Abstract
During a flood event over a karstic watershed, the connections between surface and ground watersappear to be complex. The karst attenuates surface floods by absorbing water or contributes to thesurface flood by direct contribution of karstic waters in the rivers and by diffuse resurgence along thehillslopes. If it is possible to monitor each known outlet of a karstic system, the diffuse contribution isdifficult to assess. These connections vary over time according to several factors - the water content ofthe soil and underground, the rainfall characteristics, the runoff pathways. Therefore, the contributionof each compartment is difficult to assess, and flood dynamics are not understood. We analysedsurface waters during 5 recent flood events in the Lirou watershed (a karstic tributary of the Lez riverin South of France). We were able to acquire samples from the beginning, the peak and the end of theflood. Analyses consisted on chemical ones, because of the specific chemical signature of karsticwaters, and on hydrogeomorphological and hydraulic ones, to supply information about waterpathways and flood dynamics. A first analyse allowed us to define specific chemical signatures forrunoff and karst water. Then, we used the dilution law to combine chemical results, flow data and fiel dobservations to assess the dynamics of the karstic component of the flood. We identified twodistinct karst signatures, discriminated by the magnesium composition. The magnesium has adissolution speed lower than the calcium, so these two signatures represents two different karstdynamic- a slow dynamic karst (with a high rate of magnesium) and a faster one. The separationbetween 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, wediscussed the origin of the waters responsible for the apparent runoff coefficient rise during flash karstflood. This study was funded by the French Flood Forecasting Service (SPC Med Ouest).