Water Runoff and Catchment Improvement by Nature-Based Solution (NBS) Promotion in Private Household Gardens: An Agent-Based Model

Nature-based solutions (NBS) such as rainwater gardens and permeable paving can be deployed as an alternative to conventional urban gardens to improve cities’ resilience against increasing rainfall. This study describes the application of an agent-based model (ABM) to assess the role of private gardens toward the enhancement of water management by households. The ABM simulates the process of switching from “gray” (i.e., paved) to green gardens, taking into account the effect of “soft” (garden networks and gardening workshops) and “hard” (monetary) incentives. The ABM is supported by a water balance model to consider the effect of rainfall on soil water retention. Four different cities in Europe were analyzed: Szeged (Hungary), Alcala de Henares (Spain), Metropolitan city of Milan (Italy), and Cankaya Municipality (Turkey). The results demonstrate that greening private gardens can generate impact on water run-off and catchment in cities in the order of 5–10%, reaching picks up to 20% in certain cases. While the proposed model is not devoid of limitations, the results provide useful insights in the ways different instruments (e.g., municipal subsidies and knowledge support) could assist with the greening of private gardens for NBS promotion to respond to cities’ water management challenges.