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The present material demonstrates appliance of regional studies on the area of Central Danube Plain in Bulgaria for determination the characteristic maximum flows at nonobserved river sections with focusing on for flood risk areas. River flow is characterized by annual variability in maximum runoff caused by climatic factors and anthropogenic impacts. For this reason, in most cases, information from hydrometric stations cannot be used directly. One of the main tasks of hydrological research is the transfer of information from observed river sections to unobservable ones based on the recorded flows from the monitoring network. In the present study, the statistical maximum estimates used are the time series of annual maximum flows for the period 1961-2018, which are composed of instantaneous absolute values of the maximum runoff recorded within one calendar year for each hydrometric station. For the adopted period, there are complete time series of annual maximums for almost all stations in the country, which ensures comparability of results. The used regionalization approach is based on the correlation between the specific flows (in this case the annual maximums for the hydrometric stations) and one of the main characteristics of the catchments. These links are established after detailed analysis and justification. The regression relationship used is proven to be appropriate for determining maximum runoff at non monitored stations in multiple developments. For further refinement and analysis of the regression relationships, information from closed and frozen stations from the hydrometric network was used with shorter rows but providing additional clarity for areas with fewer hydrometer stations. Regional studies were conducted for the area of the central Danube Plain, where three large watersheds are located - the Vit, Ossam and Yantra. All available hydrological information at the National Institute of Meteorology and Meteorology has been used to estimate the characteristic maximum flows at the gauging stations and afterwards their spreading at non-observed sections. The material is illustrated with maps and graphics of the hydrological zones and regional relationships in the three river systems. [ABSTRACT FROM AUTHOR]

Water resource of a given river basin is the main component of the ecosystem and water resources management system as well. Ecological flows are considered within the context of the Water Framework Directive (WFD) as "a hydrological regime consistent with the achievement of the environmental objectives of the WFD in natural surface water bodies as mentioned in Article 4(1). The so-called Heavily Modified Water Bodies (HMWB) or "regulated rivers" have modified flow, which is influenced by reservoir compensation releases and management. National Institute of Meteorology and Hydrology (NIMH) has responsibility and contributed to the implementation of the Eflow concept in Bulgaria. For the needs of the Ministry of Environment and Water (MoEW), a hydrological regionalization and a calculation scheme for the minimum acceptable flow in the rivers on the territory of Bulgaria are developed. In this report are represented the following researches: up-to-date assessment of the minimum acceptable flow (MAF) in the riverbeds after the complex dams, according to the Water Law (WL), a comparative analysis with the current state and definition of recommendations in support of the MoEW and the Basin Directorates (BD). The specificity of each reservoir in determining the MAF, the change during drought, etc. are highlighted. In the regulated rivers, the MAF is for the purpose of water ecosystems, groundwater and drinking water supply, wetlands, etc. The NIMH approach is applied and an up-to-date assessment of the MAF from the 52 complex and significant dams from Appendix 1 of the WL is carried out. The assessment is based on the results of hydrological development for the evaluation of the annual average flow and minimum monthly flows with 95% probability of exceedance of the own water catchment of dams. A comparative analysis was carried out between up-to-data assessed minimum acceptable flow (determined by NIMH) and current released MAF (by MoEW). The risks for ensuring the ecological flow are identified. Recommendations are given regarding the determination of ecological flow, definition of a minimum lower threshold, the specifics of reservoirs along the Black Sea or those for drinking water supply, etc. The research supports the MoEW and BD in determining the ecological flow in the riverbeds after the complex reservois, implementation of the Program of measures and the Eflow concept in Bulgaria. [ABSTRACT FROM AUTHOR]

The main goal of presented study is a demonstration of technological approach to determine the water resources of surface water bodies, using a sparse hydrological monitoring network not covering many of them. Resource of the water body is a flow formed in the catchment area above the studied one including the additional inflow in the frame of the same water body. Practically the flow is changing in each sector of the water body but average annual resources are specified at the final section. The water resources for the river water bodies are determined using information from the monitoring network of the National Institute of Meteorology and Hydrology (NIMH) for the Iskar River. The investigation is preceded by hydrological analysis of the information: the referent period, homogenous and suitable data set selection for statistical procedures reflecting the actual conditions in the surface water. The transfer of information from observed river stretches to non-observed ones requires a special hydrological investigation and methodological decision. In the world practice there are two main methodological approaches for flow determination in the nonobserved river stretches - regionalization of characteristic flows depending on catchment characteristics and a balance method. The balance method requires a very big amount of information linked to the various water consumptions as: water supply, irrigation, power engineering, reservoir management and water accumulation very often missing or not reliable. That is the reason for the acceptance of regionalization of characteristic flows as a suitable approach for Bulgarian conditions based on the real and reliable measured data from the monitoring network in NIMH. In the case of water accumulation after dams, where practically the flow is interrupted, the water resource determination can be based only on the information from reservoir management authorities (if this information exists). Iskar River has a biggest watershed in the Danube catchment area in Bulgaria, where the capital Sofia is located, with important economic significance and big anthropogenic impacts on the flow regime. In the study is used a GIS information for the water bodies delineation presented by the Ministry of Environment and Water. [ABSTRACT FROM AUTHOR]

