Groundwater storage variations were calculated using TWS anomalies for the Sacramento, San Joaquin and Tulare basins (Fig. 2(A)) and subtracting the anomalies of soil moisture, surface water, and SWE (Fig. 2(B)) (see Material and Methods). Figure 2(C) shows observations of precipitation (P), evapotranspiration (ET) and stream discharge (Q) for the river basins, while Fig. 2(D) shows a close correspondence between dS/dt derived from GRACE/FO, and that computed using P – ET – Q in Eq. 1. The good agreement between GRACE/FO-derived and observed dS/dt demonstrates that GRACE/FO is capable of accurately monitoring basin-wide water balance changes, and provides further confidence in the groundwater estimates described below (12). Figure 3(A) shows the monthly groundwater storage anomalies derived from GRACE/FO and the datasets shown in Fig. 2(A and B) in the Central Valley between September 2003 and December 2021. Figure 3(A) shows that three notable periods of groundwater recharge and loss were identified in the past 18 years. Recharge periods are from October 2003 to July 2006, March 2011 to July 2011, and October 2018 to August 2019. Groundwater loss phases correspond to the well-known droughts that occurred during that time period, namely August 2006 to February 2011, August 2011 to March 2017, and since September 2019. Estimated rates of groundwater gains and losses are summarized in Table 1.
Table 1
Groundwater change rate and total volume changes in Central Valley. The signs of – and + indicate groundwater losses and gains, respectively.
| Change Rate (mm/yr) | Change Rate (km3/yr) | Volume Change (km3) |
Oct. 2003 – Jul. 2006 | + 22.8 ± 16.0 | + 3.50 ± 2.46 | + 9.9 ± 4.1 |
Aug. 2006 – Feb. 2011 | -42.9 ± 7.8 | -6.59 ± 1.20 | -30.2 ± 2.6 |
Mar. 2011 – Jul. 2011 | + 257.7 ± 20.3 | + 39.60 ± 3.12 | + 16.5 ± 2.0 |
Aug. 2011 – Mar. 2017 | -42.6 ± 5.8 | -6.55 ± 0.89 | -37.1 ± 2.1 |
Oct. 2018 – Aug. 2019 | + 189.4 ± 108.2 | + 29.10 ± 16.63 | + 25.8 ± 15.9 |
Sep. 2019 – Dec. 2021 | -55.8 ± 21.8 | -8.57 ± 3.35 | -20.0 ± 5.1 |
Sep. 2003 – Aug. 2017 (Lifetime of GRACE) | -19.4 ± 2.1 | -2.98 ± 0.32 | -41.7 ± 1.2 |
Sep. 2003 – Dec. 2021 | -15.7 ± 1.4 | -2.41 ± 0.22 | -44.2 ± 0.9 |
1962–2021 | -12.3 ± 0.8 | -1.89 ± 0.12 | -113.4 ± 0.9 |
A period of groundwater recharge (22.8 ± 16.0 mm/yr; 3.50 ± 2.6 km3/yr) in the Central Valley was observed at the beginning of the GRACE mission during 2003–2006, when the precipitation amounts were close to or slightly higher than the 20-year average. The NOAA National Weather Service report (41) reveals that weak to moderate levels of El Niño events during 2004–2006 resulted in nearly normal amounts of precipitation and snow in the study region. A volume of 9.9 ± 4.1 km3of groundwater was replenished during this phase of the analysis.
This period of groundwater storage increase was followed by the 4.5 year long drought that began in August 2006. During the 2006–2011 drought, a groundwater loss rate of 42.9 ± 7.8 mm/yr (6.59 ± 1.20 km3/yr) was estimated, resulting in 30.2 ± 2.6 km3 of groundwater loss during that period. Compared with the earlier analysis in (12), an additional year of data was included here, and represented the complete drought phase through 2011, rather than through 2010, as in (12). Although the groundwater loss rate is slightly higher than the 38.9 ± 9.5 mm/yr reported in (12), the difference falls within the 95% confidence interval, confirming the consistency between the two analyses. The analysis was also conducted in each of the three basins in Fig. 1, as shown in Fig. S1 and Table S1 in Supplementary Materials. It shows that the Sacramento, San Joaquin, and Tulare basins all experienced similar groundwater loss rates of ~ 42 mm/yr (40–44 mm/yr).
