Sneed and Brandt (USGS, “Land Subsidence in the San Joaquin Valley, California, USA 2007–2014”, unpublished paper; Ninth International Symposium on Land Subsidence, Nagoya, Japan, 15–19 Nov 2015) used interferometric synthetic aperture radar (InSAR), continuous global positioning systems (CGPS), and extensometer data to determine the location, extent, and magnitude of subsidence in select areas of the San Joaquin Valley. Many of the details from their analyses are included in this section. Analysis of interferograms generated from synthetic aperture radar images from the European Space Agency’s ENVISAT satellite and the Japan Aerospace Exploration Agency’s ALOS satellite acquired between 2008 and 2010 indicated 50–540 mm of subsidence in two large agriculturally dominated areas in the San Joaquin Valley. One area is centered near the town of El Nido (2,100 km2) and the other near the town of Pixley (5,500 km2; Fig. 5). The period 2008–2010 is shown in Fig. 5 because suitable InSAR data were not available for 2010–2014. As a result, CGPS data collected during 2007–2015 were used to generate subsidence time series. CGPS confirmed the InSAR-derived rates and generally indicated that these rates persisted or accelerated through 2015 (Fig. 6). The CVHM also simulated these persistent and in some areas accelerating rates through 2014.
To help explain the variability in location and magnitude of subsidence, computed subsidence was compared with local geology information and water-level measurements retrieved from USGS and California Department of Water Resources databases (Figs. 5 and 6a). Simulated subsidence from CVHM was used to further evaluate the spatial extent and magnitude of subsidence during the period 1962–2014 (Fig. 7).
The magnitude and rate of subsidence varies based on the hydraulic and mechanical properties of the saturated geologic materials constituting the aquifer system and on the consolidation history of the aquifer system. Therefore, the extent of subsidence was compared with the extent of fluvial fans from the Sierra Nevada (Weissmann et al. 2005; Figs. 1 and 5) and also to groundwater levels (Fig. 6a). In general, valley deposits sourced from the Coast Ranges and the non-glaciated fluvial fan deposits sourced from the Sierra Nevada are finer grained and more compressible than the coarser-grained sediments, resulting in greater subsidence under equivalent applied stresses (declining groundwater levels). Conversely, the upper reaches of the large drainage area glaciated fluvial fans that are relatively coarser grained and have much lower rates of subsidence (Fig. 5). Following the theory of aquifer-system compaction embodied in the aquitard drainage model (Galloway and Burbey 2011), the consolidation history of an aquifer system establishes the preconsolidation stress, which is often represented by the previous lowest groundwater level (highest effective or intergranular stress). The relation of current groundwater levels to the previous lowest water level controls whether subsidence is inelastic (permanent) or elastic (recoverable). Permanent subsidence occurs as a result of rearrangement of fine-grained materials when the preconsolidation stress is exceeded (current water levels lower than historical lows); whereas recoverable (elastic) subsidence occurs when the preconsolidation stress is not exceeded (current water levels higher than historical lows).
Since spring 2008, groundwater levels are at all-time historical lows (for period of record) in most areas of the southern San Joaquin Valley and portions of the Sacramento Valley. These areas exhibit groundwater levels more than 15 m below previous historical lows experienced sometime prior to 2000. There are many areas of the San Joaquin Valley where recent groundwater levels are more than 30 m below previous historical lows and correspond to areas of recent subsidence. According to the California Department of Water Resources (2014), groundwater levels in 55 % of the long-term wells (1,718 of 3,124) in the San Joaquin Valley and 36 % of the long-term wells (216 of 599) in the Sacramento Valley are at or below the historical spring low levels. Groundwater levels declined during these periods in response to increased pumping, approaching or surpassing historical low levels. As groundwater levels dropped, subsidence occurred in many areas.
The large recently subsiding areas in the San Joaquin Valley include areas that subsided historically; however, some of the areas of maximum subsidence have changed. The Tulare-Wasco area (Figs. 5 and 7) had substantial subsidence both historically and recently, the Los Banos-Kettleman City area (Figs. 5 and 7) has substantially less subsidence recently compared to historically—where maximum subsidence during 1926–1970 was about 4 m (Ireland 1986)—and the El Nido area has substantially more subsidence recently compared to historically (Ireland 1986; Sneed et al. 2013; Farr et al. 2015). More than 120 mm of subsidence occurred over a large part of the southern subsiding area during 2007–2010 (Fig. 5). In some places nearly 900 mm of subsidence occurred during this period. The maximum rate of recent subsidence (250 mm/year) is about twice the maximum rate that occurred historically in the area (200 mm/year).
