Key hydraulic structures for regulating water resources are reservoirs whose efficiency depends on their water balance components which are presented in this paper. In order to exploit and preserve the water from the reservoir in the optimum way, continuous research and analysis are required. This paper analyses the use of a relatively young reservoir called Butoniga in Istrian County, whose initial planned capacity of about 2,000 l/s has never been reached. The paper presents and analyses data about the position of the reservoir and its hydrological and general characteristics. The degree of utilization of the reservoir was analysed using the SYMAC software, determining the water level in the storage reservoir depending on water demand and hydrological characteristics. Input hydrological data analysed from the period 1994 – 2013 are: precipitation, inflows, abstraction and evaporation. Modelling was carried out in such a way that a gradual increase in abstraction started after 2002, when a treatment plant was put into operation. The paper presents the results of modelling in the form of tables and figures obtained by the afore-mentioned software. Based on the obtained modelling results, it was concluded that an increase in the existing water volumes abstracted from the reservoir can lower the water level in the reservoir below the critical point.