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2. Antonov, H.; Danchev, D.: Podzemni vodi v NRB. Technika, Sofia 1980.
3. Bulgarian Academy of Sciences: Klimatichen atlas na Narodna Republika Bulgaria. Sofia 1976.
4. Bulgarian Academy of Sciences: Information Bulgaria. Pergamon Press, Oxford, UK (especially 34?44 on water resources).
5. Bulgarian Academy of Sciences: Vtora natsionalna konferentsia po hidrologia i hidrolozhki aspekti na vodostopanskite problemi, 1987. Sofia: University Press ?Kliment Ochridski,? Sofia 1990.