Among the major subsidence areas, the San Joaquin Valley in California, the United States, has been the most noticeable. It has a total of about 9 meters of subsidence between 1925 and 1977, with an area of about 6,200 square kilometers. This area is the largest agricultural production area in the United States. It accounts for 11% of the United States, so the groundwater is extracted mainly for agricultural irrigation. In 1955, about 32 billion cubic meters of groundwater was extracted for agricultural irrigation. The largest drop in groundwater level in history was about 120 meters. Geologically, it belongs to the Central valley aquifer area of the United States. The thickness of the accumulation layer is about 720 meters on average. The source of the sediment is igneous rock and metamorphic quartz sandstone. The content of fine particles in the sedimentary layer, including clay and silt, exceeds 50%. Clay and silt are easily compressible soils. It is easy to cause severe stratum subsidence in the process of position descent (USGS, 2000). In addition, the agricultural industry in the Midwest of Florida in the United States has a large increase in the amount of groundwater pumped under the condition of insufficient surface water. The continuous decline of groundwater level has caused serious ground subsidence; and in parts of Arizona, because of the long-term use of groundwater to irrigate crops, With the rapid increase of groundwater exploitation, the stratum subsidence is serious, and the development of ground fissures intensifies. All in all, there are 45 states in the United States, covering an area of more than 17,000 square miles, which have been threatened by stratum subsidence; 80% of which are due to the relationship between super pumping of groundwater; and the United States continues to use a variety of technologies (such as In addition to GNSS, InSAR) to monitor the state of stratum subsidence in various states, national policies or promotional methods are also used to encourage or control people to reduce the use of groundwater (Gallowway et al., 1999; Knudsen et al., 2014).
Citation: Monitoring and analysis of stratum subsidence in Taipei, Chiayi, Kaohsiung and Pingtung in 109 P4-2 Report from the Water Resources Department of the Ministry of Economic Affairs
Land subsidence in the world