Water in an aquifer is under tremendous pressure from the weight of soil and water above it. When an aquifer is over-pumped, the water that was supporting the soil above it is removed, and the structural integrity of the aquifer is reduced. Without water pressure to support it, the land surface begins to settle and compress in a process called subsidence. When an aquifer collapses, the pore spaces that once held water are eliminated, meaning that the storage capacity of that aquifer is lost forever. Subsidence can appear as a small local collapse or as broad regional lowering of the land's surface height (USGS, 1995).
Figure 3.1 illustrates the dramatic affect land subsidence can have. This photo, taken in California, shows the position of the land surface in 1925, 1955, and 1977. In 1977 the region's farmers stopped using groundwater, and instead switched to surface water. However, during a drought between 1987 and 1992 farmers again began using groundwater and again the land surface began to drop (Bertoldi and Leake, 1993).
Land subsidence causes substantial damage to structures such as buildings, roads, and buried pipes. Additionally, subsidence can disrupt water conveyance structures, leading to poor drainage and possible flooding. The National Research Council estimates the annual costs due to increased flooding and structural damage from land subsidence to be in excess of $125 million. This total does not include the estimated loss of property value due to subsidence or related increases in farm operating costs. According to USGS a more realistic estimate would be closer to $400 million a year (Bertoldi and Leake, 1993).