In the interest of creating a point of reference allowing to know the hydrological conditions of the karst system and for the purpose of using it as a parameter in a modeling procedure, a Hydric Indicator of the Karst (HIK) was established from the knowledge acquired on the functioning of the karstic hydrosystem of the Cèze. This indicator makes it possible to identify and qualify if at a specific moment “t” the karst is more able to contribute to the flow of the river or on the contrary to infiltrate precipitation water. The HIK is constructed from the data collected at the Ussel spring and the MétéoFrance rain gauge at the karst plateau of Méjannes-le-Clap (Gard, France). Chronic rainfall and discharge measurements from the source include two hydrological cycles. The discharge of the spring is calculated from the recordings of water amounts from autonomous water level probes. These water levels are converted into discharge through a calibration curve drawn for this study. For rain events with at least 4 mm of precipitation, ten parameters on rainfall characteristics and the flow rate of the Ussel spring are analyzed to obtain the HIK. In total, 74 events were analyzed. Finally, this indicator is a relevant entry for the simulation of flow variations in the river within its karst canyon. It is also a relevant tool for institutions which monitor the condition of the karst aquifer and thus can be a useful tool for the management of the aquifer. Our goal was to develop an approach, with which we can calculate and predict, what will be the response at karst spring, if we know current discharge of the Ussel spring and expected amount of rainfall.