# Hydrological Analysis of Extreme Rain Events in a Medium-Sized Basin
## Materials and Methods  
### Study Area: Humber River Basin  
The hydrological basin of Humber River is located in the Greater Toronto Area, in Southern Ontario, Canada. The hydrological catchment has an area of 889 km\({}^{2}\), consisting of seven sub-basins. The area of each sub-basin, along with the length of the longest flow path and its slope, are shown in Table 1.  
Figure 1 points out the extent of the Humber River Basin and the existing river network, as resulted through the watershed delineation from the digital elevation model (DEM) at a spatial resolution of 30 m \(\times\) 30 m, provided by the Ontario Ministry of Natural Resources [28], with the use of the HEC-Geo-HMS extension of ArcGIS. The elevation ranges between 75 m and 489 m above sea level, with the northwest part being characterized as hummocky topography with steep gradients [29], whereas the central part is comparatively flat. For the present study, the Thiessen polygons methodology was used to estimate the mean areal precipitation for each one of the seven sub-watersheds through the weighting of sixteen rain gauges across the total hydrological catchment. Data for the \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sub-Basin** & **A (km\({}^{2}\))** & **Longest Flow Path (km)** & **Slope of Longest Flow Path (m/m)** & **Flow Gauging Station at Exit** \\ \hline W1460 & 161.6319 & 30.490527 & 0.007277 & 02 HC047 \\ W790 & 63.2952 & 15.446972 & 0.007888 & 02 HC023 \\ W800 & 192.4848 & 53.400175 & 0.003251 & 02 HC009 \\ W900 & 140.3595 & 27.587806 & 0.004841 & 02 HC031 \\ W1030 & 60.246 & 31.909755 & 0.004202 & 02 HC027 \\ W1020 & 241.1919 & 76.009458 & 0.002489 & 02 HC003 \\ W1180 & 29.8854 & 19.110794 & 0.004919 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characteristics of the Humber sub-watersheds. observed flow were used from six flow gauging stations located at the exit of six sub-basins. The observed gauges' data at 1 h time-step were gathered from the Toronto and Region Conservation Authority [30] and Environment and Climate Change Canada [31]. Based on a 30-year observation period (1971-2000) the average annual precipitation in the basin was found to be equal to 834 mm, according to the records of a station in the City of Toronto, in the shore zone of Lake Ontario [29]. According to the General Land Uses maps of 2010, the watershed studied consists of rural and agricultural uses, urban uses, natural vegetation, and waterbodies. Specifically, the Humber River Basin is covered by urban settlement (28.1%), roads (8.8%), water (0.9%), forest (15.9%), forest wetland (0.4%), trees (0.4%), treed wetlands (0.2%), cropland (44.2%), wetlands (1.0%), wetland shrub (0.1%) and other land (0.01%), as indicated in Figure 2a, afforded by Agriculture and Agri-food Canada, Land Use [32]. Thus, the study watershed is characterized as partly rural and partly urban. In terms of urban settlement, W1460 is 35% high residential, 60% estate residential, and 5% consists of golf course; W790 is 50% high residential and 50% estate residential; W800 consists of 70% high residencies and 30% estate residencies; W900 is 50% high residential, 45% estate residential, and 5% consists of golf course; W1030 is 70% high residential and 30% industrial; W1020 consists of 60% high residencies and 40% industries; W1180 is 100% high residential, based on the interpretation of the joint effort of Civica Infrastructure Inc. (CIVICA) and Toronto and Region Conservation Authority (TRCA) [33] as well as TRCA [34]. As far as roads are concerned, W1460, W790, W800, and W900 consist of gravel roads, paved roads that pass from W1030 and W1180, whereas gravel roads and paved roads exist in W1020 at equal rates, based on the interpretation of [33]. Figure 1: Location of Humber River Basin in Southern Ontario, with the rain and flow gauge stations. The soil type of the area varies spatially.