[en] As the systematic study of tritium and oxygen-18 concentrations in surface and underground waters revealed evidence of some circulation modalities, and as it appears that the greatest uncertainty concerned the assessment of infiltration, this research thesis aims at precising this parameter by using isotopic method from the metre scale (lysimeter) to the kilometre scale (drainage basin). The notion of infiltration encompasses all the processes which govern the downward movement of meteoric water through the non-saturated zone towards the saturated zone. After having described the different types of water present in the soil, and their properties and characteristics, the author reports the study performed at the metre scale (parameters affecting evapotranspiration, computed and measured evapotranspiration), and then the study performed at the kilometre scale (presentation of the drainage basin, hydro-climatology, isotopic study of infiltration and flow over this basin)