Using a hydrological model to determine the cause of the water supply crisis for the town of Pernik in Bulgaria

In 2019 the Studena reservoir, the only water supply source for the town of Pernik in Bulgaria, accumulated less than half of its normal annual inflow. This factor, together with an inadequate and delayed response by the water resource managers caused a severe water supply crisis in the fall and winter of that year. As this situation was unprecedented, investigations were undertaken to determine the cause of the low inflow to the reservoir using the model CLM3 and available climatic and spatial data, and data that were collected in the watershed for the reservoir. The study was conducted by simulating the hydrological processes that take place in the watershed, and the monthly inflows to the reservoir for the period 2017–2019. It was found that hydrological parameters produced in the calibrated model were similar to those measured in the field and by another model. The achieved agreement was considered to be satisfactory given the complexity of their nature and assessment methods. The result of the study convincingly confirmed that the main reason for the poor inflow to the reservoir in 2019 was the low precipitation in this year and, above all, the scarce snowfall in the winter months. Although the total annual precipitation falling in the region during the period 2017–2019 has not changed significantly, but the seasonal distribution of precipitation has changed. In particular, the amount of precipitation that falls in snowfalls has declined and reached a minimum in 2019. These decreases are due to the increase in the average air temperature of 1 °C which causes the observed decreases in the runoff.