# Hydrological Analysis of Extreme Rain Events in a Medium-Sized Basin
## 4 Discussion  
Average annual and seasonal precipitation is expected to increase in the Greater Toronto Area due to the changing climate, producing more frequent and more intense severe precipitation events [29]. Thus, the progress in evaluating the hydrological basin's response under extreme rain events is of the utmost importance to the water resources management of the basin. Single storm events of high precipitation intensity, either producing a double hydrological peak flow or with already saturated soil due to the 5-day antecedent moisture, are already taking place frequently in the studied basin. The grouping of such events' hydrometric characteristics and the hydrological response of the basin based on two hydrological models with certain important differences in their setup are the objectives of our research.  
The HEC-HMS is an empirical model, with fitted parameters based on observed precipitation and discharge. The model is essentially lumped, where the spatial variations of the parameters concerning the loss method (\(CN\), %imperviousness) were averaged. The model simulates the storm during the period in which it takes place, taking into account the soil moisture conditions in a sub-basin for the last five days through the \(CN\) parameter. The methods used to describe the direct runoff via the transformation of excess rainfall, as well as the baseflow and the open channel's flow are also lumped. Evaporation losses were not considered, as these are optional for event-based simulation with the particular model but compulsory to continuous simulations [39]. On the other hand, the HBV-light is a conceptual model, based on the relevant hydrological processes that convert precipitation to discharge at the basin's exit through snow accumulation, soil moisture storage, evaporation, groundwater flow, and routing. The model is semi-distributed, allowing the separation of a sub-basin into several altitudes and vegetation zones. The parameters of the soil moisture routine were fitted for each vegetation class. Moreover, the model uses a warm-up period so that the state variables reach satisfactory initial values. Apart from precipitation and evaporation, temperature data are also used as input. In addition, the model does not work with discharge volumes, but with discharge in millimeters [54] in contrast with HEC-HMS. Even though both applied models are quite different, both fitted well on the examined events regarding the Nash-Sutcliffe and the volume error indices.  
Modeling components that may influence the output hydrographs of the studied basin are: (a) the 1 h time-step of precipitation and flow recorded measurements; (b) potential inaccuracies in the used DEM; (c) the watershed delineation; (d) changes in the land use; and (e) the selection of methods to calibrate the hydrological parameters. In other research concerning the area of interest, different calibration functions solved with other algorithms were applied and compared in order to validate the observed flow response; the results pointed out greater uncertainty for sub-basins with imperviousness higher than 80% [66]. Different methodologies of applying areal precipitation may also affect the final rain distribution per sub-watershed. In a recent study regarding the Humber River basin, various radar-gauge merging techniques were applied to ensure optimum rainfall evaluation [67]. In this research paper, we proposed certain values of parameters regarding two applied hydrological models for precipitation events with different characteristics that decision-makers and policy planners should take into account to adapt to the consequences of future extreme events, especially in the residential areas towards the basin's outlet.