## 2 Materials and Methods  
### Characteristics of the Study Area  
The investigations into stormwater quality and quantity were conducted for three urban catchments in the city of Kielce, Poland (Figure 1). In the catchments, which differ in land use, five characteristic surface types with respect to runoff were identified (Table 1): asphalt and gravel road surfaces, roofs, car parks, and green spaces. The first catchment, having a total area of 62 ha, is located in the central-eastern part of the city. It holds main transportation routes, service sector areas, and high-rise residential buildings. Overall, impervious areas with a high runoff coefficient (>0.8) constitute 51.5% of the total area of the catchment, which indicates its typical urban character. The highest point of the catchment is at 271.2 m above sea level and the lowest at 260 m a.s.l. The average slope of the land is 0.71% [15]. The stormwater from the catchment area is collected by the existing stormwater drainage system and conveyed by a main sewer to the stormwater treatment plant (SWTP) in IX Wiekow Kielc Street (Figure 1a).  
The Witosa SWTP catchment, which has an area of 83 ha, is located at the outskirts of the city. On the east, and partially north side, the catchment is surrounded by an open ditch collecting stormwater flowing from a dense forest area. The ditch turns into a \(\omega\) 800 mm closed sewer, which is connected to a sewer conveying effluent from the Witosa treatment plant (Figure 1b). All the stormwater is piped (\(\omega\) 1400 mm) to the receiving water of the river Silnica. Land development includes primarily one- and multi-family housing, and the share of impervious surfaces is 25.9% of total catchment area (Table 1). The highest point in the catchment is 365.5 m a.s.l., and the lowest is 291.25 m a.s.l. The average slope of the land is 8.2% [16].  
Jesionowa SWTP catchment occupies the largest area (400 ha). It is located in north-western part of Kielce and includes highly urbanised areas. The catchment land use is dominated by industrial zones with large commercial buildings, and low residential buildings. As a result, the share of impervious surfaces is 42.4% (169.6 ha). The highest point in the catchment is 315 m a.s.l., and the lowest is 265 m a.s.l. The average slope of the land is 2.65%.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{3}{*}{**Catchment**} & \multirow{3}{*}{
\begin{tabular}{c} **Area** \\ **A** \\ \end{tabular} } & \multicolumn{5}{c}{**Surface Type**} \\ \cline{3-8}  & & \multicolumn{2}{c}{**Roads**} & \multirow{2}{*}{**Roofs**} & \multirow{2}{*}{**Car Parks**} & \multirow{2}{*}{**Spaces**} \\ \cline{3-8}  & & & & & & **Spaces** \\ \hline \multicolumn{8}{c}{**ha**} \\ \hline IX Wieków & \multirow{3}{*}{62} & \multirow{3}{*}{26.0} & \multirow{3}{*}{–} & 14.3 & 11.2 & 48.5 \\ Kielc & & & & & & \\ Witosa & 83 & 8.5 & 1.6 & 9.4 & 6.4 & 74.1 \\ Jesionowa & 400 & 11.3 & 8.4 & 11.5 & 11.2 & 57.6 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Land use characteristics.  
Figure 1: Study area—(**a**) location in the city of Kielce; (**b**) IX Wieków Kielc stormwater treatment plant (SWTP) catchment; (**c**) Witosa SWTP catchment; and (**d**) Jesionowa SWTP catchment.  
In all three catchments, a separate sewage system is used, in which drains collect stormwater and sanitary wastewater is carried by sewers.