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Changes in discharge regimes in the middle course of the Sava River in the 1931-20101 period

In order to analyse discharge regimes in the middle course of the Sava River (Croatia and Bosnia and Herzegovina), three hydrologic stations were chosen, having homogenous as well as continuous data in the observed 1931– 2010 period. Those stations are Rugvica at the beginning of the middle course of the Sava River, station Jasenovac located just forehead of the Una River mouth and all the other large tributaries from Bosnia and Herzegovina and station Županja near the end of the middle course of the Sava River. At all three stations, there are notable negative trends of annual mean (all stat. significant) and minimum discharges for the whole period. At the same time, only in the upper part of the middle course of the Sava River maximum annual discharges have a positive trend, contributing to more unfavourable regime characteristics. At all three stations a cyclic variation of mean annual discharges is observed, likely to be linked to the cyclic variation of annual precipitation in the area of the basin. According to the latest discharge regime typology of the region the discharge regime at Rugvica is of Peripannonian pluvial-nival type, while the discharge regimes downstream, at Jasenovac and Županja fall into Pannoninan pluvial-nival type. A comparison of discharge regimes established for two past climatological standard reference periods 1931–1960 and 1961– 1990 as well as for the contemporary 30- year period 1981–2010 lead to a conclusion that important changes in mean monthly discharges yearly course already occurred in the climatological standard reference period 1961– 1990 compared to the previous one. In 1961–1990 period the autumn season discharge decreased, which is almost solely a result of a large fall in the November mean monthly discharge at all stations. Furthermore, in the same 1961–1990 period a transition of the primary maximum in the annual course of mean monthly discharges from November to December and of the secondary maximum from March to April occurred, which is preserved in discharge regimes in the middle course of the Sava River. Discharge regimes in the middle course of the Sava River in the contemporary period are characterised by decreased summer and even spring time discharges, which is linked to the trend of raising air temperatures, an increase in potential and real evapotranspiration and to a slowly decrease in annual precipitation, particularly in the summer season as established in spatially relevant previous climatological research.

discharge regime, discharge trends, the Sava River basin, hydrogeography, hydrology

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