# Hydrological Analysis of Extreme Rain Events in a Medium-Sized Basin
## Materials and Methods
### Hydrological Model HEC-HMS, Setup, and Methods  
The Hydrologic Modeling System (HEC-HMS), developed by the U.S. Army Corps of Engineers' Hydrologic Engineering Center, formed to simulate the precipitation-discharge processes in dendritic catchments such as the Humber River basin, was one of the two models applied to calibrate the six studied single flood events. For the present study, the produced hydrographs resulted from the rain gauge data, with the subtraction of infiltration losses, the transformation of the excess precipitation into surface discharge, and the addition of the baseflow. Open channel flow simulation is also included in the basins required (W1020, W1180) to calculate the output hydrograph [38]. The selected modeling methods for the hydrological response of each sub-basin were: (a) the SCS curve number loss model for counting the cumulative losses; (b) the Clark Unit Hydrograph model for the representation of the basin's direct runoff via the transformation of excess rainfall; (c)the exponential recession model for counting the basin's baseflow; and (d) where needed, the Muskingum routing model for the computation of the open channel's flow [39]. The parameters regarding the SCS Curve Number loss method are the curve number (CN) and impervious (%). The CN is a sensitive parameter used at single flood event modeling, which incorporates the effect of soil type and land use of a watershed, together with the antecedent soil moisture conditions [37]. High residencies are considered as those residential districts by mean lot size equal to \(1/8\) acre or less, referring to townhouses. Estate residencies are assumed as residential districts by an average lot size of \(1/2\) acre. Regarding golf courses, these are considered to be in fair hydrologic condition, with grass cover between 50% and 75%. Gravel roads are counted as those that also include right-of-way. Pavel roads, together with curbs and storm sewers exclude right-of-way [37]. Water bodies, forest wetland, treed wetlands, wetland, and wetland shrub have a CN equal to 100 at any soil type [40]. Concerning forest, woods are considered to be in fair hydrologic condition, being grazed but not burned, with some forest litter covering the soil. On the other hand, trees are counted as woods and grass (pasture) combination in fair hydrologic conditions and at equal rates. Finally, cropland is considered as row crops planted in straight rows, contoured and terraced, also in fair hydrologic conditions [37]. The hydrologic conditions specify the influence of covering the category on percolation and discharge through components such as canopy, the density of cultivations, quantity of grass, and ground surface coarseness. Poor hydrologic conditions suggest decreased percolation and are prone to raise discharge, whereas good hydrologic conditions point out raised infiltration with a reduced trend of runoff [39]. Curve numbers (CN) per every found land use in the studied sub-basins of the Humber River basin, are given for every met soil group, fair hydrological conditions, and average moisture conditions in [37]. Maps of land use (LUI) and soil hydrologic groups (SG) were intersected in ArcGIS, producing a new map that displayed the combination of the above properties. An attribute table that gathered all the data per each unique land use-soil group polygon was constructed, along with every polygon's area and resulting curve number [37] from the aforementioned joining of land use and soil group, referring to normal (average) runoff conditions (Type II). The final CN of each one of the sub-basins was determined by summing up each resultant CN multiplied with the area-weighted land use-soil group plane element, given as:  
\[CN=\frac{\sum_{i=1}^{n}(A_{i,\;LUI-SG}\times CN_{i})}{A_{total}} \tag{2}\]  
Antecedent moisture conditions (AMC) are defined as the previous 5 days' rainfall and are important for the computation of the final CN.