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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. [ABSTRACT FROM AUTHOR]

The water resources of the Danube region for basin management represent 31 % of the whole water resources in the country. The flow regime of river is Mild-Continental type - high water phase in spring, low water phase in summer-autumn and transitional water during winter. [ABSTRACT FROM AUTHOR]

Numerous studies have projected that devastating natural disasters (i.e., droughts) will become more severe under changing climatic conditions. This study employed a new drought indicator namely Normalized Ecosystem Drought Index (NEDI) which calculates the dryness conditions according to transitional ecosystem patterns. To assess the response of the ecosystem to water availability, modified Variable Interval Time Averaging (m_VITA) was used. The NEDI coupled with the m_VITA was validated for over 102 in situ stations and three vegetation types in the selected study area. The results indicate that the NEDI can effectively represent ecosystem evapotranspiration reduction and negative precipitation anomalies that occur in response to limited water availability. NEDI displayed high correlation with precipitation anomaly (0.52–0.62) and evapotranspiration deficit (0.33–0.73) compared to other drought indices. Three widely used drought indices, namely, the Standardized Precipitation Evapotranspiration Index (SPEI), the Standardized Precipitation Index, and the Palmer Drought Severity Index (PDSI), were employed to assess the transitional ecosystem response. Although these indices successfully capture the drought conditions, they do not identify the response of the ecosystem to limited water availability. Moreover, further analysis of indices to evaluate the sensitivity to vegetation types revealed that use of SPEI and PDSI have a lack of presenting response of ecosystem at a different level of water availability for all selected vegetation types. This study shows that the application of the NEDI and the m_VITA may be helpful for decision-makers when they are revising irrigation plans because the drought conditions identified by the NEDI show how an ecosystem responds to limited water availability. • An ecosystem drought indicator was employed to identify drought conditions. • NEDI effectively represent ecosystem evapotranspiration reduction. • Water availability is a key factor that regulates evapotranspiration in dry regime. • NEDI well reflects the response of ecosystem to limited water availability. [ABSTRACT FROM AUTHOR]