The Central Valley in California (USA) covers about 52,000 km 2 and is one of the most productive agricultural regions in the world. This agriculture relies heavily on surface-water diversions and groundwater pumpage to meet irrigation water demand. Because the valley is semi-arid and surface-water availability varies substantially, agriculture relies heavily on local groundwater. In the southern two thirds of the valley, the San Joaquin Valley, historic and recent groundwater pumpage has caused significant and extensive drawdowns, aquifer-system compaction and subsidence. During recent drought periods (2007-2009 and 2012-present), groundwater pumping has increased owing to a combination of decreased surface-water availability and land-use changes. Declining groundwater levels, approaching or surpassing historical low levels, have caused accelerated and renewed compaction and subsidence that likely is mostly permanent. The subsidence has caused operational, maintenance, and construction-design problems for water-delivery and floodcontrol canals in the San Joaquin Valley. Planning for the effects of continued subsidence in the area is important for water agencies. As land use, managed aquifer recharge, and surface-water availability continue to vary, long-term groundwater-level and subsidence monitoring and modelling are critical to understanding the dynamics of historical and continued groundwater use resulting in additional water-level and groundwater storage declines, and associated subsidence. Modeling tools such as the Central Valley Hydrologic Model, can be used in the evaluation of management strategies to mitigate adverse impacts due to subsidence while also optimizing water availability. This knowledge will be critical for successful implementation of recent legislation aimed toward sustainable groundwater use.

Nineteen percent of the global population may face a high probability of subsidence

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This report summarizes hydraulic and mechanical properties affecting groundwater flow and aquifer-system compaction in the San Joaquin Valley, a broad alluviated intermontane structural trough that constitutes the southern two-thirds of the Central Valley of California. These values will be used to constrain a coupled groundwater flow and aquifer-system compaction model of the western San Joaquin Valley called WESTSIM. A main objective of the WESTSIM model is to evaluate potential future land subsidence that might occur under conditions in which deliveries of imported surface water for agricultural use are reduced and groundwater pumping is increased. Storage values generally are components of the total aquifer-system storage and include inelastic and elastic skeletal storage values of the aquifers and the aquitards that primarily govern the potential amount of land subsidence. Vertical hydraulic conductivity values generally are for discrete thicknesses of sediments, usually aquitards, that primarily govern the rate of land subsidence. The data were compiled from published sources and include results of aquifer tests, stress-strain analyses of borehole extensometer observations, laboratory consolidation tests, and calibrated models of aquifer-system compaction.

California's Central Valley covers about 52,000 square kilometers (km 2) and is one of the most productive agricultural regions in the world. More than 250 different crops are grown in the broad alluvial filled structural trough, with an estimated value exceeding $20 billion per year (Faunt 2009) (Figure 1). Central Valley agriculture depends on state and federal water systems that divert surface water, predominantly originating from Sierra Nevada snowmelt, to agricultural fields. Because the valley is semi-arid and the availability of surface water varies substantially from year to year, season to season, and from north to south, agriculture, as it grew, developed a reliance on groundwater for irrigation. The extensive withdrawal of groundwater caused water levels to decline on the west side of the southern two-thirds of the Central Valley, also known as the San Joaquin Valley. Long-term groundwater-level declines resulted in a one-time release of "water of compaction" from compacting fine-grained deposits, which caused land subsidence (Galloway et al. 1999). More than half of the thickness of the Central Valley aquifer system is composed of fine-grained sediments, including clays, silts, and sandy or silty clays (Williamson et al. 1989; Faunt 2009), that are susceptible to compaction if depressurized by groundwater pumping. Land subsidence in the Central Valley from groundwater pumping began in the mid-1920s, and by 1970 about half of the San Joaquin Valley, or about 13,500 km 2 , had subsided more than 0.3 meters (m) (Poland et al. 1975). Locally, magnitudes reached 9 m by the early 1980s (Ireland 1986). Partially in response to these groundwater-level declines and associated subsidence, an extensive surface-water delivery system was developed to redistribute some of the water from north to south and east to west. Surface-water imports from the Delta-Mendota Canal (DMC) since the early 1950s and the California Aqueduct since the early 1970s resulted in decreased groundwater pumping in ESSAY