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Based on 29 climate projections, we find that both the sign and magnitude of potential changes in naturalized streamflow of the Nile in 2021–2050 are highly uncertain. These uncertainties spark the need for an adaptive and cooperative approach. We show that cooperative adaptive management of the GERD yields compromise solutions with economy-wide benefits to Ethiopia, Sudan and Egypt compared with a proposal discussed in Washington, D.C. in 2020 (Fig. 1). Under an example compromise solution (Fig. 1), the mean (based on 29 projections) discounted (at 3%) real gross domestic product (GDP) increases by US$0.77, 0.67 and 0.18 billion in 2020–2045 for Ethiopia, Sudan and Egypt, respectively, relative to the Washington draft proposal. These benefits are more pronounced under extreme climate scenarios, with rises in discounted real GDP of up to US$15.8, 6.3 and 3.0 billion over 2020–2045 for Ethiopia, Sudan and Egypt, respectively. Our results should be complemented by evaluating the impacts on ecology, groundwater and riparian populations.Fig. 1: Ethiopian, Sudanese and Egyptian economic and river system performance under the best-performing designs of an adaptive GERD operating approach, considering 29 climate change projections for 2020–2045.Each line of the parallel coordinates plot shows the performance achieved by one of the Pareto-efficient adaptive designs or policies, that is, a policy that, if further improved for one performance metric, would imply a reduction in one or more other performance metrics. All change values are calculated from a baseline in which the GERD is operated based on the Washington draft proposal. The upward direction on each axis indicates better performance (that is, a ‘perfect adaptive plan’ would be a straight line across the top); diagonal lines between neighbouring axes imply tradeoffs, whereas horizontal ones show synergies. The firm power values are calculated based on a 90% reliability, and the real GDP values are discounted at a 3% rate. bcm, billion cubic metres.Full size image More

Efficiency estimationTo compute the efficiency scores of WCs based on the DEA-CSW approach, the methodology proposed by Wu et al.25 was employed. It was assumed that there are n units (left( {j = 1,..,,d,..,,n} right)) ((WC = left{ {d|d,is,a,water,company} right})) and each WC uses m inputs (left( {i = 1,….,,m} right)) to produce s outputs (left( {r = 1,….,,s} right)).To evaluate the efficiency of WCd, the basic DEA-CCR model proposed by Charnes et al.17 was used (Model 1):$$Max,E_d = mathop {sum}limits_{r = 1}^s {u_{rd}y_{rd}}$$
(1)
s.t.$$mathop {sum}limits_{r = 1}^s {u_{rd}y_{rj}} – mathop {sum}limits_{i = 1}^m {omega _{id}x_{ij} le 0}$$$$mathop {sum}limits_{i = 1}^m {omega _{id}x_{id} = 1}$$$$begin{array}{*{20}{c}} {u_{rd} ge 0} & {r = 1,2, ldots ,s} end{array}$$$$begin{array}{*{20}{c}} {omega _{id} ge 0} & {i = 1,2, ldots ,m} end{array}$$where (u_{rd}) is the weight of the output r for the WCd (observation evaluated) and (omega _{id}) is the weight of the input i for the water company evaluated (WCd). Model (1) is an output-oriented DEA model because within a regulatory framework, the objective of WCs is to improve the quality of their services (outputs) keeping constant economic costs (inputs).Model (1) selects the set of input and output weights that maximize the efficiency of WCd. In other words, the efficiency score for the water company d in the DEA-CCR model ((E_d)) is the best that the WCd can obtain. The WCd is efficient if (E_d = 1) and is not efficient (i.e. has room for improvement) if (E_d ,, left| { mp 3} right|$$
(9)
Table 3 Correlations (Pearson coefficient) between input and output variables.Full size tableTable 4 provides an overview of the statistical data employed to compute the efficiency scores of the WCs evaluated in Chile.Table 4 Main descriptive statistics of variables used to evaluate the efficiency of water companies.Full size table More

