Climatic characteristics and hydrological zones

The climate of Bulgaria is influenced by intensive atmospheric circulation from the Atlantic Ocean coast, continental Europe, the Aegean Sea and the Black Sea. The transfer of air masses is determined by the location of mountain chains in the territory of the country. In this respect, we should mention the Balkan range (Stara Planina mountain). The Balkan range is a barrier stopping the cold continental air masses and their propagation to the south. The Rila-Rhodopes massif, which is situated in the central southern part of the country, is a barrier to the Mediterranean influence. Important to climate variability are horizontal air movements such as vertical air drifting due to altitude. A mountain climate occurs at high altitude.

The specific orographic characteristics of the country are reasons for climatic differences and definition of the main drainage basins. Three main drainage basins or so-called hydrological zones are distinguished (Fig. 1).

Danube hydrological zone

The zone occupies north Bulgaria without its eastern part (number 1 in Fig. 1). The southern border of the basin is the Balkan range. This zone includes the drainage basin of the Iskar River going through the Balkan range. The upper recharge zone of the Iskar River is located in the Rila mountains. The hydrological zone is characterized by a temperate climate to a (European) continental climate. The Danube zone covers around 45% of the territory of Bulgaria.

Black Sea zone

This zone (number 2 in Fig. 1) is characterized by a temperate climate, and a climate with Mediterranean influence south of the Balkan range. The Black Sea zone covers around 13.8% of Bulgaria.

Aegean Sea zone

This zone (numbers 3 and 4 in Fig. 1) occupies south Bulgaria. It is possible to divide it into Western Aegean and Eastern Aegean basins. The zone is characterized by a transition from temperate climate to Mediterranean (continental-Mediterranean)

Fig. 1. Hydrological zones in Bulgaria: 1, Danube; 2, Black Sea; 3, Western Aegean; 4, Eastern Aegean. Dots indicate location and numbering of studied springs.

climate. The southern parts of the zone and river valleys are influenced by hot air masses moving from the south. The Aegean Sea zone covers the remaining 41.2% of the territory of the country.

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