The Yzeron catchment (150 km2) is located to the south-west of Lyon city, France. It forms part of the Observatoire de Terrain en Hydrologie Urbaine (OTHU, http://www.graie.org/othu/index.htm ) long term observatory. It is representative of French periurban areas and is characterized by a marked topography (162 m at the outlet when it reaches the Rhône River and 917 m for the highest point) et 50% of the catchment with slopes larger than 10% (from a 25 m resolution DTM). The geology is contrasted with crystalline formations (granite, gneiss ) in the western part of the catchment and more alluvial and glaciar formations in the eastern part. The land use is heterogeneous. The upstream and western part of the basin is limited by a range of hills covered with forests. The intermediate part is mainly covered with grassland and cultivated lands, mixed with urban nucleus. Thin green corridors remain along rivers, covered with deciduous trees. The downstream part is mainly covered with densely urbanized areas. A fast progression of urbanization is observed since the seventies. The area is prone to sharp Mediterranean-type flood events due to its steep topography in the upstream part and limited soil water storage capacity overall. The rain water is mainly collected through combined sewer systems, equipped with combined sewer overflow devices that overflow in the river when the sewer capacity is exceeded. This water is rich of sediments and pollutions, causing quality problems in the rivers, especially during summer storms, where most of the water reaching the river comes from urbanized areas via SODs. Increased erosion of the river banks has also been evidenced with impact on the ecosystems. The catchment is instrumented since 1997 with nested sub-catchments, in particular two sub-catchments with different land uses (Mercier: agriculture and forest) and Chaudanne (agriculture and urban) with rainfall and discharge data. Information about physico-chemical characteristics and microbiology of the water is also available, in particular before and after a SOD in the Chaudanne catchment.

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