4.1.3. Quantity/Scale Determination

In order to make up for the existing shortcomings, it is essential to analyze the mechanism of how GSI provides ES. The dominant features covered in this review are water quantity regulation and water quality regulation ESs that GSI provides by making full use of natural elements, especially vegetation (e.g., BR, GR, VS), to carry out stormwater infiltration, retention, transmission, evapotranspiration, and purification processes; this is exactly the essential difference between GSI and conventional methods; in other words, GSI planning can introduce ecological processes (mainly related to vegetation), and as there are complex interactions between ecological and hydrological processes, changes in hydrological processes will occur, thus the production of various ESs such as water quantity and quality regulation. There are mutual influences (Figure 2) between ecological and hydrological processes. On the one hand, the evapotranspiration process of vegetation transports water to the atmosphere; the growth of vegetation roots affects the structure of soil porosity, thereby changing the distribution of surface and deep soil moisture [87]; the existence of rhizosphere microorganisms and vegetation absorption facilitates the removal of heavy metals, nitrogen, phosphorus, bacteria and other pollutants in the water body; the increase in vegetation cover with the canopy interception contributes to the redistribution of precipitation; compared with other land cover types, vegetation cover generally has a lower runoff coefficient, which helps reduce the peak runoff and delay the peak time [88]. Conversely, hydrological elements, such as flow volume, flow velocity, water quality, and water level, affect the structure, dynamics, distribution, and succession of vegetation communities; hydrological processes such as infiltration, abortion, and confluence affect the flow of nutrients, pollutants, minerals, and organic matter in the ecosystem and its distribution in soil and water bodies; moreover, hydrological processes contribute to improving hydrological connectivity, recharging watershed water volume, making hydrological gradients smoother at the large scale (e.g., watershed and country), and the integrity of ecosystems [89]. The water cycle takes the atmosphere, vegetation, and soil as basic media for the migration and conversion of water, while the material circulation and energy flow of the ecosystem driven by the water cycle maintain its critical structure and function.

There are many differences in hydrological, ecological, and other features of society, economy, and environment among the study areas, so different responses between hydrological and ecological processes are bound to be involved, which leads to different performances of GSI. The above-mentioned idea of referring to the default values given by existing research or hydrological models [70,90,91] is still dedicated to obtaining the direct quantitative relationship between GSI and ES, which ignores the discrepancy of the hydrological and ecological process responses in different planning scenarios, and it should be amended. We suggest an indirect method to determine the quantity or scale, i.e., exploring the interaction mechanism between local hydrological and ecological processes in each planning area, then identifying the response and variation of hydrological processes driven by the ecological processes that are introduced by GSI, so as to determine the extent of ESs that these responses and variations can produce, as shown in Figure 3. We are not committed to obtaining a simple quantitative relationship between GSI and ES once and

for all, proven to be unattainable, but to provide an indirect quantitative approach that can be used for the future GSI planning step of quantity/scale determination, where the identification of the interaction mechanisms between local hydrological and ecological processes in each planning is encouraged.

**Figure 2.** The interaction between hydrological processes and ecological processes [80–82].

**Figure 3.** The method of quantifying the relationship between GSI and ES by eco-hydrological coupling model.

The eco-hydrology proposed by the United Nations Conference on Water and Environment [92] in 1992 provides an understanding of the complex interaction between hydrology and ecological processes quantitatively. The method that has been adopted is to construct a coupling model of hydrology and ecology, where the two-way feedback of hydrological and ecological processes can proceed. Specifically, numerous variables and parameters in hydrological and ecological models are used to simulate the hydrological and ecological processes, and the variables and parameters that exist in the two types of models at the same time support the feedback. Related research on the coupling of these two systems has been extensively carried out. Marshall et al. [93] applied the Simultaneous Heat and Water (SHAW) model, loosely coupled with the Geophysical

