The diploma thesis discusses the mass balance of Cerknica Lake, known as the most famous intermittent lake in the world. The lake is in avegare present for three quarters of a year, and for a quarter of a year, the bottom of Cerknica Polje is dry. Nearly every year, the lake expands to the size of 20 square kilometers, and the volume of 29 million cubic meters of water. Before calculating the mass balance of the lake, I have designed a terain model with the programme AutoCAD Civil 3D, based on the data from the digital land model. According to the designed terain model and a daily change of water level on the hydrometric station Dolenje Jezero, I have calculated daily change of the volume of water and by using this data, I have calculated the water ballance of the lake. The established mass balance of the lake is based on the rate of flow and concentration of parameters, observed on the inflows and swallow holes, by the Slovenian Environment Agency. Various quality parametes have been observed (ammonium, ortophosphates, nitrates, nitrites), with the emphasis on
phosphorus and nitrogen which are the most important indicators of water quality. More nutrients come to the lake than flow out of it which means the lake has the capacity to clean itself. The only exceptions are the nitrites and ammonium which are produced in the lake. I have also calculated the impact on the lake of phosphorus and nitrogen which come to the lake from the hydrology. The calculations have been made for the period between years 2004 and 2014, In the year 2014, 29 tones of phosphorus and 457 tones of nitrogen have come into the lake. The calculation of phosphorus concentration has also been done with the Vollenweider model. By comparing calculations with observed values, observed by the Slovenian Environment Agency, I have established that this model is not suitable for calculations, due to the intermittent nature of the lake.