Human-induced subsidence probably began in the middle 1800's when peat soils in marshes of the Sacramento--San Joaquin Delta were first drained for cultivation. In the delta, shallow ground water is drained into ditches to dry the fields before planting and then pumped from the ditches into nearby natural channels. During the growing season, water is siphoned back into the drainage ditches to raise the water table to the root zone. When the peat soils are drained and exposed to the atmosphere they oxidize and compact, and (or) are reduced in thickness by wind erosion; thus, the land surface is permanently lowered with each yearly cycle. To dry the fields each year, the water table must be lowered below that of the previous year, which requires an increase in withdrawals and a decrease in the volume of ground water in storage. Subsidence due to oxidation and compaction of peat soils has lowered the land surface in the delta as much as 6 to 15 feet.
Hydrocompaction is caused when formerly unsaturated soils become saturated, which allows the soil particles to reorient into a more compact form. Irrigation of clayey alluvial-fan soils has resulted in hydrocompaction and subsidence of 3 to 15 feet on the western and southern margins of the San Joaquin Valley (fig. 93). Soils in many areas crossed by the California Aqueduct were intentionally hydrocompacted before aqueduct construction to avoid subsidence problems. Subsequent subsidence due to hydrocompaction in these areas has been minimal.
The primary cause of land subsidence in the Sacramento and the San Joaquin Valleys has been the compaction of fine-grained sediments (predominantly clay) in the aquifer system following severe, long-term withdrawal of ground water in excess of recharge (fig. 93). The amount of such subsidence in an area is related to the amount of withdrawal and the percentage of the withdrawal zone composed of clay beds. Compaction occurs when the hydraulic head in the confined parts of the aquifer system is lowered, thus reducing the hydraulic head in the clay beds, which, in turn, reduces the pore pressure in the clay. The weight of overlying sediments compacts the clay and squeezes water out of the clay until equilibrium is reached with the pore pressure in the clay. Compaction seems to happen more readily when the wells are open only to the confined part of the aquifer system than when they are open to the shallow water-table aquifer as well. When ground water is withdrawn from above and below the confining units, head differential is less between the shallow and deep aquifers and reduction in pore pressure in the clay is less.
Subsidence due to compaction of fine-grained sediments began in the San Joaquin Valley in the 1920's and in the Sacramento Valley in the 1950's. The area most affected has been in the southern and western parts of the San Joaquin Valley (fig. 93). Approximately one-half of the valley, or about 5,200 square miles, had subsided at least 1 foot by 1977; the total volume of subsidence was greater than 17 million acre-feet. The land surface declined nearly 30 feet from the 1920's to the late 1970's in an area southwest of Mendota (fig. 94). Importation of surface water and reduction in ground-water withdrawals during the 1970's slowed or stopped the decline of ground-water levels. In many cases, this allowed recovery to pre-1960's water levels and prevented further land subsidence.
Compaction and declining water levels are directly related (fig. 95). As water levels declined severely during the 1960's, fine-grained sediments lost water from pore spaces and became compacted. When withdrawal rates decreased and water levels were allowed to recover, compaction rates slowed significantly. Increased withdrawals during the 1976--77 drought caused additional subsidence, some of which was the result of compaction of coarse-grained sediments. When water levels recovered, the fine-grained sediments remained compacted; however, the land surface rebounded in 1978 because the compacted coarse-grained sediments regained some of their original volume when the former or near former pore pressure was attained.
The three areas most affected by subsidence are, in order of total volume of compaction, the Los Banos--Kettleman City, the Tulare--Wasco, and the Arvin--Maricopa areas (fig. 96). The direct relation between withdrawals and subsidence in the Los Banos--Kettleman City area is apparent in figure 97. Clay has a high percentage of pore space and, when compressed, can yield large amounts of water relative to its volume. Approximately one-third of the withdrawals in the Los Banos--Kettleman City area were derived from ground water released from clay beds when the clay was compacted.
Because compaction of the clay is permanent, the volume of storage lost because of compaction will not be regained, except for the small amount that was the result of the compaction of coarse-grained aquifer materials. Thus, the land surface cannot rebound to any significant degree, but when the demand for ground water increases because of drought, water levels can be lowered nearly to those of the late 1970's without the threat of additional land subsidence. For example, during the 1976--77 drought, ground-water withdrawals increased markedly, and water levels declined rapidly. However, from 1968 to 1976, water levels had risen to such an extent that hydraulic heads remained above the extremely low levels of the 1960's. Compaction occurred only in the sand and gravel and was relatively insignificant and, to a degree, reversible (fig. 95).