Institute Permafrost Laboratory (GIPL) model to simulate the soil moisture dynamics. Cristiano et al. [94] used an eco-hydrological streamflow model for urban areas (EHSMu), taking into consideration water and soil dynamics, vegetation types, evapotranspiration fluxes, and aquifer recharge, and simulated the runoff formation, evapotranspiration, and aquifer recharge on an hourly scale. These studies are helpful to analyze the migration of water in the soil–plant–atmosphere continuum (SPAC) [95]. Under the drastic process of climate change and urbanization, it is inevitable to consider climatic and social disturbance factors. Yu et al. [96] coupled the vegetation interface processes (VIP) model with the China AgroSys model to simulate eco-hydrological processes, such as crop yield, evapotranspiration, and runoff yield, and discussed the human impact on hydrology and ecology within the basin and region. Li et al. [97] coupled an eco-hydrological model (GBEHM-HEIFLOW) with a socio-economic model (WEM-HRB), taking into account the impact of the socioeconomic system, and developed a watershed system to simulate the coevolution of natural and social systems with water–land–air–plant–human nexus. Among the coupling models, there are differences in the degree of simplification of eco-hydrological processes, the use of empirical equations, and the choice of parameterization schemes. However, it is still possible to use the same equation formula, such as the soil temperature diffusion equation for calculating soil temperature [98], the Richard equation for calculating water movement of unsaturated soil [99], and the Farquhar and Collatz photosynthesis models for simulating vegetation photosynthesis [100,101]. Eco-hydrological models may involve many variables [102], e.g., meteorological variables (rainfall, radiation, and evaporation, etc.), hydrological variables (water level, water discharge, and flow velocity, etc.), and ecological variables (vegetation and its net primary productivity, plankton, and benthos, etc.). Existing studies mainly focus on experimental observation, mechanism exploration, and numerical simulation toward ecosystems to discuss carbon flux, soil water transfer, evaporation, and soil water-related parameter observation at a point or field scale [102], but the interaction mechanism of eco-hydrological processes at the watershed and even the global scale is worthy of more exploration. For future research on coupling models, we suggest the following:


related to model input, model structure, parameters, and observations used for model calibration [104]. It is estimated that the sources of uncertainty in complex models are still in the initial stage, and more experimental research and summary can be conducted to reduce the uncertainty.


#### 4.1.4. Site Selection

Site selection by means of remote sensing is a widely used method; the most common approach is to generalize the planning area via the Geographic Information System (GIS) and to construct a GSI evaluation system suitable for construction with the MCDA framework to obtain the evaluation results, then to select the sites based on economic, social, and ecological conditions in areas with higher suitability. Similar to the studies in type/scenario evaluation, site selection also involves multi-standard evaluation. MCDA can logically structure complex issues and specify various uncertainties; therefore, all specifications for the site selection of GSI involve MCDA [73]. AHP is also widely used in the index weight determination process [33,106]. The layer-cake theory proposed by McHarg [107] guides the site selection plan. Although planners select different numbers and types of social, economic, and environmental indicators, the overlay analysis of construction suitability indicators is evaluated on land grids of different resolutions essentially. It is worth mentioning that related studies use different terms, such as suitability, sensitivity, or vulnerability. We do not make a distinction because these terms are similar; planners are all committed to choosing the most vulnerable areas with the highest demand for GSI construction. GIS provides convenience to visualize the evaluation results of the suitability of GSI construction. It is worth noting that most of the existing suitability studies still focus on the environment dimension, and therefore, the evaluation framework for suitability may only deal with environmental indicators (e.g., slope, elevation, water body, and ecological land). However, GSI should never be separated from human society with the exclusive consideration of the environment. Although the objective of GSI planning is water quantity and water quality regulation services, the complex interaction of hydrological and ecological processes existing in GSI in the social–ecological system makes the comprehensive consideration of hydrological, ecological, and social benefits indispensable. The key to site selection is to harvest more potential benefits on the basis of achieving planning objectives. Researchers have begun to incorporate GSI into the social–ecological system, considering the interaction of multiple processes of ecology, hydrology, and social economy involved in GSI, with more attention paid to public participation. Therefore, both the construction suitability indicators (slope, elevation, land use type, etc.) and the requirement indicators of hydrological, ecological, and social benefits (runoff coefficient, ecological sensitivity, social sensitivity, etc.) all need to be considered in the evaluation framework. In addition, for the site selection of different types and scales of GSI, the evaluation should be further adjusted in each planning to ensure that the sites of GSI are located in the most suitable and most needed areas.
