# Hydrological Analysis of Extreme Rain Events in a Medium-Sized Basin
## 5 Conclusions  
Several hydrological model parameters may influence the accuracy of the observed flow hydrographs at the exit of the gauged sub-basins. Regarding the HEC-HMS model, the time of occurrence of maximum runoff mostly depended on the value of \(T_{c}\), but also on the storage coefficient. Specifically, raising the storage coefficient might produce a forward shift of the peak runoff time, accompanied by a significant reduction in the runoff magnitude. Lower peak runoff also resulted by increasing \(T_{c}\). The decrease in the \(CN\) as well as of the % imperviousness reduced the magnitude of peak runoff too. The Muskingum \(K\) was also critical, producing changes in the peak discharge as well as the Nash-Sutcliffe efficiency coefficient (NSE). Calibration of the \(T_{c}\) calculated by using different formulas generated the major time shifts and variations of peak simulated discharge. Concerning the HBV-light model, the magnitude of peak discharge depended mainly on the values of \(FC\). In particular, raising the \(FC\) resulted to lower maximum runoff. The _MAXBAS_ routing parameter affected substantially the time of occurrence of peak flow; its rise resulted to a shift of the peak runoff time forward, along with a decrease in the runoff. Since the HBV-light is a conceptual model, there is no clear trend among the other parameters, which depend not only on their own values but also from other parameters' values. While the sub-basins W800 and W900 have approximately the same percentage of urban development, concerning the studied precipitation events, the sub-basin W900 mostly produced higher peak runoffs because the sub-basin had a smaller surface area by 27% and a greater slope of the longest flow path, with the slope being taken into account in the HEC-HMS model through the time of concentration (\(T_{c}\)). However, the channel slopes were relatively flat and did not differ significantly in the sub-basins. The most obvious differences in the parameter values of the aforementioned sub-basins were in the HBV-light model, especially regarding the soil moisture routine. On the other hand, W790 is a rural sub-basin with 13.1% urbanization, while the sub-basin W1030 was 97.7% urbanized; both sub-basins had a similar areal extent. In comparing the two sub-basins, it is evident that the peak discharge was significantly higher in the urban sub-basin and the time of appearance of the modeled peak discharge occurred earlier in the sub-basin W1030 due to higher flow velocity in both applied hydrological models. Observing both examined statistical indices, the events of 28-30 May 2013, 7-15 April 2013, and 26-30 June 2010 were better calibrated with the HBV-light model. The 28-30 May 2013 and 26-30 June 2010 events had less total precipitation volume as well as smaller maximum precipitation intensity than their comparable pairs (4-10 July 2013 and 22-25 June 2017); those characteristics seem to have contributed so that the corresponding flow hydrographs were better approached by the HBV-light model in which the initial states took their proper values due to the one-year warm-up period. Regarding the 28-30 May 2013 event, both rural sub-basins had better Nash-Sutcliffe indices with the HBV-light model and almost similar volume error indices with the ones of the HEC-HMS model. The rural sub-basins with fairly or some urban development had both indices almost similar in both models. Regarding the urban and semi-rural to semi-urban sub-basins, the Nash-Sutcliffe indices were similar in both models, but the volume error indices were decreased with the HBV-light model. Concerning the 26-30 June 2010 event, the W800 sub-basinhad both slightly better indices using the HBV-light model, whereas in the W1030 and W1020, the resulting indices from HBV-light had a better fit. For W790 and W900, the NSE coefficient was improved by using the HBV-light in comparison to the volume error index, but the peak hydrographs were more fitted with the HBV-light.