WATER RESOURCE ANALYSIS OF THE OMBLA SPRING

The paper carries out a complex water resource behaviour analysis of the Ombla karst spring. The Ombla is a typical coastal karst spring. It is located in the vicinity of the town Dubrovnik, and reliably supplies it with good quality drinking water. It appears on the surface at the attitude of 2.5 m asl, so its water almost immediately flows into the Adriatic Sea. Its minimum discharge is estimated at 4.30 m(3) s(-1), and its maximum at 117 m(3) s(-1). Considering the size of its recharge area and the abundant, intense precipitation which appears in it and also due to its karst character, it has been concluded that this is a spring with a limited maximum discharge capacity. With regard to its minimum discharges, it can be concluded that it is recharged from a stable, large karst aquifer. Its medium discharge in the natural state in the observation period from 1968 to 1980 equalled 28.01 m(3) s(-1). From 1981 to 2012 (with the exception of the 1991-1992 period), i.e. in the period when the river bed of the Trebisnjica River, which flows through its catchment area, was regulated and turned into a concrete canal in the length of over 60 km, its medium discharge was reduced by 4.29 m(3) s(-1), to the value of 23.71 m(3) s(-1). The paper presents the most recent hydrological analyses of water discharges and temperatures. A special attention was paid to the definition of the spring's catchment area. In the literature, this surface is estimated at about 600 km(2), which is determined mostly through regional geological analyses. Due to a lack of reliable data, this paper could not determine the catchment area with certainty either, and even less reliably its boundaries. However, by application of a hydrological method, it was estimated that the catchment area is probably much larger, ranging between 850 km(2) and 1100 km(2). The behaviour of groundwater levels, measured in several deep piezometers drilled in its hinterland, was also analyzed. It was proven that the watershed between the Ombla spring catchment and the neighbouring catchment of the Zavrelje spring suddenly changes as a result of intense precipitation, and that waters from the aquifer of the Ombla spring occasionally spill over to the neighbouring springs.