engleski

Hydrology of two coastal karst cryptodepressions in Croatia : Vrana lake vs Vrana lake

This paper compares hydrological systems of two coastal karst cryptodepressions which represent the two largest lakes in Croatia, and are known by the same name of Vrana Lake. The paper analyzes Vrana Lake in Dalmatia (surface 30 km² lowest point at – 3.0 msl ; occasional records of increased salinity with chloride content up to 4500 mg/L), and Vrana Lake on island Cres (surface 5, 75 km² lowest point at – 61.3 msl ; and constant salinity values with chloride content of about 100 mg/L). Both lakes represent valuable water resources on a water-poor karst area and have a status of protected localities – Vrana lake in Dalmatia as a nature park, and Vrana lake on Cres (the only source of island's drinking water) as a strict zone of sanitary protection. The lakes have different hydrological, limnological and ecological characteristics, which are compared in this work for the first time. The aim of this work was to assess the hydrological stability of the lake systems, define their relation with karstic aquifer and the sea and determine the risks of undesirable changes (salification due to present anthropogenic influence and global climate changes), all based on simple analysis of basic lake systems' dynamics. The area of study is hydrology of coastal karst systems, with an emphasis on limnological research. The methods used are simple stochastic analysis of data time series, appropriate for modest background resources available – continuous data on sea and lake water level oscillations, and results from occasional monitorings of water conductivity and salinity. Attained results are interpreted in the context of Ghyben-Herzberg rule on freshwater – saltwater equilibrium. Results of conducted analysis showed different sensitivity of analyzed lakes towards hydrological conditions, and demonstrated different trends during the same observed period (1948-2005). Vrana Lake on the island Cres, whose water level oscillations range between 9.11 – 15.93 msl, demonstrated a siginficant trend of decrement in minimum annual water level values (0.41 % / year), while Vrana Lake in Dalmatia, with water level oscillations ranging between 0.03 – 2.24 msl, demonstrated a trend of increment in minimum annual water level values of 1.28 % / year. It is concluded that different trends can be related to local anthropogenic influence (water usage from the lake and its catchment), but also to global climate changes which show significant influence on this fragile hydrological systems, even in this short-term analyzed period.

karst lakes; water level oscillation dynamics; water management; anthropogenic risks; salification

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