During a flood event over a karstic watershed, the connections between surface and ground waters appear to be complex. The karst attenuates surface floods by absorbing water or contributes to the surface flood by direct contribution of karstic waters in the rivers and by diffuse resurgence along the hillslopes. If it is possible to monitor each known outlet of a karstic system, the diffuse contribution is difficult to assess. These connections vary over time according to several factors - the water content of the soil and underground, the rainfall characteristics, the runoff pathways. Therefore, the contribution of each compartment is difficult to assess, and flood dynamics are not understood. We analysed surface waters during 5 recent flood events in the Lirou watershed (a karstic tributary of the Lez river in South of France). We were able to acquire samples from the beginning, the peak and the end of the flood. Analyses consisted on chemical ones, because of the specific chemical signature of karstic waters, and on hydrogeomorphological and hydraulic ones, to supply information about water pathways and flood dynamics. A first analyse allowed us to define specific chemical signatures for runoff and karst water. Then, we used the dilution law to combine chemical results, flow data and fiel d observations to assess the dynamics of the karstic component of the flood. We identified two distinct karst signatures, discriminated by the magnesium composition. The magnesium has a dissolution speed lower than the calcium, so these two signatures represents two different karst dynamic- a slow dynamic karst (with a high rate of magnesium) and a faster one. The separation between runoff and karst water is obvious with all parameters (Ca2+, Mg2+, HCO3- and conductivity). By using the dilution law, we evaluated the contribution of karst water to surface flood. To end, we discussed the origin of the waters responsible for the apparent runoff coefficient rise during flash karst flood. This study was funded by the French Flood Forecasting Service (SPC Med Ouest).