Green and Blue–Green Infrastructure: From Definitions to Relationships

With climate change intensifying and space for new greenery becoming scarce, cities need their green spaces to work more effectively. Yet it remains unclear how green infrastructure (GI) and blue–green infrastructure (BGI) differ and whether combining their elements provides additional benefits. This conceptual review examined how sources position GI in relation to BGI (RQ1) and what evidence they provide for functional links between elements (RQ2). We conducted two consecutive targeted searches. The first, on conceptual relationships (46 sources), showed that element labels do not reliably distinguish the concepts. In the second, on functional links (29 sources), elements were therefore coded by their function rather than their labels. Most sources treated GI and BGI as nested concepts or synonyms. None treated them as mutually exclusive. Element labels reflected water management roles more than physical form. Five sources described a functional link, usually one in which a BGI element supplied water to vegetation. Two tested such a link, but neither appears to have compared the same receiving element with and without this supply. In the reviewed sources, GI and BGI are therefore mostly nested rather than distinct concepts (RQ1), and the added effect of linking their elements has not yet been demonstrated (RQ2). Functional links were tested only at the local scale. City-scale studies reported only combined effects, possibly because of how they were designed. At the metropolitan scale, effectiveness may depend on a mosaic of spatial arrangements of GI, BGI and natural systems. Rather than introducing new terms, effective integration requires planning and research to specify which element supplies water to which, and to test whether this link adds benefits.