engleski

Karst spring hydrogaph analisys : Una spring case example, Croatia

Application of traditional hydrogeological methods in defining properties of karst aquifers is limited due to pronounced heterogeneity and anisotropy of the aquifers. Possible solution of the mentioned problem is application of global karst system respond analysis (karst spring hydrograph and/or chemograph analysis, isotopic investigations). Una Spring catchment area is composed mainly of karstified carbonate rocks whereas impermeable clastic rock appearances form groundwater circulation barriers. Locations of major springs are conditioned by tectonic contact of carbonate rocks with evaporite-clastic rock complex which compose Una and Srebrenica river valley. Una Spring catchment area is hydrogeologicaly poorly defined and its waterdivide to Adriatic sea catchment area (catchment areas of rivers Zrmanja and Otuča) position is only approximately determined. It is estimated that 70-80 % of total water that infiltrates karst aquifer appears at Una Spring. Large proportions of the main conveying karst channel and its depth of more than -205 m indicates great depth of karstification and water circulation within Una Spring karst aquifer. Both hydrograph recession analysis and time series analysis have been utilized in order to investigate hydraulic and retention properties of the considered aquifer, to define a hydrograph respond of the karstic system to rainfall input measured on available rain gages in the spring watershed and to investigate linkage of the karstic aquifers that drains in main Una Spring and nearby Joševica Spring. Obtained relatively high values of the Una baseflow recession coefficient are probably a consequence of dense and well connected karst conduit network developed over the whole karst system and, consequently, poor water retention capability of the aquifer. Quickflow recession parameters show much greater variations than baseflow parameters because of spatial and time rainfall variations influence on the quickflow stage of hydrograph. Una Spring hydrograph autocorrelation and spectral density function reflect dual character of the karstic system. Rapid decrease in value during first eight days reflects fast infiltration and short duration of groundwater flow through karst conduits. Gentle decrease that follows the eight day indicates slow flow through less permeable rock mass. Shape of the crosscorrelation function shows mean system lag time, which is shorter than 1 day and mean quickflow duration (2 days). Crosscorrelation function also shows strong correlation between hydrographs of Una and Joševica springs. Cross-spectral density function additionally signifies pronounced system nonlinearity, mean delay period of system reaction shorter than 1 day and quickflow duration of 7-8 days. Results of the conducted Una Spring hydrograph analysis show both usability and usefulness of presented methods in identifying karst system characteristics and quantitative comparation to other karst systems with similar analysis results available.

karst; spring; hydrograph

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