Mažos upės nuotėkio ypatybės metais su sausu šiltuoju laikotarpiu

Abstract

Such decrease in precipitation amount of the warm period might be related to the changes in atmospheric circulation due to the warming climate. As no longer-lasting measurements were taken in meteorological stations, there is no evidence to suggest whether the changes are of anthropogenic origin or are only a part of a longer natural cycle. During the last decades the part of runoff of the warm period is also decreasing in total yearly runoff rate. At the end of the dry period (in September) nearly all dam-free tributaries of the river Šušvė (including the tributary Ažytė with the basin area of 98 km2) were dried out. In the middle of October the runoff from the Ažytė and upper reaches of the Šušvė recurred, however it did not exceed 11/s km2. During the operation of hydroelectric power station the eco-discharge of water was not impeded, though the allowable intensity of water level decrease was exceeded in certain cases. The paper gives the analysis of the distribution of precipitation in the basin of a small river and presents certain approaches of the runoff formation. The object of the study - the River Šušvė (the length 130.5 km, basin area 1165 km2) basin — itself contains no meteorological stations therefore precipitation was evaluated on the basis of the data collected in the nearby stations located in Dotnuva, Raseiniai and Šiauliai. An obvious difference was observed in the long-term rates of precipitation amounts of certain months. In 2002, the average values of precipitation and runoff were close to the long-term rate though the rainfall distribution was very uneven within the year, what resulted in some extreme seasonal and months by values. The beginning of the year was rather wet; in Kaunas and Raseiniai February was wettest during the whole study period. The warm period was extremely dry. In Dotnuva the April-September period was driest within the whole study period. Considering the data of the meteorological station in Raseiniai, the period mentioned was least abundant in rainfalls in 2002 compared to the annual precipitation rate. During the last 50 years the portion of the total warm period also decreased in the annual precipitation rate. A statistically reliable difference (p <0.05) between the beginning and the end of the period was obtained in two meteorological stations (except for the meteorological station in Šiauliai). Average air temperature of the year increased by 1°C within the period from 1951 to 2002 (p <0.01).