A recent study (Gerassimov et al. 2001, 2004a, 2004b) concerned water resources in Bulgaria during the 1982-1994 drought period. An investigation was made of precipitation (P, mm) and river discharge (runoff depth, h, mm) for two periods: one with a duration of 13 years (the
T lïïl TTT TTT1 T ~TT TTTT TTTT TTTT TTTT 1TTT ÎTTT TTTT 11 0.01 0.05 0.1 0.5 1 2 5 1 0 2 0 30 40 5 0 6 0 70 80 90 95 98 99 99.5 99.9
Fig. 3. Probability distribution curves for springs 48 and 63 located in the Black Sea basin.
drought period) and the other with a duration of 106 years (1890-1995). In these studies the river discharge is calculated and used in total depth of runoff. The authors concluded that the drought period had a strong negative impact on river runoff in the country.
The statistical descriptors of data such as the (arithmetic) mean (X) and standard deviation (o-) of discharge and precipitation, with comparison to the long period of 106 years, are presented in Table 2 according to Gerassimov et al. (2004a). From Table 2 it is possible to conclude that during the drought period the average values and standard deviations are considerably lower compared to those of the long series. The reductions of these values are most significant for the Danube basin, followed by the Aegean Sea and Black Sea basin.
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