1. Hydrology and Water Management Research Center, National Institute of Geophysics, Geodesy and Geography, Bulgarian Academy of Sciences (NIGGG-BAS), Acad. G. Bonchev Str., bl. 3, 1113 Sofia, Bulgaria
2. Department of Geography, National Institute of Geophysics, Geodesy and Geography, Bulgarian Academy of Sciences (NIGGG-BAS), Acad. G. Bonchev Str., bl. 3, 1113 Sofia, Bulgaria