In this paper a case of very special hydrological behaviour of two neighbouring sinking karst rivers, Lika and Gacka, (Dinaric karst of Croatia), is analysed. The Lika River has a torrential hydrological regime. At the Sklope gauging station its minimum, mean and maximum measured discharges in the 1951-2005 period were: 0 (dry) m3/s : 24.5 m3/s : 729 m3/s. During the same period the Gacka River, at the Vivoze gauging station, had the following characteristic discharges: 2.29 m3/s; 14.7 m3/s; 71.0 m3/s. While the flow regime of the Lika River is characterised by extremely and very quick changes of discharges, the Gacka River flow regime is unusually uniform. The objective of the investigations made in this paper was to analyse the extremely different hydrological behaviour of the two neighbouring sinking rivers in order to find its reasons. Master depletion curves defined for the two analysed rivers shows that the karst aquifer of the Gacka River is much more abundant than Lika’s. The difference in the water temperature regime of the two neighbouring rivers is extremely high. At the Lika-Bilaj gauging station the minimum, mean and maximum measured water temperatures in the period of 1964-1991 were: 0.6°C; 9.3 °C; 21.4°C. During the period of 1964-2005 the Gacka River, at the Čovići gauging station had the following characteristic water temperatures: 6.4°C : 9.1°C : 11.6°C. The resident time in the karst underground of water discharging from the Gacka karst springs is much longer than in the case of the Lika River. The most probable explanation for this unusual hydrological behaviour of the two neighbouring karst rivers is that water from the Lika River and its catchment recharges some karst springs of the Gacka River. It is concluded that the Lika River feeds the Gacka River with an average annual discharge of about 5.35 m3/s. This value is different during each year and depends on the hydrological situation. It is very probably higher during the wet years than during the dry ones.

Abstract:This paper describes the hydrological changes caused by inter-basin water transfer and the reservoir development on the hydrological regimes of two rivers. The Sabljaki Reservoir in the Zagorska Mrežnica River and the Bukovik Reservoir in the upper Dobra River began operation in 1959. Both are part of the hydroelectric power plant (HEPP) Gojak, whose installed capacity is 50 m 3 /s. Their water volumes at the spillway altitudes of 320Ð10 and 320Ð15 m a. s. l. are 3Ð3 ð 10 6 and 0Ð24 ð 10 6 m 3 respectively. Both the Dobra and Mrežnica Rivers are losing, sinking and underground karst rivers. A 9376-mlong tunnel provides water from the Sabljaki Reservoir to the HEPP Gojak, which was constructed in the Lower Dobra River. The Sabljaki Reservoir is located in the Plaški karst polje, while the Bukovik Reservoir is located in the neighbouring Ogulin karst polje. The consequences of the inter-basin water transfer are strong and have caused abrupt changes in the hydrological regimes of the downstream sections of both rivers. At the same time, the construction and development of both the reservoirs have also caused hydrological changes to the upstream section of the Upper Dobra River.

Abstract:The aim of this paper is to analyse hydrological measurements obtained from Blue Lake located near the town of Imotski (Croatia), during the period from 6 November 2009 at 10:26 h to 4 September 2010 at 03:26 h (7230 h or 302 days). The water depth, water temperature and electrical conductivity of the water were continuously measured during this period. The measurements were obtained with a CTD DIVER (Schlumberger Water Services). The instrument was fixed at the bottom of Blue Lake, which means that all the measured data refer to a single point of measurement. The data represent the first systematic and continuous monitoring of the hydrological parameters of this fascinating karst phenomenon. The hydrological analysis also involved daily rainfall data and daily mean air temperatures recorded at the nearby Imotski meteorological station. The rate of water level rise and fall and the hourly and daily average inflow and outflow into and from the lake were calculated using the available data. The analysis led to the conclusion that Blue Lake is mainly recharged by water coming from the karst aquifer, the dimensions and the characteristics of which have not yet been adequately studied.

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Ivo Andrić & Ognjen Bonacci: Morfološka študija Crvenega jezera v Dinarskem krasu na podlagi terestičnega laserskega skeniranja in sonarnega sistema Crveno jezero (Dinarski kras, Hrvaška) je izjemen kraški pojav, po vsem svetu poznan po svoji lepoti in skrajnih globinah. Kljub temu je bilo v zgodovini raziskav Crvenega jezera veliko polemik v sklepih in teorijah o njegovem nastanku, geomorfologiji in hidrologiji. Namen tega dela je podati pregled obstoječih spoznanj o jezeru, kakor tudi predstaviti najnovejše rezultate raziskav, pridobljenih s pomočjo novih tehnologij, ki temeljijo na terestičnega laserskega skeniranja in hidroakustike. Raziskave so potekale septembra 2013. Nova generacija opreme, razvite za napredovanje geoznanstvenih raziskav, je bila uporabljena med delom na terenu. Zbrani podatki so omogočili temeljito analizo, ki je privedlo do novih pomembnih ugotovitev o Crvenem jezeru. Uporabili smo podvodno vozilo na daljinsko upravljanje (ROV), opremljeno z imaging sonarjem, ki je prineslo prve hidroakustične raziskave jezera. Nekateri rezultati so potrdili že znane in dobro dokumentirane značilnosti jezera, medtem ko so drugi izpodbijali splošno sprejete predpostavke znanstvene skupnosti in širše javnosti. V raziskavi prvič predstavljamo DEM za Crveno jezero, kar je omogočilo oceno o njegovem obsegu. Za najvišji zabeleženi nivo vode na 311 m nadmorske višine je ocenjena količina shranjene vode 8,24 x 10 6 m 3 .Temperaturni profil jezera je bil posnet med delom na terenu in je ponudil vpogled v možno mešanje vode s kraških kanalov, blizu površini vode. Uporaba TLS in ROV sonarnega sistema je precej izboljšal razumevanje morfologije jezera.