# Geohydroology of a Reference Mediterranean Catchment (Cilento UNESCO Geopark, Southern Italy)
## 2 Materials and Methods
### Connection between the Hydro-Chemical Relations and Hysteretic Loops  
In order to evaluate the hysteretic loop, 11 events were selected from the 5-min Q-EC dataset. The events are shown in Figure 11, where the first column contains the selected hydro-chemograph events. In the middle field, the events are characterized by using the daily chart of [8] for identifying the runoff mechanisms. The daily chart was derived from the application of the mixing law equation by using the Q-EC end-members collected during the hydro-geomorphic monitoring activities. The mixing law was applied considering that following prolonged and intense rainstorms, occurring during the wet period, different runoff mechanisms are progressively activated from different runoff sources and mixed to the previous one. Based on the above assumptions, we derived that the upper hyperbolic 1(UH1) curve that describes the Q-EC relation for a groundwater and groundwater ridging mechanisms (Q = 1-3 l/s and EC = 350-240 uS/cm). The second upper hyperbolic curve (UH2) starts when the UH1 reaches its horizontal asymptote (Q = 3 l/s and EC = 220 uS/cm) and the sub-surface mechanism starts to enlarge and expand, mixing with the previous one. The upper linear curve (UL3) describes the Q-EC relation when the direct runoff and soil pipe mixes with the previous components; this condition begins when the UH2 reaches the horizontal asymptote for Q = 400 l/s and EC = 120 uS/cm. In the last field the hysteretic loop is shown. It is widely accepted that during a purely natural event, only affected by precipitation input, the hydro-chemo-graph shows an inverse relationship between the two variables Q and EC, therefore when the discharge reaches its maximum value, EC reaches its lowest value, while the EC values increase (recovery phase) during the recession limb of Q. Following this assumption, the first five events of Figure 11 exhibited a hydro-chemograph pattern typical of rain-influenced events and these events fell within the boundary curves of the daily chart. They differed from each other in their hydrologic characteristics: the 14 th February 2016 event was a compound event with several peaks that fell under the linear curve of the daily chart (UL3) describing the prevailing direct runoff, so called Hortonian flow and the hysteretic loop exhibit overlapping curves due to the presence of several peaks and a clockwise loop. The event that occurred on the 13 th March 2016 was a sub-event (Q = 237 l/s, EC = 173 uS/cm) extracted from a multiple peaks event. Its position on the daily chart confirms the nature of sub-events because it is not completed with the groundwater (first hyperbolic curve) and Hortonian flows (linear relation). For this event, the hysteretic loop was clockwise. The events that occurred on the 20 th of May 2016, 1 st of March 2016 and 23 rd of March 2016 were all single events with well-defined peaks of 960 l/s, 750 l/s and 500 l/s, respectively. In the daily chart, the event data points were well described by the characteristic curves identifying the active runoff mechanisms and their hysteretic loops were anticlockwise. ridging and sub-surface flow; UL3--upper linear curve 3, describing the Q-EC mixed value of groundwater, groundwater ridging, sub-surface flow and direct runoff; LH--lower hyperbolic curve typical of the Q-EC response when direct runoff is suddenly added to the groundwater, following the heavy showers occurring during the dry period; WET-DRY 1 and 2--the hyperbolic curves defining the transition from the dry to wet season (under the curves UH1 and UH2, respectively). By analyzing the Q-EC pattern, the three events occurring on September 2016 were deemed to be \"affected\", as defined in [48]: the event that occurred on September 14 th 2016 began with a constant horizontal EC value even if Q increased and a sudden decrease of EC was delayed with respect to the peak value. The hysteretic curve had an anticlockwise loop and a square shape.