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Annual streamflow of Bulgaria in the beginning of XXI century. Estimating the annual streamflow under the climate change is a challenge for hydrology science and an important task for geography as well. There is no analysis about the basic statistical parameters of the annual runoff time-series after 2000 in scientific investigations in the country. This paper describes Bulgaria's annual streamflow during 2000 -2016 for the main catchment areas in the country (Danube watershed, Black Sea catchment area and the Aegean Sea watershed). The work is based on publicly available data of annual streamflow. The screening of the hydrological data for homogeneity, randomness, probability distribution and the trend is based on statistical methods. The obtained results show regional differences in scale of fluctuation of hydrological variables around the norm for the referent periods, in the years with maximum and with the minimum volume of the water resources, in the probability distribution of empirical data. All timeseries are homogeneous, randomly and with the absence of the trend. The work draws a conclusion that annual streamflow of main catchment areas of Bulgaria in the beginning of XXI century is in the normal frame and with larger fluctuation than the referent periods and that the time-series statistical models need confirmation using the larger base of hydrological data. This work is the first estimation of the annual streamflow during XXI century in Bulgaria by major watersheds and it gives a new information about hydrological processes in the country.

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The minimal stream flow is the most sensitive characteristic of stream flow. The aim of the paper is the assessment of the duration and the frequency of low flow through threshold method on daily discharges. The daily data of the two Danube tributaries (the Topolovets and Voinishka) have been used. The results present the following: low flow typically appears in the summer and episodically in the spring and winter; the duration varies between some days to month; the spatial distribution of extreme minimum flow is discrete and depends on climatic and anthropogenic activities. The method of quantiles gives good threshold level for defining extremely low stream flow, but have to apply in keeping with concrete conditions and purposes.