Generating Regional Models for Estimating the Peak Flows and Environmental Flows Magnitude for the Bulgarian-Greek Rhodope Mountain Range Torrential Watersheds

The flood magnitudes with 25, 50, and 100 years return periods and the environmental flows (Q(env)) are of outmost importance in the context of hydraulic and hydrologic design. In this study, 25 watershed characteristics were linked with the aforementioned recurrence intervals, peak discharge values, as well as Q(env) for 15 pristine torrential watersheds with more than 10 years of streamflow records in the Rhodopi mountain range with a view to generating regional relationships for the assessment of discharge annual peaks and environmental flows regarding the ungauged torrential watersheds in the region. The Log-Pearson Type III probability distribution was fitted in the discharge annual peaks time series, so as to predict Q(25), Q(50), and Q(100), whereas the Tennant method was utilised so as to estimate the environmental flows magnitude. Similarly, the Kolmogorov-Smirnov and the Anderson-Darling tests were performed to verify the distribution fitting. The Principal Components Analysis method reduced the explanatory variables number to 14, whilst the stepwise multiple regression analysis indicated that the exponential model is suitable for predicting the Q(25), the power model best forecasted the Q(50) and Q(100), whereas the linear model is appropriate for Q(env) prognosis. In addition, the reliability of the obtained regression models was evaluated by employing the R-2, the Nash-Sutcliffe efficiency, and the Index of Agreement Statistical Criteria, which were found to range from 0.91-0.96, 0.88-0.95 and 0.97-0.99, respectively, thereby denoting very strong and accurate forecasts by the generated equations. Thus, the developed equations could successfully predict the peak discharge values and environmental flows within the region's ungauged watersheds with the drainage size not exceeding 330 km(2).