River hydrology is a key component in river resources planning and river engineering. Our recent study is focused on flow metrics that best describe the registered flow regime and hydrological characteristics of rivers within the Tundzha watershed located in South Bulgaria. The flow regime will be described using 5 broad categories: magnitude of river flow; frequency; duration; timing and rate of change. A special attention in the research was given to the low flows. Thirteen hydrometrics stations were selected for the analysis. Data were obtained from the monitoring network of the National institute of meteorology and hydrology, Bulgaria. A flow duration analysis was done for each station in Tundzha River basin in order to estimate the probability of occurrence of different flows. Low flow frequency analysis was conducted. Baseflow indices were calculated to estimate the relative contribution of ground flow or baseflow. Regional regression equations were calculated between flow metrics and drainage basin area. Hydrological analyses play an important role not only in water resources projects but also in instream flow and fish habitat studies. Furthermore, we believed that such a regional characterization of river hydrology will help water resources managers to better deal with water issues. [ABSTRACT FROM AUTHOR]

Modeling is important for river system analysis and management. The aim of the research is to apply disaggregation models to data with recovered natural river flow of Osam River located in North Bulgaria. As a result river discharge synthetic sequences are generated on monthly and seasonal discretization. We want to test applicability of the models for specific climatic, hydrological and orographic conditions in Bulgaria. Disaggregation models are from the family of stochastic models. Disaggregation models could be divided into the three main domains: temporal, spatial and temporal-spatial. The research has been done to apply selected temporal disaggregation models for flow data generation. Two basic models are executed - the Original model of Mejia and Rousselle and the Corrected extended Lin model, one-stage disaggregation. The objects for disaggregation are data with recovered natural river discharges from drainage basin of Osam River. Data from five stations along the Osam River are executed. As a result synthetic flow sequences with monthly values were generated. The comparison between the two applied models is made and discussed. We can conclude that both models are suitable for disaggregation of the recovered natural river flows with snowmelt conditions together with rainfall in the catchment areas. The models are suitable for the specific conditions in the hilly and valley regions in the country. They were tested for drainage basins with small, medium and relatively large for Bulgaria sizes. [ABSTRACT FROM AUTHOR]

Floods are natural phenomena which cannot be prevented. In order to avoid and reduce the adverse impacts of floods for human health, the environment, cultural heritage and economic activity it is important to provide flood risk management plans. Development of flood risk management plans under Directive 2007/60/EC are elements of integrated river basin management. For the implementation of Directive 2007/60 / EC, Member States need to carry out a preliminary flood risk assessment based on available or readily derivable information. Based on this flood risk assessment, areas are identified where a potential significant flood risk exist or might be considered likely to occur. As part of a preliminary flood risk assessment, it is necessary to describe floods which have occurred in the past and which had significant adverse impacts on the protected categories "Human Health", "Environment", "Cultural Heritage" and "Economic Activities" and where there is a likelihood of recurrence of similar future events. In order to obtain as much information as possible about past floods, different sources of information were used for data collection. The results of the following data collection methods are taken into account: survey of municipalities through a questionnaire on past floods, a list of information of specialized services and departments and evaluation of available materials from literature sources, websites, scientific articles and monographs. After processing, analysing and evaluating the information provided by the various sources, it was found that, for the Eastern Aegean Basin, the main reason for the floods is intense, prolonged rainfall. Another type of floods, common for the basin, is flash floods resulting from intensive (up to 6 hours) rainfall. A small part of the documented floods (three) are as result of sewage overflow. Criteria for the number of affected inhabitants and material damages in the settlements, infrastructure and agriculture are applied. As a result of the analysis and evaluation of the information, 23 sensitive areas have been identified in the region. When identifying potential flood risk areas based on the provided information, there is a risk of missing such areas for which insufficient or complete historical information has been collected. To approximate these areas, an approach with the introduction of a criterion for horizontal distance from the river bed was used. 1D hydraulic modeling and GIS were used to determine the flooded areas in the defined areas at risk in terms of future flood risk. [ABSTRACT FROM AUTHOR]

The management of protected territories in Bulgaria is a subject to the Bulgarian and European environmental legislation. The used plans for management provide basic information on the condition of abiotic and biotic components of the natural environment in the protected territories and Guidelines for their management, control and conservation. The water resources with their specificity and dynamics of alteration are a major abiotic component that impacts on functioning, development and conservation of ecosystems in these territories. According to the Bulgarian legislation, the surface water resource is determined on the basis of information from the hydrometer stations of the monitoring system in the country maintained by National Institute of Meteorology and Hydrology. The stations of the monitoring network record the actual and available water flows. The major problem of the present investigation is the absence of a monitoring network in the frame of the whole National Park and necessity of realistic water resources determination. National Park Rila is situated in one of the highest mountain catchment areas on the territory of the country with significant water sources. There begin the biggest and the longest Bulgarian rivers as Iskar and Maritza. Despite the status of protected area in the Rila National Park there are no functioning hydrometric stations providing up-to-date information required for the assessment of water resources. [ABSTRACT FROM AUTHOR]