Pres ent-Day Vari a tions of the Min i mum Run off of the Volga Ba sin Rivers

Carried out is the analysis of stationarity of minimum runoff series in summer and winter for the Volga basin rivers. Two conditionally homogeneous periods are singled out within the temporal variations of these characteristics, and the date of their change varies throughout the territory of the basin. The considerable rise in air temperature in winter on the whole territory of the Volga basin is demonstrated as a result of the analysis of meteorological parameters. The relationship between runoff variations and wintertime temperature variations is proposed for predicting the minimum runoff. The distribution of minimum values of runoff is computed using this dependence and the forecast method based on the sum of distributions. DOI: 10.3103/S1068373914030078 The values of the minimum runoff belong to the category of major computed hydrological characteristics; therefore, the special attention has always been paid to studying formation processes and the methods of computation of the minimum runoff. Under the conditions of permanent increase in water consumption, the minimum water content can be considered as an extreme event because it limits water consumption and water use; the deficit of potable water and its unsatisfactory quality are associated with the periods of the minimum runoff. The estimation of the minimum runoff for the future is one of the most urgent problems of hydrology gaining a special stress under the current conditions of changing climate. The data of hydrological monitoring indicate that the considerable variations (mainly the intraannual ones) of runoff are registered in some regions of Russia. The reasons for the registered trends towards increase (decrease) in the water content of rivers have not been revealed yet using the available data because hydrometeorology has no reliable ideas about the secular changes of climate and runoff being its product. In the immediate past, the solution of practical problems was facilitated by the fact that the hypothesis on the stationarity of runoff processes was assumed for the hydrological substantiation of hydroeconomic activities for the short (at geological scale) periods of time that was of principal importance for runoff computation techniques. The appropriateness of this hypothesis for major hydrological characteristics was corroborated by numerous results of studying the variations of runoff and other components of water budget till the last quarter of the 20th century. At present, a concept of the stationarity of long-term runoff variations is called in question due to the intensification of global warming processes in recent decades. In the nonstationary case, some new problems arise requiring new methods and approaches for their solution. The nature of the occurred warming and the high probability of its continuation (2) force to approach in