Correlation between groundwater level and altitude variations in land subsidence area of the Choshuichi Alluvial Fan, Taiwan

One of the prominent features observed in the groundwater-overdraft area is the land subsidence that creates a permanent damage to the land surface. Severe land subsidence has occurred in the western Taiwan due to excessive extraction of groundwater in the past decades. The areas of land subsidence have been gradually expanded and caused many problems such as flooding and building damage. This study examines the elevation changes on surface, which is the vertical component from the Global Positioning System in the Choshuichi Alluvial Fan of central Taiwan and describes the land-subsidence phenomena quantitatively. The Choshuichi Alluvial Fan has a high density array of wells monitoring the groundwater level variations of various aquifers automatically in an hourly basis. Through an assumed linear relationship, the inferred results show that the Global Positioning System data are closely correlated to the groundwater level variations of deep aquifers. The highest correlations between elevation changes and groundwater level variations have been found in the land subsidence area, even if a drought period is present. The responses on the surface vertical displacements from the groundwater level changes of deep aquifers can be quantitatively estimated. Furthermore, the long term subsidence trends can be derived and be used as useful reference for land and water resources management.