## 4 Results
### Spring Recharge Area Identification by Using Isotope-Driven Model  
Starting from the digital elevation model (DEM) of the study area, the basin has been discretized into finite square elements (FSE) with the size of \(100\times 100\) m. Therefore, a unique elevation value has been attributed to each of the cells of the grid. In this way, it was possible to define the elevation classes distribution of the topographic surface of Capodacqua di Spigno Spring basin as represented in Figure 7, which shows that the topographic surface distribution of this basin is typical of a carbonate karst aquifer. In fact, the depression areas are due to karst landforms, such as dolines, karren, dry valleys, swallow holes and poljes. In this regard, the irregular elevation distribution of the area is mainly due to the presence of a karst-prone large depression area at higher altitudes. In particular, the most representative classes are around 950 and 1155 m a.s.l., while the classes at a lower altitude range (95-255 m a.s.l.) show a low-frequency distribution that is due to the presence of steep topographic slopes in that range. These slopes evolve towards more flat topography in the higher elevations. According to the aforementioned conceptual model, we propose, about the study area and similar areas, an analytical  
Figure 6: Comparison between spring discharge values (Q) and \(\delta^{2}\)H‰ and \(\delta^{18}\)O‰ values and monthly cumulative rainfall data (Esperia Pluviometric Station, 302 m a.s.l.) during the study months. model aimed at identifying the main recharge areas according to the isotopic rates variation, detected in groundwater samples, taken at the spring. The model is a not time depending model, this one by using the relationship between the natural elevation distribution of the feeding basin and the isotopic rate distribution of groundwater samples, identifies the main recharge zones that contribute to the spring discharge. Regarding the distribution, the model defines the recharge areas considering two different output assignment conditions, in a non-combined way. These conditions are referred to as: (i) uniform rainfall distribution on the same elevations classes, on the topographic surface of the basin; (ii) rainfall events that involve specific and limited zones within the basin, both depending only on elevation. In addition, in the distribution of the recharge area with the elevation, the proposed model does not take into account the different soil permeability and runoff effects depending on the slope of the surface. However, these effects can be considered by an evolution of the proposed model regarding their influence on elevation classes. The proposed model is based on setting up a linear relationship between discharge values and \(\delta^{2}\)H and \(\delta^{18}\)O rates at the same time monitored at the spring (Figure 5). Therefore, concerning the discharge, it is possible to assume that the average elevation of the recharge area, obtained by using isotopic concentrations, can be referred to as a prevalent sector of the topographic surface of the basin that is depending on the elevation. In this study case (Figure 5), the minimum discharge value of 657 l/s refers to the average recharge area altitude of 878 m a.s.l., while the maximum discharge of 2935 l/s refers to the average recharge area elevation of 633 m a.s.l. These values are defined as the average of the two average recharge area elevations, obtained using \(\delta^{2}\)H and \(\delta^{18}\)O concentration values. Considering the topographic distribution of the area in relation to the elevation, it is possible to associate a theoretical regular aquifer having the same equivalent area, \(\rm{A_{eq}}\), for each altitude.