Rainwater harvesting systems can be designed to mitigate stormwater runoff by incorporating features such as oversized gutters and downspouts, first flush devices, and overflow management systems. These features help to capture and store rainwater during intense rainfall events, reducing the amount of runoff entering sewers and storm drains. Additionally, the system can be designed to slowly release excess water into the ground or a nearby waterbody, further reducing stormwater runoff.