In the present study, we have assessed low-flow discharge deficits, and the characteristics of the low flows at a constant and variable (monthly) low-flow threshold. The low-flow truncation level was identified based on the 70th percentile from the flow duration curve (Q70%). Q95% was assumed to be the threshold for the deep low-flow discharge. Thirteen catchments in the Vistula basin, varying in size, river regime, and physico-geographical conditions, were selected for the study. The input data was a series of daily discharge data sets from the period 1951–2016, made available by IMGW-PIB (Polish Institute of Meteorology and Water Management – National Research Institute). Analyses were conducted of drought flow discharge deficit volume in absolute and relative values, and low-flow duration and deficits contributing to severe low-flows were calculated for temporal scales of months and years. Comparison of the obtained results led us to the determination of the basic factors affecting the formation of low-flow deficits, and the indication of restrictions when applying fixed and variable criteria for identifying river low-flows.
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