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Fig. 12.3 Spatial differentiation of selected hydrological drought estimators in Poland (1984–
2015). Mean annual number of days with hydrological drought (N D ) (days): 1 – 121–135, 2 – 136–
150, 3 – 151–165, 4 – 166–180, 5 – 181–195; 6 – N D changes in larger transit rivers (outside the map
scale), CS D – mean annual coefficient of severe drought contribution (%); statistically significant
(α = 0.05) linear trend in multiannual course of annual number of days with hydrological drought
(A) and the coefficient of severe drought contribution (B), a – trend line slope, R 2 – coefficient of
determination, arrow direction denotes positive (up) or negative (down) sign of the slope
As mentioned in the statement opening this chapter, it was assumed that river
low flow occurrence indicates the development of the hydrological drought in the
catchment. Drought identification criteria were as follows:
• Events of hydrological drought per year, if the number of days with low flow
(Q 70% ) exceeded 90 in a given hydrological year,
• Events of hydrological drought per month, if the number of days with low flow
(Q 70% ) exceeded 10 in a given month.