Influence du bocage sur le climat d'une petite region: Resultats des mesures effectuees en bretagne

Abstract

Within a more general frame of a multi-disciplinary study initiated in Brittany (north-west France), a special programme was carried out by the INRA Station de Bioclimatologie d'Avignon-Montfavet to determine the influences of hedge networks (“bocage”) on the climate of a small region. The study, based on theoretical studies described elsewhere, consisted of comparative micrometeorological and meteorological measurements taken in nearby sites, one within the old original ‘bocage’ landscape and the other where land management operations had led to more or less total destruction of the hedge system. The study first considers the aerodynamic effects due to the modification of air flow by the hedge system, which may be related to the general aerodynamic roughness corresponding to the height and density of obstacles. Then follow the results regarding the radiation balance, which showed no noticeable modification of net radiation, the slight increase in infra-red loss being compensated by a similar slight increase in absorbed solar radiation due to a smaller albedo. These combined effects generally induced higher daytime and lower night temperatures, resulting in an increase in the diurnal range (0.5°–1.5°C). An increase in air saturation deficit also occurred, especially on the sea border (0.5-3 mbar). Although evaporation measured by a Piche evaporimeter showed a marked reduction (about 30%), no such effect appeared in potential evapotranspiration ( PET), which can be explained by the low weight of the advective term (to which the Piche instrument is sensitive) in the Penman formulae in comparison with the radiation term in the specific context of the climate of Brittany.