## 3 Data and Methods  
### Research Methods  
3.1.1. SWAT Model  
The Soil and Water Assessment Tool (SWAT) is a distributed hydrological model for watersheds, created by the United States Department of Agriculture, capable of simulating intricate hydrological processes including surface water, soil water, and groundwater. It is mostly utilized to forecast variations in watershed water quantity and quality under diverse land use and soil type situations [23]. The entire natural basin in the study area can be delineated into several natural sub-basins based on the actual terrain, thereby mitigating the impact of spatio-temporal variations in natural factors on simulation outcomes. Additionally, this approach facilitates the further subdivision of corresponding hydrological units within each sub-basin for the simulation of collaborative change characteristics. The hydrological process of the SWAT model (version10.6) is categorized into the following two phases: road stage and river surface stage [24]. The road stage represents the phase of runoff generation and slope convergence, regulating the influx of water and solutes from each sub-basin to the primary river. The water surface stage refers to the channel confluence stage, encompassing both the main channel confluence and reservoir confluence, and mimics the transport process of water flow and solutes to the basin's total output [25]. The water balance equation derived from the simulation of the hydrological cycle is presented in Equation (1).  
\[SW_{t}=SW_{0}+\sum_{i=1}^{t}\Big{(}R_{day,i}-Q_{surf,i}-E_{a,i}-W_{sep,i}-Q_{ gu,i}\Big{)} \tag{1}\]  
The \(SW_{t}\) in the formula is the final soil water content at the end of the specified period, mm; \(SW_{0}\) is the initial soil water content; \(t\) is the period of calculation, mm; \(R_{day,i}\) is the precipitation on day \(i\); \(Q_{surf,i}\) is the surface runoff for day \(i\), mm; \(E_{a,i}\) is the evaporation for day \(i\), mm; \(W_{sep,i}\) is the permeability of day \(i\), mm; and \(Q_{gu,i}\) is the underground runoff of day \(i\), mm.  
3.1.2. InVEST Model  
The InVEST (Integrated Valuation of Ecosystem Services and Trade-Offs) (version 3.14.2) methodology is mostly utilized for ecosystem evaluation. Owing to the specificity of the study location, the model employs meteorological data, including temperature and precipitation, to illustrate the impact of a cold environment, while modifying the hydrological characteristics of frozen soil to accommodate these alterations. This study utilizes the \"annual water quantity\" module in the model to estimate the water volume for each grid unit, calculated by subtracting actual evapotranspiration from precipitation. The yearly water volume of the basin is approximated [26]. The water production evaluation tool relies on the Budyko hydrothermal coupling balancing theory and yearly mean precipitation data. The principle is illustrated in Equation (2).  
\[Y(x)=(1-\frac{AET(x)}{P(x)})\cdot P(x) \tag{2}\]  
The \(Y(x)\) in Equation (2) is the annual water capacity (mm); \(AET(x)\) is the annual actual evapotranspiration in each grid cell; and \(P(x)\) is the annual precipitation (mm) within the grid cell (mm).  
3.1.3. Groundwater Runoff Modulus Method  
The runoff modulus method calculates the annual groundwater recharge in the research region. The equation for the runoff modulus is as follows:\[Q=M\cdot F\cdot T\cdot 10^{-7} \tag{3}\]  
In Equation (3), \(Q\) is the natural replenishment amount (\(10^{4}\) m\({}^{3}\)); \(M\) is the groundwater runoff modulus (L/(s\(\cdot\)km\({}^{2}\))); \(F\) is the catchment area (km\({}^{2}\)); and \(T\) is the time (s). The formula for calculating the groundwater runoff modulus of \(M\) is as follows:  
\[M_{year}=\frac{W_{year}}{F}\cdot t\cdot 1000 \tag{4}\]  
In Equation (4), \(W_{year}\) is the average underground runoff in the time step (m\({}^{3}\)); F is the catchment area (km\({}^{2}\)); and \(t\) is the time (s)