Prior to the second drought, a short, rapid recharge phase (March - July 2011) replenished 16.5 ± 2.0 km3 of groundwater (257.7 ± 20.3 mm/yr; 39.60 ± 3.12 km3/yr), as a result of the strong El Niño in 2010 that brought abundant precipitation in early 2011 (42).
The groundwater loss rate for the second phase of drought in the GRACE/FO record (2011–2017) was − 42.6 ± 5.8 mm/yr (6.55 ± 0.89 km3). Although a similar groundwater loss rate was estimated for the drought of 2006–2011, the second drought lasted a year longer, resulting in roughly 7 km3 more groundwater loss (-37.1 ± 2.1 km3 total), equivalent to about 23% of surface water storage in the Central Valley, and greater than the volume of Lake Mead (35 km3) at full capacity. The GRACE/FO-based groundwater storage changes estimated in this study reached an 18-year low by late 2016. This phase of drought was notable for widespread water conservation efforts across California, and for the passage of SGMA in 2014. The drought ended with atmospheric river events that brought heavy precipitation to California in early 2017 (43). Table S1 shows that during this period, the Tulare basin suffered more severe groundwater losses than the other basins, with a loss rate of 62.6 ± 4.4 mm/yr (-2.66 ± 0.18 km3/yr). The total groundwater loss in the Tulare basin was 15.1 ± 0.4 km3, which was nearly 40% of the total loss in Central Valley, yet the area of the Tulare basin only occupies about one quarter of the study region.
The original GRACE mission was decommissioned in late 2017 and transitioned to GRACE-FO after its launch in May 2018. Hence there is year-long data gap in the combined GRACE/FO record from August 2017 – September. 2018. Studies of that time period (19, 27) suggest that groundwater recharge occurred during this data gap. We estimate that during the lifetime of original GRACE mission (2003–2017), 41.7 ± 1.2 km3 of groundwater were lost (Table 1).
We assume that the groundwater depletion followed the 18-year historical trend (2003–2021), but made no assumption about its seasonal dynamics during the data gap between the GRACE and GRACE-FO missions. From October 2018 to August 2019 we estimated that groundwater storage increased by 25.8 ± 15.9 km3 (189.4 ± 108.2 mm/yr; 29.10 ± 16.63 km3/yr).
The third phase of drought in the GRACE/FO record began in September 2019. After the recharge event in the winter of 2018, the major water inputs in the region, including SWE and precipitation, significantly decreased in winters of 2019 and 2020 (Fig. 2 (B and C)). These two winters rank the years 2019 and 2020 as fourth driest consecutive 2-year period on record (44). In particular, precipitation reached an 18 year low in the winter of 2020 (Fig. 2(C)), and TWS shows this same time period as the driest wet season in the GRACE/FO record. Between September 2019 and December 2021, total groundwater losses in the Central Valley were 20.0 ± 5.1 km3 (55.8 ± 21.8 mm/yr; 8.57 ± 3.35 km3/yr), which is roughly31 % faster than the previous two droughts.
Overall (2003–2021), groundwater storage changes observed from GRACE/FO in the Central Valley show a trend of groundwater depletion of 15.7 ± 1.4 mm/yr (2.41 ± 0.22 km3/yr), resulting in a total groundwater loss of 44.2 ± 0.9 km3, an amount that is greater than 1.25 times the capacity of Lake Mead. Figure S1 and Table S1 show that the depletion rates in the Sacramento, San Joaquin, and Tulare basins, were 12.9 ± 1.8, 16.2 ± 2.0, and 20.6 ± 1.5 mm/yr (0.89 ± 0.13, 0.67 ± 0.09, and 0.85 ± 0.07 km3/yr), respectively, indicating that the southern Central Valley (combined San Joaquin and Tulare) lost more groundwater than the north, similar to the findings of earlier studies (12, 23). However, the situation was reversed in the drought that began in September 2019, during which we found higher groundwater loss rates of 76.1 ± 28.1 mm/yr (5.48 ± 2.02 km3/yr) in the Sacramento basin compared to those of 38.1 ± 25.2 and 60.1 ±14.0 mm/yr (1.55 ± 1.03 and 2.57 ± 0.60 km3/yr) for the San Joaquin and Tulare basins, respectively.
