Environmental engineering often uses hydrologic rainfall-runoff models that simulate surface runoff and can be used for flood prediction. One of the methods to set-up hydrologic models is the unit hydrograph theory, which can be used if adequate data about precipitation and discharges is lacking. In practice, sufficient precipitation and discharge data is often not available. In this case, we can obtain results by creating synthetic unit hydrograph, which is based on the characteristics of drainage basins. For the purposes of this graduation thesis we chose twenty larger Slovenian catchments with known characteristics (e.g., land use, slope) and for each catchment five biggest flood events were selected. Based on the characteristics of the catchments we calculated lag time using existing methods such as Synder’s method, Denver method, Tulsa District method, etc. The mentioned equations were developed at the area of the United States of America where the characteristics of the catchments differ from the characteristics of the ones in Slovenia. Consequently, we can conclude that the equation developed based for the catchments in Slovenia could provide more accurate results. With this in mind, we attempted to develop a new equation by testing seventy different cases using R programming language. The selected equation, which gave the most adequate result was further verified with the HEC-HMS hydrological model where we processed data of the additional catchment (i.e. Medija River catchment). The results confirmed our hypothesis, which stated that the equation developed based on the characteristics of the catchments in Slovenia would yield better modelling results.
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