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Object of the study is surface water bodies from category "rivers" according to Water Framework Directive 2000/60/EC. Surface water assessment is important for number of activities such as: water management in the country, making reports to international agencies, determining the change of the resources in the light of upcoming climate changes. The determination of water resources is based on information of hydrometric stations from the monitoring network system in the National Institute of Meteorology and Hydrology -- Bulgarian Academy of Sciences (NIMHBAS) in which real ongoing and available water flows that are subject of management are registered. In the study a technology for surface water bodies in the Bulgarian basins of the lower Danube is applied which has been developed in the frame of cooperative project together with the Ministry of Environment and Water. This is absolutely true for the Bulgarian section of the Danube River basin which is expressed in big number and variety of hydrological homogeneous sections. The river flow is characterized with annual and inter-annual variability determined by climatic factors and anthropogenic influences. The main obtained results of the present hydrologic studies are the usage of transferred information from gauged to ungauged watersheds and the estimation of the surface water bodies' resources using original regression relationships based on multiannual hydrological information from the NIMH-BAS monitoring network. The relationships delineate the hydrological homogeneous areas with similar conditions of flow formation. The estimated resources have significant usefulness for all State institutions managing the water in the Danube basin and have already been introduced in the operative and management practice. [ABSTRACT FROM AUTHOR]

A revision of river and lake typology in Bulgaria has been done within the Ecoregion No. 12 Pontic province and Ecoregion No. 7 Eastern Balkans according to the Water Framework Directive 2000/60/EC. Certain geographic changes between the Ecoregion No. 12 and Ecoregion No. 7 have been proposed based on existing biogeographic data, at the beginning. The whole part of the Danube and Black Sea River Basin district have been associated to the Pontic Province (ER No. 12) as well as the rest part of the Southern Bulgaria (Maritsa river basin, Mesta RB and Struma RB) has been allied to the Eastern Balkan (ER No. 7). A serious reduction of the total number of river types (from 33 to 16 types) was justified using clear hydromorphological and biological criteria, e.g. ecoregion, vertical factors (4 altitude zones, main substratum characteristics, slope, and other supplementary factors such as fish and vegetation zonation, climate maps) and horizontal factors (calcareous geology, salinity, size category). Similar reduction of the number of lake types (from 33 to 17 types) was proposed in parallel with a significant modification of the lake typology method. Lake typology in Bulgaria was based on the obligatory factors (4 altitude zones, size typology based on surface area, depth, salinity and geology) and optional factors (residence time, mixing characteristics, e.g. monomictic, dimictic and polymictic, presence of profundal zone, reference trophic status). Seven lake types were identified as "reservoir types", which were only presented by heavily modified and artificial water bodies without any possibility for natural lake equivalent within the country, or region. Four coastal lake types with various salinity (from freshwater <0.5 ‰ to hypersaline >40‰) have been reviewed as belonging to the category "transitional waters". [ABSTRACT FROM AUTHOR]