Water losses from a reservoir built in karst: the example of the Boljunica reservoir (Istria, Croatia)

The paper presents the case of the Boljunica reservoir, which began operation in 1973. It is situated on the Istria peninsula (Croatia). This is a multipurpose reservoir which was built in order to protect the downstream area from flood, to store water for irrigation, and to control sediment transport. The reservoir is situated on the contact zone between water impermeable Eocene flysch and deep Eocene and Cretaceous limestone. The bottom of the reservoir is covered partly by both flysch and quaternary deposits. Water losses from the reservoir bottom are so large that the main service intended for the reservoir, the storage of water for irrigation, is impossible. After every intensive precipitation, which occurs often in this region, the reservoir fills very quickly. The problem is that its retention of water is very short, and lasts only a few days. The water volume of the reservoir at the spillway altitude of 93.00 m a. s. l. is about 6.5 x 10(6) m(3). Because of water losses from the reservoir bottom, the mean annual volume of water stored in it, during the period of 1977-2005 was only 0.5 x 10(6) m(3), which is less than 8% of the full reservoir volume. On the reservoir bottom, many new swallow holes opened through the sediment cover after each time it filled and emptied with water. Special attention is paid to the groundwater level analyses. Interdisciplinary analyses and investigations of hydrological and hydrogeological factors causing the formation of swallow holes and water losses from the Boljunica reservoir are discussed. The example given in this paper explains one unsuccessful case of building a reservoir in Dinaric karst, caused mainly due to insufficient geological, hydrogeological and hydrological investigations. In order to prevent water losses from the Boljunica reservoir, very complex and expensive work needs to be done, but its success regarding the reasonable reduction of water losses from the reservoir is in question.