*3.2. Objective Formulation*

Among the selected 67 planning-related articles, there were 60 studies that contained objective formulations, which mainly addressed water quantity regulation services (e.g., runoff volume reduction, peak flow reduction, and combined sewer overflow) and water quality regulation services (i.e., the removal of runoff pollutant), as well as referring

to greenhouse gas emissions (1), temperature regulation (3), biodiversity enhancement (5), cultural services (2), and social and economic benefits (5). The planning objectives were formulated as one or more of the abovementioned ES, and 56 articles (93%) formulated their objectives as the water quantity regulation or water quality regulation services; these two types of ES are still the dominant objectives of GSI planning. Only 3 articles put forward clear quantitative objectives, and the other 57 articles briefly summarized the planning goals in a qualitative way based on the functions confirmed by the relevant research of GSI. Another 7 studies did not mention the concept of planning objectives and directly proceeded to other steps in planning. This may be attributed to the fact that the authors utilized the relevant functions of GSI that were widely confirmed, and therefore, there was no need to specifically propose the objective. Current planning objectives lack quantitative and precise considerations, which may be caused by two reasons: Firstly, researchers are accustomed to directly providing qualitative planning objectives that may be feasible in explaining the functions of GSI. However, the ambiguous expression of such objectives may induce a decrease in the interest of investors and stakeholders, thus hindering the development of GSI. Secondly, the quantitative research on the function of GSI is still immature. For example, in the literature review, as regards the runoff volume reduction using strategies such as bioretention (BR), green roof (GR), infiltration trench (IT), permeable pavement (PP), rain barrel (RB), and vegetative swales (VS), the research results show high spatial heterogeneity, as shown in Table 1.


**Table 1.** Runoff volume reduction, implementation cost, and annual maintenance costs of six types of GSI.

Since a quantitative relationship between GSI and ES cannot be clearly known, it is formidable to determine an appropriate clear objective based on existing studies or to determine whether the GSI planning objective can be achieved after it is formulated quantitatively. Moreover, based on the literature review regarding water quality regulation, the initial conditions (e.g., climate, geology, and the composition and number of pollutants in stormwater runoff) of different study areas are inconsistent, which makes even a certain type of GSI show different water quality regulation capabilities, as shown in Table 2. Researchers adopt a variety of facility configurations in actual operations, such as filler and plants. Furthermore, GSI planning is increasingly inclined to address a combination of different types of facilities, which makes objective formulation more complicated.


**Table 2.** The runoff reduction and pollutant removal performances of bioretention.