The largest subsidence magnitude in the San Joaquin Valley during 2007–2014 was measured and simulated near El Nido (Figs. 5 and 7). The interferograms are the only measurements that captured the maximum magnitudes of subsidence because the CGPS stations and extensometers are located on the periphery of the most rapidly subsiding area (Fig. 5). The interferograms indicated a local maximum of about 540 mm during January 2008–January 2010, or 270 mm/year, which is among the highest rates ever measured in the San Joaquin Valley. The subsidence measured at nearby CGPS station P303 was about 50 mm during the same time period, indicating a large subsidence gradient between the two locations (Figs. 5 and 6b). The years 2010–2012 was a non-drought period but a continued high rate of subsidence occurred during this period near El Nido (Fig. 6b). Much of the area where subsidence is occurring near El Nido has little access to surface water for irrigation supplies regardless of climate conditions. This fact, coupled with changing land use explains the continued high rate of subsidence. Residual (delayed) compaction due to the slow equilibration of fluid pressures in relatively thick, interbedded, low-permeability fine-grained units in the aquifer system also may be a factor. Vertical displacement at P304 indicates that most subsidence occurred during drought periods and very little occurred between drought periods (Fig. 6a). This area received surface water, when it was available between drought periods. The cessation of subsidence between drought periods, when water levels recovered, indicates that residual compaction was not very important in this area. Assuming the same rate of subsidence occurred during 2007–2014 as occurred during 2008–2010 at the local subsidence maximum near El Nido, about 2 m of subsidence may have occurred during 2007–2014. The CVHM simulates slightly more than 2 m of subsidence in this area (Fig. 7).
In parts of the El Nido subsidence area, where the planting of permanent crops has increased, groundwater was either the primary source of water or groundwater pumping increased when surface-water availability was reduced, and groundwater levels declined to near or below historical lows during 2007–2010 and 2012–2014. The area with the highest rate of subsidence is correlated with rates of groundwater extraction where groundwater is used to irrigate (year-round) permanent crops (vineyards and orchards) that are replacing non-permanent land uses such as rangeland, field crops, or row crops (USDA 2000–2013; Fig. 3). The correlation between high rates of subsidence and water levels near or below historical lows indicates that the preconsolidation stress was exceeded and the subsidence is mostly permanent near El Nido.
The Pixley subsidence area is really more extensive than the El Nido subsidence area, but subsided at a lower rate during 2007–2014. Similar to the El Nido area, the interferograms provided the only measurements that captured the maximum subsidence magnitudes because the CGPS stations and extensometers are located on the edges of the most rapidly subsiding area (Fig. 5). The interferograms indicated a maximum subsidence of about 180 mm during January 2008–January 2010 (Fig. 5). If it is assumed that the rate of subsidence during 2007–2014 was equivalent to the rate during 2008–2010 at the local maximum near Pixley, about 0.7 m of subsidence may have occurred there during 2007–2014. Published subsidence rates during 2007–2010 ranged from about 0.2 to 0.25 m/year (Farr and Liu 2015; Farr et al. 2015), which are smaller than the 0.34 m/year rates, described as preliminary by LSCE et al. 2014). Farr et al. (2015) utilized InSAR to estimate subsidence rates in the Central Valley between May 2014 and January 2015, i.e., in the third year of California’s ongoing severe drought. They measured as much as 0.35 m of subsidence near the local maximum subsidence area near Pixley for the 8-month period (equivalent to a rate of about 0.5 m/year).
Data from the four CGPS stations and two extensometers near the periphery of the Pixley subsidence area show seasonally variable subsidence rates, with different interannual characteristics. Vertical displacement at P564 and P565 indicated that most subsidence occurred during drought periods and very little occurred between drought periods. This suggests that this area received other sources of water, most likely surface water, when it was available between drought periods, and also that residual compaction was not very important in this area. Vertical displacement at P056 and P566 indicated subsidence at fairly consistent rates during and between drought periods. These fairly consistent subsidence rates are in areas with limited surface water availability and where groundwater is the primary water source. CGPS and extensometer data indicated an increased subsidence rate during 2014, the third year of drought. In the Pixley area, groundwater pumping continued or increased when surface-water availability was reduced, and groundwater levels declined to near or below historical lows during 2007–2010 and 2012–2014. Similar to the El Nido area, because the high rates of subsidence in the Pixley area are correlated with groundwater levels near or below historical lows, the subsidence is interpreted to be mostly permanent. Similar subsidence magnitudes for these periods are simulated by CVHM but the spatial patterns are somewhat different. These differences are attributed in part to the grouping of farms and agencies accepting surface-water deliveries and calculating demand for the water accounting in this part of the CVHM.
Groundwater pumping has resulted in subsidence which has caused damage to infrastructure in the San Joaquin Valley. Bridges, roads, buried irrigation pipelines, land leveling of fields, and wells have been altered and/or damaged by subsidence in the San Joaquin Valley (Sneed et al. 2013). In particular, serious operational, maintenance, and construction-design problems for the California Aqueduct, the Delta-Mendota Canal, the Outside Canal, and other regional and local water-delivery and flood-control structures have been documented (Sneed et al. 2013; LSCE et al. 2014). Costs to address damage to surface-water conveyance infrastructure are estimated at more than $1.3 billion (2013 dollars) during 1955–1972; cost estimates for subsidence-related damages incurred in subsequent years are unavailable (LSCE et al. 2014).