Use of recession-curve analysis for estimation of karstification degree and its application in assessing overflow/underflow conditions in closely spaced karstic springs

Abstract

In the southern part of the Slovenský Kras Mountains, located on the Hungarian–Slovakian border, a transboundary karstic aquifer Dolný vrch/Alsóhegy underlies the structure and discharges groundwater flow to 15 major karst springs around its margin. Hydrograph recession curves from nine-gauged springs on the Slovak part of the aquifer were analysed, and for each individual spring, depletion hydrograph equations were classified into different categories based on recessional parameters quantitatively describing individual groundwater flow sub–regimes. Discharge depletion was used to create recessional equations, and these were linked to karstification degree, a qualitative scale ranging from 1 to 10. A new application of hydrograph separation was based on examining and combining pairs of springs that likely fit into the category of overflow/underflow springs for a single groundwater reservoir. Recession-curve analysis performed on coupled discharges of two pairs of neighbouring springs was conducted to examine their possible linkage as a single groundwater reservoir, with an overflow outlet and underflow outlet. In the process of discharge coupling, discharges of the springs, hypothesized to be branches of the same source and observed at the same time were simply added together. By analyzing the resulting new time series, a new classification of sub-regimes could be generated. Surprisingly, new facts were revealed about the overflow/underflow springs, which primarily were not recognized previously as being connected components of a single groundwater reservoir. Summations of combined overflow/underflow discharge volumes of a single spring reservoir led to discovery of apparent presence of turbulent flow sub-regimes on the coupled recession curve. Presence of possible overflow/underflow springs of a single groundwater basin should be considered during hydrograph analyses if the springs have attributes that suggest they may be part of a combined flow system. Treating these springs as separate entities as a result can produce significant misinterpretation of drainage parameters. Furthermore, identification of overflow branches facilitates the generation of new research ideas for further speleological investigations nearby, and for assessing the system in a more effective manner.