THE INFLUENCE OF LITHOLOGICAL STRUCTURE ON RIVER RUNOFF IN THE SOUTH-EASTERN LITHUANIAN HYDROLOGIC AREA

Due to the recently increasing frequency of extreme changes in river runoff regime, scientific literature deals with the characteristics of runoff formation. Works are carried out in analyzing climate changes and a lot of attention falls on land-use structures. Following thorough analysis of the lithological structure of river basins in South-eastern Lithuanian hydrologic area in separate costal zones, this article aims at evaluating river runoff formation characteristics. The basin lithological factor was calculated based on Quaternary map of Lithuania M 1: 50000 and Lithuanian river map M 1: 50000 using ArcGis software. In order to carry out more thorough analysis of the influence of lithology in given territories, sections of 0-20 m, 50-200 m, 200-500 m, 500-800 m, 800-1000 m and >1000 m were established, calculating the distance in meters from the riverbank. River basins of typical lithological structure (sandy, loamy, argillaceous) were selected and examined. The period of the year 1960-2007 was analyzed as this period saw the greatest amount of precipitation (up to 33% probability) and relation between the flow and precipitation was established as well as that with the lithological structure, established following the derivation of a hydromodule. Research has shown that basin areas, separate parts or sections of basins containing over 50% sand have an inversely proportional influence on the river runoff. Larger (more than 7 km(2)) homogenous argillaceous areas have double impact on the runoff: direct one during the period of 11.78 - 55.17% probability of precipitation and inversely proportional one during the period of 69.63 - 98.55% probability of precipitation. In cases of river basins, parts or sections of basins with loam percentage lower than 70%, precipitation and runoff correlation characteristics are similar to those of sandy basins, whereas in case of loam percentage higher than 70%, precipitation and runoff correlation resembles that of argillaceous basins.