Les étiages en milieu océanique : exemples dans le bassin de la Maine (France) Low water flows in oeanic regions: examples in the Maine Basin (France)

Résumé

L’analyse des données hydrométriques des cours d’eau du bassin de la Maine permet de déterminer la durée et la gravité des étiages durant les années de sécheresse. Les grandes différences dans l’espace mais aussi dans le temps s’expliquent plus par la diversité géologique que par les variations climatiques. Les aquifères jouent un rôle efficace dans le soutien des étiages. Hormis les aménagements réalisés pour régulariser l’écoulement, les autres activités humaines tendent à accentuer les étiages, surtout du fait de l’augmentation des prélèvements d’eau et de la diminution de l’infiltration liée à l’utilisation du sol.

Mots-clés:

• étiages,
• sécheresse,
• bassins hydrologiques,
• activités humaines,
• bassin de la Maine

Summary

Droughts in France during the years 1976, 1989‑1992, 1996‑1998 and 2003 had a marked effect on its hydrology. Analysis of the hydrometric data collected by DIREN Pays de la Loire made it possible to assess the duration and the importance of low water flows during these years in the Maine watershed. We used low flow limits (half of the minimum monthly discharge) to measure the duration of low flow events. A low flow duration frequency model, hydrological profiles and drought coefficients were used to characterize the low water levels.

Low flows in 1976 were the most pronounced, but did not last the longest. Maps of rainfall, flow and discharge coefficients for the Maine basin show high spatial and temporal fluctuations. These differences can be accounted for more precisely by geological diversity than by climatic changes. All watersheds with an oceanic climate suffered from pluviometric deficits and ground water played an efficient part in sustaining low water levels. Three types of stream response could be distinguished. First, the drought affects secondary streams flowing on the clay soil of the Armorican basement (Vaige, Oudon, Chéran, Uzure), but also in the Paris basin, such as the Rhonne River, where structure is responsible for the capture of a part of the ground water by other rivers. Second, the Sarthe, Huisne, Braye, Veuve and Narais rivers, which have appreciable ground water reserves, did not dry up and where low flows were not important. Third, between these two groups, the other rivers had more or less pronounced low flows, according to morphostructural conditions and also water consumption patterns.

In oceanic regions, low flows mainly take place during the summer when pluviometric deficits and strong evapotranspiration occur. Cumulative rainfall and discharge values show that low water flows occur at the end of the descending stages of the hydrograph. Seasonal rainfall distributions indicate that low flows are strongly favoured by pluviometric deficits occurring during cold seasons. From 1989 to 1992, four consecutive years with pluviometric deficits and especially with insufficient rains during the winter, an increase in the duration of low flow events and droughts was observed. In regions with sedimentary geology, low waters occurred when rainfall during the cold season was insufficient to recharge the aquifers that control base flow. In non-sedimentary basement regions, low flows are more pronounced and lengthy because of the lack of groundwater inputs. Therefore, the schistose areas of the Armorican Massif are particularly susceptible to drying up. Moreover, smaller watersheds appear to be more sensitive to drying up than are larger ones. Note too that apart from work that has been carried out specifically to regularize river flows, human activities tend to accentuate low flows, especially because of increases in irrigation pumping and decreases in water infiltration linked to changes in land use.

The study of low flows is a useful method for understanding watershed hydrology and for the management of water resources. The recorded spatial differences suggest that measures that must be taken to reduce the occurrence of low flows are different for the basement and sedimentary regions. For the former, it is necessary to build water reserves, whereas for the latter, the monitoring of water table levels and the implementation of controls should be sufficient for predicting and managing drought periods.

Key words:

• low flows,
• drought,
• hydrological basin,
• human activities,
• Maine Basin