Hill reservoir
A hill reservoir corresponds to a small lake located at the bottom of a valley, whose purpose is to store rainwater and runoff water. The structure generally fills up in winter, and the water can then be used in summer. The hill reservoir thus allows to reduce river withdrawals in summer.
Why build a hill reservoir
Water storage helps to overcome droughts
The reservoir allows to recover surface waters and use them for irrigation in summer. [1]
The reservoir is connected and fed by rivers and streams in winter and is disconnected from them in spring. It is therefore necessary to manage the collected volumes as best as possible from spring onwards.
In general, hill reservoirs supply drip or micro-jet irrigation networks.
Securing production
Water shortage and drought strongly affect crop yields. Unpumped and available water is a way to ensure good yields even during drought episodes.
Hill reservoirs should not be confused with water basins : the latter require pumping groundwater while hill reservoirs capture runoff and surface waters.[2]
Building a hill reservoir
Construction
The reservoir must be dug in a valley bottom where it can be fed by runoff and nearby rivers.
It is necessary that the hole is watertight so that water does not infiltrate : if the soil is not clayey enough or compacted, the bottom of the hole must be lined with a liner. This liner (also called geomembrane) is generally polyethylene and its thickness depends on the characteristics of the reservoir.
A settling basin can be installed upstream of the reservoir to avoid bringing too many sediments into the reservoir.
Building a hill reservoir requires calling on a professional in the sector and can be quite lengthy.
Maintenance
Maintenance (4 to 5 days per year) is necessary to clean the banks and the dam, notably to remove woody vegetation growing there.
The reservoir must also be emptied and dredged every 3 years to remove deposits that have formed at the bottom. These maintenance tasks are heavy and difficult but contribute to a longer lifespan of the structure.
Cost and regulations
Reservoirs are quite costly, on the order of 5€/m3 for a reservoir without liner, up to 10€/m3 for a lined reservoir.
Administrative and regulatory fees are included in these prices, including preliminary studies, preparation of a Water Law File, etc.
It is also necessary to plan for a parcel large enough to accommodate the reservoir, about 1 hectare for 20,000m3 of water.
Advantages and disadvantages
| Advantages | Disadvantages |
|---|---|
| Water availability during drought periods | Harmful to natural ecosystems
Prevents natural infiltration of water into the soil[3] |
| Low frequency maintenance | Difficult maintenance |
| Production security | High cost and required area |
| Losses by evaporation |
Sources
- ↑ CA Alpes de Haute Provence, 2014 : Reducing river withdrawals in summer by storing water in winter : hill reservoirs https://www.bio-provence.org/IMG/pdf/fiche5_retenues.pdf
- ↑ Louis Delatronchette, 2023 : Water management : hill reservoirs, better than basins? https://www.ouest-france.fr/economie/agriculture/gestion-de-leau-les-retenues-collinaires-mieux-que-les-bassines-d63c2334-c7d4-11ed-80c7-e23f0fe16482
- ↑ Amblard, 2020 :Water reservoirs worsen drought and agricultural vulnerability
https://reporterre.net/Les-retenues-d-eau-aggravent-la-secheresse-et-la-vulnerabilite-de-l-agriculture
