Summary:

The paper presents the analysis of hydrological drought of the Biebrza river to the Burzyn gauge and Narewka river to Narewka gauge in multiyear 1951–2002. Hydrological drought is usually expressions of deficiencies is surface water supplies. This phenomena is the last stage the drought process and is identify with low flows in river bed. The examined catchment area is part of the Podlaska Lowland where droughts occurred most often in Poland. The research described in this paper was concentrated on the Biebrza valley, which is a unique wetlands area in Central Europe and on the Narewka valley within the Bialowieza Forest, the latter being a natural heritage in the global scale. The Biebrza Wetlands and Bialowieza Forest hydrogenic sites conditions depends on ground and surface water levels and thus, a hydrological droughts is a danger to them. The methodology used in this paper describes the hydrological drought as the situation, where river discharges are below some arbitrarily chosen threshold level. The size of the threshold discharge was established in dependence on aim of study. Truncation level value of drought was applied as statistical value Q_90% determined from flow duration curve with the upper discharges. This paper include the proposition the other truncation level which depends on water organism survival rate. The droughts were described with the following parameters: minimum discharge, duration and water deficit volume. Drought parameters were computed from daily flow date for using Distributions of Low Flow Extremes (Nizowka2003) computer program. Results of 52 years studies prove that most of droughts appears in summer half-year. The average duration time of summer droughts is 2 months in Biebrza river and over month in Narewka river. Since 1973 to 2002 year no observed any drought in winter half-year in Biebrza valley. The biggest droughts were note in the 50’s and 60’s and in last decade.