Many Mediterranean watersheds are composed by or linked with karstic systems. During a rainfall event, different parameters influence their recharge and drained runoff such as: the state and humidity of soil or the epikarst, the rainfall intensity, the cumulative precipitated water and the internal structure of the karstic system. As a consequence, karsts can be connected with rivers or the hill slopes during flood events. Karstic systems may attenuate surface floods or boost overland and river flows. This latter possibility may arise from the activation of ephemeral streams or an increase of the runoff coefficient. A precise characterization of the interactions between surface and subsurface flows is important for the understanding and anticipation of flash floods. This study proposes a generic method based on an analysis of the geological structure, rainfall, runoff, tracers tests and geochemical parameters. The method was applied to 3 watersheds in Southern France, which are connected to a complex karstic system. This paper presents first results obtained on this site.