C-DRONE GUIDE · 8 SEPTEMBER 2026
Agricultural water reservoirs by drone: volume, siltation and water-law permits, price
An agricultural hillside reservoir is not just a blue dot on a map: it is a fully-fledged hydraulic structure, subject both to French water-law nomenclature and, depending on its size, to the regulatory dam-safety classification. With the ongoing debate over substitution reservoirs, it has also become an object under closer scrutiny, from regulators and the public alike. A drone photogrammetry survey has a precise role to play here: measuring a volume at the design stage, tracking siltation from one season to the next, documenting the structure's condition to support a statutory technical inspection. Here is what it concretely brings a farmer, an irrigators' association or a hydraulic engineering firm in 2026.
Published on 8 September 2026, reviewed on 8 September 2026 — regulations in force as of September 2026.
A structure with two regulatory hats: water law and dam safety
Any permanent water reservoir at least 2 metres high falls under heading 3.2.5.0 of the French water-law nomenclature (Article R.214-112 of the Environmental Code), which classifies it against two combined criteria: its height H, measured between the top of the structure and the natural ground, and the volume V retained at normal level. Crossing certain thresholds shifts a project into a full water-law authorisation regime, with an impact study and a public inquiry, rather than a simple declaration — a distinction that weighs heavily on the timeline and budget of a hillside reservoir project from the design stage onward.
On top of this environmental classification, the largest structures also fall under a dam-safety classification in three classes, A, B and C, set by decree n° 2015-526 of 12 May 2015 — a text that simplified the earlier regime under decree n° 2007-1735 by scrapping the former class D for the smallest structures. Because of their modest height, most French agricultural hillside reservoirs fall into class C once they reach the classification threshold, with obligations that scale up with class: an in-depth technical inspection every 5 years for class C, every 2 years for class B, and annually for class A, on top of monthly visual surveillance (weekly for class A). Our guide to drone dam and levee inspection covers this safety side for larger structures; this one focuses on what is specific to the small agricultural reservoir: its volume and how it evolves.
What a drone survey measures: volume, capacity curve, siltation
A photogrammetric flight over an empty or low-level reservoir produces a digital terrain model of the basin floor, accurate to the centimetre over the exposed areas. Combined with a survey of the water surface, it yields a capacity curve — the volume available at each water level — essential for a design file or a water-law regularisation, and for setting discharge and release rates against the thresholds of heading 3.2.5.0.
Repeated from one season to the next, the same survey makes visible a phenomenon a farmer usually notices too late: siltation. A study by Manuel Erena, José Francisco Atenza, Sandra García-Galiano, José Antonio Domínguez and José Miguel Bernabé, published in 2019 in the journal Water, combined drone photogrammetry (5 to 20 cm resolution), surface-vehicle bathymetric surveys and water-quality measurements across 21 reservoirs in the Segura river basin, Spain: it puts the average annual loss of storage capacity from sedimentation at 0.33 % across these structures (see the study on Google Scholar). A modest-looking figure, but one that, compounded over ten or twenty years, meaningfully shrinks the volume actually available for irrigation in a reservoir its owner assumed was unchanged since it was first filled. Tracking that drift early allows a targeted dredging job to be scheduled before it turns into a costly emergency.
Substitution reservoirs: a topic under closer scrutiny
A typical agricultural hillside reservoir, holding a few tens of thousands of cubic metres filled by winter runoff, has little in common with the large substitution reservoir projects that have dominated public debate in recent years — but both fall under the same regulatory family, and the latter has made the former more closely watched. The high-profile Sainte-Soline project in the Deux-Sèvres département, led by the Coop de l'eau 79 irrigators' cooperative, plans 16 reservoirs for a combined volume of around 6 million m³, filled by winter groundwater pumping to cut withdrawals during low-water periods; it triggered strong opposition, including the March 2023 protest. The French government's 2023 Plan Eau now makes this type of reservoir conditional on a collective commitment to a 10 % cut in agricultural withdrawals across the relevant basin, with monitoring meant to make the volumes actually stored and withdrawn verifiable.
In that climate, whether for a stand-alone hillside reservoir or a substitution-reservoir project led by an irrigators' association, having a dated, georeferenced survey of the structure's actual volume — rather than a design-stage estimate never checked again — carries value beyond mere compliance: it is an objective, defensible record that a farmer, a chamber of agriculture or an engineering firm can produce just as well in a permit application as in response to an inspection or a challenge.
Method, aviation rules and 2026 pricing
The flight itself is usually a standard agricultural flight under the open category, sub-category A1 or A3 depending on distance from third parties — a hillside reservoir is generally isolated in a rural area, far from housing. As with any mission, the Géoportail drone map is the first thing to check for nearby protected zones (Natura 2000 sites or riverside nature reserves are common around water bodies), a point covered in our guide to drone wetland mapping and water law. The volume-calculation method itself follows the same comparative-model logic covered in our guides to stockpile volume measurement in quarries and to siltation monitoring in stormwater retention ponds — applied here to an agricultural hydraulic structure with its own regulatory thresholds.
Ranges observed in France in 2026, excl. VAT:
| Service | Range (excl. VAT, 2026) |
| Initial survey for a water-law file (reservoir up to 5 ha) | €400 to €900 |
| Annual siltation monitoring (compared capacity curve) | €350 to €700 per campaign |
| Survey supporting a class C in-depth technical inspection | €600 to €1,200 |
| Multi-year package (irrigators' association, several structures) | quote-based on scope |
These amounts cover the flight, photogrammetric processing and delivery of the model and capacity curve; they exclude the impact study or the regulatory file itself, as well as the in-depth technical inspection, which remains produced by the qualified people appointed by the structure's operator. For a farmer, an irrigators' association or a hydraulic engineering firm, request a quote stating the reservoir's surface area, its approximate dam height and the goal sought — design, siltation monitoring or support for a statutory inspection.