Over-pumping groundwater has been leading to serious problems in the western coast of Taiwan, especially for Yunlin area. Yunlin has the most extensive land subsidence area, and the subsidence rate ranks the second place in Taiwan. Land subsidence has not only caused the supply and demand of groundwater unbalanced but also affected geo-engineering constructions of the country.
The purpose of this thesis is to study the amount of land subsidence in Yunlin area. The numerical simulation groundwater flow software MODFLOW is applied to construct numerical models of groundwater flow at Chou-Shui alluvial fan. By setting hydraulic parameters, the combined hydrological model with the code (Interbed Storage Package) can be built the model of land subsidence in Yunlin. Then, we evaluate the difference in amount of land subsidence under three types of water level observation data: average water level of the month (model 1), the highest water level of the month (model 2), and the lowest water level of the month (model 3). Finally, the difference between collected data and the simulation results of the land subsidence is compared.
The comparison between field measurement data and simulation results with three models indicates that the trend of simulation results of the land subsidence corresponds to the filed measurement of land subsidence. First, at subsidence stations in inland area of Yunlin, such as Yuan-Chang elementary school, Hu-Wei elementary school, and Ke-Tsuo elementary school, the simulation results of subsidence in model 1 had more accurate results of subsidence compared with field measurement. Next, the subsidence stations, like Jian-Yang elementary school and Jin-Hu elementary school of southern coast, the simulation results of subsidence in model 2 had better correction results of subsidence. Last, the subsidence stations of northern coast area, for example Lun-Feng elementary school, Shin-Shing elementary school, Lung-Yan elementary school, and Feng-An elementary school, the simulation results of subsidence in model 3 also showed better correction of subsidence.