3.3 SWI assessment of study-site based on monthly GALDIT index
For the monthly GALDIT index to which the theoretical weight was applied, the average of observations for 10 years was applied for G, A, and D, whereas the parameters that changed monthly were applied for L, I, and T. The colors of the histogram in Figure 11 express the range of the calculated GALDIT index. The y-axis represents the cumulative ratio of the area according to the index range, the secondary y-axis represents the GALDIT index, and the value indicated by the broken line represents the average GALDIT index of the entire study area. The score range that occupied the highest proportion was 2–3, which accounted for 78 % in February, 78 % in May, and 78 % in July. The calculation results show that the most vulnerable month was September, when the average GALDIT index of the study area was 3.03. The ratio of areas with scores ≥ 5 was highest in September (8.87 %), followed by October (8.64 %), and April (8.63 %). In September, L was relatively robust at 1.19, as the third place from the bottom. However, it was the most vulnerable month in terms of I and the third from the top in terms of T. Thus, it was the most vulnerable month when the theoretical weight was applied. Figure 12 (a) shows the calculation result of the original GALDIT using the quartile method, and Figure 12 (b) shows the result of September, which is the most vulnerable month in the monthly GALDIT results using the decile method. In Figure 12 (a), the vulnerable areas are the coastal area in the south of Incheon and the areas near Incheon and Siheung, where the index value is approximately 8–9. Upon comparison of the monthly GALDIT map for September in Figure 12 (b) with the original, the index of the Gimpo area indicated in orange in the northeastern side of the study area is 5–6 in most periods, exhibiting moderate vulnerability. The western coast area of Gimpo, which does not appear in the original GALDIT, shows an index value of 7–8, indicating the boundary between moderate and high vulnerability. The SWI vulnerable area on the southern coast of Incheon has an index value is 6–7 at both edges around the area protruding to the coast, showing a different pattern from the original GALDIT map. In the Pyeongtaek area, the eastern inland area exhibited low vulnerability. However, in the monthly GALDIT map, the yellow parts showed index values of 3–5, revealing differences in the seawater intrusion vulnerability. Most of the areas excluding the coast showed little differences in the degree; however, in the monthly GALDIT map, they showed partial differences, indicating low vulnerability. In the entire study area, the inland areas up to 1 km away from the coastline are considered areas of moderate to high vulnerability.