Spatial Variability in Land Subsidence and Its Relation to Groundwater Withdrawals in the Choshui Delta

In this article, the characterization of land subsidence with the spatial variability of soil formation and groundwater withdrawals in Choshui delta, Taiwan, is presented. Levelling surveys, borehole logging, multi-layer compaction monitoring network, multi-layer groundwater level monitoring network, and the electricity consumption of wells in the study area are adopted. Various factors, including the percentage of coarse-grained soil, percentage of fine-grained soil, length of average maximum drainage path, percentage of agricultural land use, electricity consumption of wells, and accumulated depth of land subsidence, are applied. Thematic maps based on these factors are established using geographic information system spatial analysis. A principal component analysis (PCA) is then employed to obtain the dominant factors for land subsidence. The results indicate that the largest subsidence rate is observed in the region that has both a high electricity consumption of wells and a large percentage of fine-grained soil. The PCA results reveal that the electricity consumption of wells is highly correlated with the accumulated depth of land subsidence. The first principal component is the acquired factor causing land subsidence, such as the excessive withdrawal of groundwater. The second principal component is the congenital factor causing land subsidence, which corresponds to fine sand, silty and clayey soils.