Hill reservoirs are rain water-harvesting structures that have been increasingly adopted in arid and semi-arid regions, such as North Africa, to capture and conserve runoff water and for use as alternative water resources in agricultural development. Currently, process-based information on reservoir hydrology is needed to improve reservoir management practices. The study aims to develop an approach to estimate the reservoir-subsurface exchange flux and its associated error at the annual, monthly, and intra-monthly time scales to better understand the hydrological functioning and dynamics of hill reservoirs. This approach is based on a hydrological water balance of the hill reservoir by considering all water input and output fluxes and their associated errors. The results demonstrate the ability and relevance of the approach in estimating the net reservoir-subsurface exchange flux and its error estimations at various time scales. Its application on the Kamech catchment (Northern Tunisia) for the 2009-2012 period demonstrates that the net reservoir-subsurface exchange flux is positive, i.e. the infiltration from the hill reservoir to the aquifer dominates over the discharge from the aquifer to the reservoir. Moreover, reservoir-subsurface exchange constitutes the main output component in the water balance.