Longer-term trends and comparison to observations. The dynamics of GRACE/FO groundwater estimates were compared with water table depth anomalies observed from groundwater wells, as shown in Fig. 3(B). We removed seasonal variations using climatology to avoid seasonal inconsistencies and to examine long term trends. Overall, the dynamics of the two measurements demonstrate similar trends from 2003 to 2021. While there is a greater difference between the well and GRACE/FO estimates following 2017, Fig. 3 (B) shows that the groundwater estimates using GRACE/FO are capable of capturing the periods of loss and recovery observed on the ground, and in particular, the greater rate of groundwater loss since 2019, which appears even stronger in the observations than in the GRACE/FO estimates.
Figure 3(C) shows cumulative groundwater losses from 1962–2021 using the CVHM (13) and GRACE/FO. From 2003 to 2014 when both CVHM and GRACE data were available, the groundwater depletion rate for the CVHM was 17.3 ± 6.2 mm/yr (2.66 ± 0.95 km3), matching that from GRACE, 16.8 ± 5.9 mm/yr (2.58 ± 0.90 km3), indicating that the two methods are compatible and may be combined for the further analysis. The combined CVHM-GRACE/FO groundwater depletion rate from 1962 to 2021 was 12.3 ± 0.8 mm/yr (1.89 ± 0.12 km3/yr), resulting in a total groundwater loss of 113.4 ± 0.9 km3. In addition, Fig. 3(C) shows that the periods for groundwater recovery were shorter, and mostly driven by extreme weather events (42–43, 45) in the nearly two decades of the GRACE/FO record. Although groundwater was recharged, these extreme wet events typically generated flooding, and had significant negative social, environmental and economic consequences. This sequence of extreme hydrological events - long-term extremely dry conditions with considerable groundwater losses, punctuated by short-term extremely wet conditions with short bursts of groundwater recharge - underscores the challenge of sustainable groundwater management under changing climate.
Figure 3(A) and Table 1 show that the rate of groundwater loss is accelerating in the Central Valley. Groundwater loss rates observed from GRACE/FO (15.7 ± 1.4 mm/yr; 2.41 ± 0.22 km3/yr) between 2003 and 2021 are 28% faster than the longer-term (1962–2021) depletion rate of the combined CVHM-GRACE/FO record (12.3 ± 0.8 mm/yr; 1.89 ± 0.12 km3/yr). The most recent phase of groundwater loss, between September 2019 and August 2021 (55.8 ± 21.8 mm/yr; 8.57 ± 3.35 km3/yr), is nearly 31% faster than GRACE/FO estimated losses the previous two drought phases during the GRACE/FO record, and nearly 5 times faster than the long-term depletion rate.
Surface water allocations and groundwater use. Figure 4 compares GRACE/FO estimated groundwater storage changes to annual surface water allocations via the two primary aqueducts in the Central Valley, the California State Water Project (SWP) (46) and the federal Central Valley Water Project (CVP) (47). The two aqueducts transport surface water from northern California to the south. When surface water is abundant, greater allocations are made to farmers, relieving stress on groundwater and allowing for recovery, and vice versa. Between 2003 and 2007, surface water storage was increasing (Fig. 2(B)), allowing for larger allocations (> 60%) from both aqueducts, less reliance on groundwater, and hence increasing groundwater storage. Surface water storage and then allocations decreased between 2007 and 2009, resulting in significant groundwater storage decline.
The second drought in the GRACE/FO record began in August 2011, triggering decreasing surface water allocations that resulted in heavy groundwater demand. During this period, CVP cut its allocation to 0% in 2014 and 2015, while the SWP reached its historic low allocation of 5% in 2015 and 2016. These led to intensified groundwater pumping through 2016.
Groundwater storage variations continued to reflect surface water allocations, increasing in 2017 and 2019 with above-average surface water storage, followed by major losses in both surface water allocations, and groundwater storage, through the end of 2021. For example, in 2020, aqueduct allocations decreased to 20% for both projects, and to 0% and 10% in 2021 for the CVP and SWP, explaining in part the increased rate of groundwater loss during this time period.