Session B7 - Stormwater harvesting: evaluating the potential

This study explores an integrated approach to managing rainwater and groundwater in the city of Kinshasa, DRC. In a context of rapid urbanization and climate disruption, the risks related to flooding and the overexploitation of aquifers are increasing. The study proposes a methodology combining the analysis of rainfall, piezometric levels, and infiltration data to design effective and sustainable aquifer recharge (artificial) solutions, while reducing runoff, and highlighting the importance of integrated urban planning based on local hydrological indicators. This approach promotes resilience to extreme rainfall events and supports the quantitative balance of water resources. The results show that extreme rainfall has increased by 23% since 2012, whereas a seasonal drop in the water table of 1.2 to 2.5 meters is mainly observed in certain areas.Simulations of groundwater recharge measurements have led to an improvement by reducing the annual water deficit by 20 to 35%.

In the context of increase of water scarcity due to climate change, wastewater reuse (REUT) – which involves recovering part of the wastewater after appropriate additional treatment – is one of the solutions proposed to reduce pressure on natural water resources. While it presents a clear opportunity as a non-conventional water source, REUT raises questions concerning low water flows downstream of WWTPs, since WWTP discharges can in some cases contribute significantly to maintaining low water flows in rivers. Based on databases of WWTP discharge flows (Rhône Méditerranée Corse Water Agency), river flows from HydroPortail or simulated using a hydrological model, this study presents a map of the contributions of WWTPs (average discharges during dry weather from July to September) to low flow indicators (VCN10 and QMNA5) across the entire hydrographic network of the Rhône river basin. The results show that these contributions vary, thereby helping to better identify opportunities for diversifying water sources in different territories.

Climate change increasingly demands robust strategies that can guide society toward more sustainable pathways. Rainwater Harvesting Systems (RWHS) constitute a proven and effective tool in this transition, contributing not only to the preservation of water resources but also to the mitigation of urban flooding and the reduction of demand on public water-supply networks. The present study applied a methodology to assess the potential of each Portuguese municipality for RWHS implementation by calculating the C Index, incorporating built-area typology (Class 1, Class 2, and Class 3), precipitation, and water consumption. All spatial analyses were carried out in QGIS. The results demonstrated the feasibility of RWHS deployment, particularly in municipalities located in northern and central Portugal. A total of 22 Class 1 municipalities and 2 Class 2 municipalities were identified as having the capacity to meet their population’s water demand exclusively through harvested rainwater.