## 2 Materials and Methods
### Data Preparation and Geospatial Layer
#### 2.2.1 Percentage of Coarse-Grained Soil  
Based on the unified soil classification system, coarse-grained soils are gravelly and sandy in nature with less than 50% passing through the No. 200 sieve [29]. As listed in Table 2, two groups, the coarse-grained soils and the fine-grained soils, were classified from the WRA [26]. The coarse-grained soils contain three types of soils, namely gravel, coarse sand, and medium sand. The percentage of coarse-grained soil is the ratio of the thickness of gravel, coarse sand, and medium sand to the total drilling depth.  
\begin{table}
\begin{tabular}{c c c} \hline
**No.** & **Factors** & **Source Data** \\ \hline
1 & Percentage of coarse-grained soil & \\
2 & Percentage of fine-grained soil & \\
3 & Length of average maximum drainage path & \\
4 & Percentage of agricultural land use & \\
5 & Electricity consumption of wells & \\
6 & Accumulated subsidence depth & \\ \hline \end{tabular}
\end{table}
Table 1: The source data used in this study [26; 27; 28].  
Figure 1: Administration map of Yunlin County, Taiwan.  
Figure 2: _Cont._  
Figure 2: Cont.  
Figure 2: Thematic maps of Yürlin County, Taiwan, and relevant factors: (**a**) percentage of fine-grained soil; (**b**) length of average maximum drainage path; (**c**) percentage of agricultural land use; (**d**) distribution of the managed wells; (**e**) electricity consumption of wells; and (**f**) accumulated subsidence depth from 1992 to 2020.  
Using 75 borehole logging data from WRA and the CGS, we established the geological histogram, as demonstrated in Figure 3. Figure 3a illustrate examples of borehole logging data from WRA. Based on the soil classification as listed in Table 2, we classified the borehole logging data into six different soil type, as depicted in Figure 3b. The soil composition of the geological histogram was then used to classify and calculate the proportion of coarse-grained and fine-grained soils, as shown in Figure 3c. The coarse-grained soils were categorized as the aquifer. On the other hand, the fine-grained soils corresponded to the aquitard.  
To evaluate the percentage of coarse-grained soil [29], we utilized the following equation.  
\[S_{c}=\frac{H_{c}}{H_{t}}, \tag{1}\]  
where \(S_{c}\) denotes the percentage of coarse-grained soil, \(H_{c}\) denotes the soil thickness of coarse-grained soil, and \(H_{t}\) denotes the total drilling depth.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Soil Properties** & **Soil Type** & **Classification** & **Main Type** \\ \hline Gravel and coarse gravel & Gravel & 1 & \\ Grey coarse sand and coarse sand & Coarse sand & 2 & \\ Coarse to medium sand and medium sand & Medium sand & 3 & Coarse-grained soil (Aquifer) \\ Grey silty sand, grey fine sand, blue-gray silty & Fine sand & 4 & \\ sand, and fine to very fine sand & & & \\ Mud, muddy sand, sandy mud, & & & \\ silty sand, surface soil, silt, backfill sand, & & 5 & Fine-grained soil (Aquitard) \\ brown-gray sandy silt, and grey sandy silt & & & \\ Grey silty clay, grey clay, and clay & Clay & 6 & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Soil classification from the Water Resources Agency [26].  
Figure 3: Examples of borehole logging data: (**a**) Water Resources Agency; (**b**) this study (six soil type); (**c**) this study (two main type).