Climate change is often represented by increase of extreme phenomena, like storms, torrential rains, and floods. Their regional and local dimensions vary from one area to another. This is especially valid for the regime and distribution of precipitation. Even for a country with relatively small territory, like Bulgaria, precipitation in some areas has increased during the last decades, while in others it has decreased. The only precipitation characteristic with undoubted trend of increase for most of the territory is the heavy and torrential rains (VELEV 2005, BOCHEVA et al. 2007). This means that, even in cases with no increase in the annual precipitation, there are less storm events but their quantity and intensity are higher. Torrential rains are one of the most important factors for flood formation and there is a significant increase in the number of this hazardous phenomenon during the last few years. The causal relationship between the increasing number of torrential rains and flood events for the last decades in Bulgaria can be illustrated with an example from Yantra River basin (fig. 1). The problem is more serious in the mountainous areas, where the amount of precipitations is normally larger and the topography facilitates a rapid increase of surface runoff and formation of disastrous peak flows. Spatial distribution of precipitation in the mountains is usually discontinuous. This is even more typical for the torrential rains, the influence of which is often concentrated in some parts of the river basin. They seldom exert simultaneous effect on all the tributaries of the main river (NIKOLOVA 2007).
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