The calculation of low flows is crucial for monitoring streams and ensuring their quality, because they
are habitat for many animals and plants. There is a number of different indices for calculation of low
flows, such as flow duration curve, streamflow deficit, base flow index, ninety-five percentile flow Q95
mean annual minima etc. In USA most commonly used methods for calculation of low flows are
hydrological (xQy) and biological (xBy). Both methods can be implemented with DFLOW software.
Hydrological method is statistical, biological one is empirical and uses aquatic life water quality
criteria directly for its implementation. Criteria are expressed as two concentrations, instead of one, so
that criteria can more exactly reflect toxicological and practical realities. The two concentrations are
CCC, which provides the highest concentration of pollutants that could be maintained indefinitely in
the stream and CMC, which determines maximum concentration of pollutants in the stream.
In graduation thesis calculations of low flows and excursions with DFLOW sofware are conducted for
56 Slovenian gauging stations. For calculated flows 4B3, 7Q10, 1B3 and 1Q10 an analysis and
comparison with calculated Q95 were made. It has turned out that the calculated flows with software
DFLOW compared to Q95 are significantly lower. For 15 selected gauging stations a comparison with
ecologically acceptable flow by the Slovenian methodology and methods of selected European
countries was also made. The smallest difference have occurred by comparison of flows 4B3, 7Q10,
1B3 and 1Q10 with the return withdrawal by the Slovenian methodology and Austrain method Q361.
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