AN ISOTOPIC STUDY (H-2 AND O-18) OF WATER MOVEMENTS IN CLAYEY SOILS UNDER A SEMIARID CLIMATE

We applied the natural variations of stable isotopes in water to the s tudy of infiltration in clayey soils and to the study of groundwater r echarge in the small drainage basin of Barogo in West Africa. The comp arison between the variations in isotopic contents of soil water and r ainwater in 1988 and 1989 led us to characterize two infiltration proc esses: (1) an infiltration through the soil matrix of rainwater that m ixed with mobile water which was held in the superficial part of the s oil and was enriched in heavy isotopes by evaporation and (2) a fast a nd deeper infiltration by preferential flow along macropores that bypa ssed the superficial part of the soil. The study of the variations in groundwater isotopic content led us again to separate the recharge pro cess into two components: (1) a slow infiltration through the soil mat rix and the weathered basement (distributed recharge) and (2) a fast a nd direct recharge through conducting fissured zones (localized rechar ge). A schematic and local representation of groundwater recharge in t he Barogo basin allowed us to estimate that direct recharge represente d around 70% of the total recharge in 1989.