losses.
- Localized systems, whose efficiency far exceeds that of cannons, are less regulated because their consumption per hectare is significantly lower.
A Breton farmer quoted by France 3 Régions summarizes the tension: “We jeopardize our farms by not irrigating as we wish.” Some operators admit to violating restrictions to avoid losing their crops, which fuels the debate between resource preservation and the economic survival of farms.
Technical and logistical constraints of night irrigation
The idea of irrigating exclusively at night seems logical from the perspective of an individual programming a garden sprinkler. At the scale of a farm, the logistical reality complicates this option.
An irrigation cannon covers a limited area and must be moved several times to treat an entire plot. The time needed to irrigate an entire farm often exceeds the available nighttime hours. A farmer with several dozen hectares to irrigate cannot concentrate all the work between 9 PM and 6 AM.
The pressure in the collective supply networks poses another problem. When all irrigators in the same watershed irrigate simultaneously at night, the pressure drops. The flow at the cannon decreases, irrigation becomes less uniform, and the time spent per position increases, which mechanically pushes part of the work into the daytime.
The human factor and safety
Moving heavy equipment in fields at night increases the risk of accidents. Rolling cannons weigh several hundred kilograms, and repositioning them on waterlogged soils requires tractors. Jean-Louis Durand also highlights another aspect: “It is important for farmers to work in view of everyone”, both for transparency reasons and for workplace safety.
Collective water management and imposed water turns
In many basins, irrigators do not freely choose their time slots. Unique collective management organizations (OUGC) allocate volumes and time slots among operators to prevent simultaneous withdrawals from exceeding the environment’s capacity.
This system of water turns assigns each farm a specific time slot, sometimes during the day. The farmer irrigates when their turn comes, not when they wish. Refusing the assigned time slot means losing their volume for the relevant period, which can jeopardize the harvest.
Collective management also explains why irrigation occurs at seemingly absurd hours. A farmer whose turn falls between 1 PM and 5 PM has no other option but to irrigate at that time, even if the temperature exceeds 35 degrees. The collective schedule takes precedence over general recommendations.
The gap between public perception and technical reality remains the crux of the problem. Evaporation losses during spraying are documented as marginal, the orders distinguish between techniques without banning everything during the day, and the water turns impose time slots that the operator does not control. The next time a cannon turns under the sun, the likelihood that it is waste is much lower than that of a constraint being imposed.