Influence of Hydrogeological and Climatic Conditions on Piston Flow in Karst Aquifers Using Statistical Approach

Piston flow is a term used to describe the phenomenon of the pressure pulse during pipe-full flow in karst conduits. During piston flow, as dilute meteoric recharge displaces water present in the conduits, discharge increases and specific conductance decreases at karstic springs. The aim of this research is to gain a deeper understanding of how piston flow can be identified on a global scale. To achieve this, the study has defined six phases (stable, lag, hydraulic pressure, mobilisation, dilution and recovery) based on the pattern of hydrographs and specific conductivity time series. Data from 69 flood events at 42 different locations worldwide have been collected and analysed. This analysis considered various factors such as lithology, aquifer type, allogenic or autogenic recharge, precipitation amount and intensity, dry period before storm, hydrograph shape and recession coefficient, specific-conductance time series parameters, memory effect, regulation time and typology of hysteresis loop in order to determine if groups could be differentiated using these parameters. The flood events were classified into two main groups: those with piston flow and those without. Furthermore, the cases with piston flow were categorised into five subgroups based on the location of mobilised stored water. The results show that piston flow can be observed in any karst setting, climatic region and hydrological situation, although it is more likely to occur during extended dry periods preceding a storm, periods of high precipitation and intense rainfall. The mobilised stored water is most likely found in the phreatic zone, but could also be present in the soil or epikarst zone. The occurrence of piston flow depends on the storage capacity of the matrix as well as the development of conduit systems that do not significantly weaken the hydraulic function of the matrix. The typology of hysteresis in piston flow is primarily influenced by the presence of pre-event water or a mixture of pre-event and event water, whereas in systems without piston flow, it is mainly affected by event water.