What is land subsidence?
Land subsidence is a gradual settling or sudden sinking of the Earth's surface owing to subsurface movement of earth materials. The principal causes are aquifer-system compaction, drainage of organic soils, underground mining, hydrocompaction, natural compaction, sinkholes, and thawing permafrost (National Research Council, 1991). Three distinct processes account for most of the water-related subsidence--compaction of aquifer systems, drainage and subsequent oxidation of organic soils, and dissolution and collapse of susceptible rocks.
How is land subsidence measured?
Measurements of elevations, aquifer-system compaction, and water levels are presented, interpreted, and integrated to improve understanding of the processes responsible for land-surface elevation changes. Elevations, and elevation changes, have been measured using Interferometric Synthetic Aperture Radar (InSAR), Continuous GPS (CGPS) measurements, campaign Global Positioning System (GPS) surveying, and spirit-leveling surveying.
Importance of Monitoring Subsidence
Continued groundwater-level and land-subsidence monitoring in the San Joaquin Valley is warranted because groundwater levels are poised to decline when surface-water deliveries do not meet demand, which may result in additional land subsidence. Even in precipitation record-setting years such as 2010-11, water deliveries fell short of requests in the Central Valley. Therefore, it is likely that groundwater levels will decline in the future. Integrating subsidence, deformation, and water-level measurements-particularly continuous measurements-permits analysis of aquifer-system response, which enables identification of the preconsolidation head and calculation of aquifer-system storage properties. This information could be used to improve numerical models of groundwater flow and aquifer-system compaction, to refine estimates of governing parameters, and to predict potential aquifer-system compaction which could be used to manage water resources while considering land subsidence.Explore the data map!