Moreover, cities can target their policies in a way that national or regional schemes may not be able to, leading to less costly policy action. A case in point is flood mitigation activities, which are generally interdependent or generate risk externalities, for instance, channelling rivers upstream can increase flood risk downstream, and hydraulic infrastructures and land-cover composition can also affect flood risk in urban areas. Cities and their surroundings also share significant uncertainties associated with managing water resources, both quantitatively and qualitatively.