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The main object of the paper article is water resources of Lithuanian rivers, which were calculated using a long data series of the runoff. Water resources are based on the balance equation. This means that this equation includes only the runoff formed in the Lithuanian territory, from neighboring countries to Lithuania and through them into the sea. The average and dry annual water resources have been calculated at a 95% probability. The average perennial water resources of Lithuanian rivers are 24.844 km³ / year, of them 8.883 km³ / year is the transit from Belarus, Poland and Kaliningrad Region to Lithuania, 15.961 km³ / year is formed in Lithuania and 7.471 km³ / year are effluents through Latvia into the Baltic Sea from Kaliningrad Region into the Curonian Lagoon. The Nemunas basin floats 21.501 km³ / year to the Curonian Lagoon. The present water resources are by 4.5% less in comparison with water resources of in the year 1996. Cyclic water change is typical of the Lithuanian rivers. The Nemunas-Smalininkai hydrological station has noted 7 cycles in a long data series. The average term of a cycle is 27 years (14 years of a wet and 13 years of a dry period). The runoff of the Nemunas river differs in the duration of dry and wet periods (by about 16%). The runoff of rivers in Southeast and Western Lithuania differs by 21-23% and in Middle Lithuania by 28%. The long data series of the Nemunas-Smalininkai runoff have a negative trend because of a decrease in the last four decades. The runoff measured by this water management station consists of river runoff from the southeastern and middle parts of Lithuania. The annual runoff has a positive trend in the majority of Western Lithuanian rivers in the period 1961-2003. The minimum runoff of a dry period shows no trend in most Lithuanian rivers, except Western Lithuania where a positive trend was estimated in the period 1961-2003. [ABSTRACT FROM AUTHOR]