## 5 Discussion  
Generally, \({}^{18}\)O and \({}^{2}\)H isotopes are used only to identify the main recharge areas of aquifers. However, in this study case, the contemporary measurement of the spring discharge and the concentrations of the aforementioned stable isotopes permitted us to obtain a good correlation between these parameters. Therefore, \({}^{18}\)O and \({}^{2}\)H isotopes can be good natural tracers of the flow rate supplied by the spring, also without taking into account the rainfall regime that is the main factor influencing the spring discharge. Nevertheless, in this paper, we argue for the possibility to use the \(\delta^{2}\)H and \(\delta^{18}\)O isotopic values as tracers for identifying the main recharge areas related to different discharge values detected in the spring, during the hydrological year. Specifically, we introduce a model able to identify the percentage of the topographic area-elevation classes involved in recharge. This model defines the recharge areas considering two different output assignment conditions, both in a non-combined way. These conditions are referred to as: (i) uniform rainfall distribution on the topographic surface of the basin; (ii) rainfall events that involve specific and limited zones within the basin, both depending only on elevation. Besides, in consideration of the nature of the phenomenon that influences the concentration of isotope values, the model is non-dependent on the delay time between infiltration and spring flow rate. In addition, the model defines the recharge areas but not the temporal distribution of the rainfall events that determined the isotopic concentration measured in the spring. In this regard, the isotopic groundwater signature suggests that the lower spring discharge values are related to the rainfall water infiltrated at previous period and at the highest altitude (higher distance from the spring). Despite the aforesaid limitations, the proposed model correlates the spatial distribution of the recharge area (infiltration area) with the spring discharge.  
In general, the model is based only on the isotopic concentration measured in the spring. Theoretically, this concentration remains constant if the rainfall intensity increases and the recharge area remains the same. On these bases, the fact that the isotopic concentration increases with discharge values admits that the discharge increases with an increase of the recharge area (isotopic ratio dependent) in relation also to the increasing of the rainfall intensity (discharge value dependent). In the mountain karst aquifer, the increase of the recharge area geometrically involves the area at lower elevation, as well as marked by isotopic concentration. Therefore, the correlation between the isotopic concentration and discharge values is based on the assumption that the recharge area increases with lowering altitude; in this way, the discharge increase depends on the progressive involvement of the recharge area at the lower elevations. On the other hand, discharge low values are attributed to recharge areas that remain generally confined to the high elevations. The model considers this progression as non-linear, according to the Weibull function, and it depends on the amount of the area disposable at a specific elevation. Finally, for a specific spring, the use of the concentrations of the isotopes can be a useful marker able to give an adequate estimation of the recharge area effectively involved in the spring discharge supply; this area changes for different discharges and so for different rainfall regimes. Therefore, considering the aforesaid framework, the \({}^{18}\)O and \({}^{2}\)H isotope rate values can be used in a preliminary phase to define the aquifer vulnerability related to the prevalent recharge area.