SUMMARY
The knowledge of the infiltrated water amount after a flood is very important to avoid an over-exploitation of the groundwater aquifer. This data, which is also used to calculate the water balance of a reservoir dam, is very difficult to determine.The goal of the article is the exploitation of the infiltrated water quantity calculated by the daily five parameters hydrological model GR5J, to fill the lack of data concerning the infiltration variable in the computation of the water balance of Meffrouch dam. The latter is located at the outlet of Oued N'chef watershed situated in north western Algeria with the feature particularity of having a karstic aquifer. The GR5J model was selected after that the modelling by GR4J (not presented here) was unsuccessful. The results of the GR5J simulation are very satisfactory with a NASH criterion above 80%. The model shows that most of the precipitation infiltrates, the equivalent of 62%; the exchanges are about 31% and only 7% for the runoff. The water balance calculation of the Meffrouch dam was based on the Gûntner (2002) formula whose results seem to be consistent with the infiltration values calculated by GR5J.