Expansion of desalinationWhile reliance on desalination may allow Israel to meet basic domestic and agricultural needs, doing so will have considerable environmental consequences. Foremost, increased production of desalinated water will lead to a correspondingly steep rise in energy demand. The world’s most efficient desalination facilities currently require 3–3.5 kWh to desalinate 1 m3 of seawater10,11,12. Assuming the lower end of this bound, under the high-growth scenario, Israel will need an additional 11 TWh per year, or about 15% of the country’s current electrical generation. For comparison’s sake, this amount of electricity would require the equivalent of a 1600 MW natural gas power plant operating with an 80% capacity factor. Of course, technological improvements can be expected to make the desalination process less energy intensive, but the rate of reduction is expected to be limited13. We emphasize that these numbers include only the electricity required for the reverse osmosis process. They do not include the cost or energy associated with pumping water from the Mediterranean, distribution within Israel, and eventual wastewater treatment, all of which are energy intensive in their own right14. While renewable energy generation holds great promise, it could take decades before Israel has a low-carbon electricity system. Indeed, Israeli pledges at the Glasgow COP 26 envision only 30% of electricity coming from renewable sources by 203015. Should Israel pursue a solar PV-based decarbonization strategy, it will require substantial amounts of open space, in a country that already suffers from land shortages12. If solar PV is to become a main source of electricity generation in Israel, then demand will far exceed what is viable to produce on rooftops. At present, Israel generates over 90% of its electricity from fossil fuels, mostly natural gas and in the near-term, desalination will lead to increased greenhouse gas emissions14.The construction of new desalination facilities has the potential to negatively affect Israel’s coastal landscape and aquatic coastal ecosystem. At present, Israel’s major desalination plants are all located along the country’s Mediterranean shoreline, with the future Haifa and Sorek II plants also planned for the coast. Significant future construction has the potential to limit public access to coastal recreation areas. While the construction of future desalination facilities at inland locations may alleviate the environmental impact on Israel’s coast, the feasibility of such construction is still being evaluated. An inland approach could increase the energy requirements for desalination, since it would require pumping seawater further inland. Moreover, scientists and environmentalists have voiced concerns that increased reliance on the pumping of seawater and discharge of brine following desalination over the long term has the potential to damage Israel’s coastal ecosystems, including plant and animal life16,17,18. For almost two decades Israel’s Oceanic and Limnological Research Institute has carefully monitored the effects of desalination facilities in these areas and has yet to detect signs of consequential ecological damage19,20. Notwithstanding, this is an issue that requires continuous observation and analysis.A transition to desalinated water as the primary source of drinking water also raises a number of potential health concerns. Notably, desalinated water is lacking in certain minerals, such as magnesium, considered essential for human health21,22. The long-term consequences of consuming water that does not contain these elements are unknown21,22.Sustainability of treated wastewater for agricultureDespite the boon that treated wastewater has been to Israeli farmers—ensuring a steady supply of low-cost water—leading voices within the scientific community have raised potential environmental and health concerns that question the sustainability of Israel’s practices23,24,25,26.Treated wastewater, including Israel’s relatively high-quality effluents, remains high in salt content, which can lead to reduced plant yield and increase the risk of long-term soil degradation26,27,28. In particular, recent research has demonstrated that irrigation water with a high relative fraction of sodium can cause irreversible breakdowns in soil structure, such that the affected land can no longer support agricultural production. Long-term use of saline water also has the potential to pollute Israel’s aquifers27.Treated wastewater is additionally known to contain various chemical compounds—ranging from pharmaceuticals to heavy metals—that may present risks to human health. When used for agriculture, pharmaceuticals and heavy metal compounds can be taken up by food crops and consumed by people29,30,31,32. Crucially, the health consequences of long-term exposure to these compounds are uncertain and further research in this area is needed33.Should Israel determine that reliance on treated wastewater for agriculture is too great a liability, this would create a twofold stress for the country’s water infrastructure. First, additional water supplies would be required to sustain the country’s farmers, with the only viable alternative likely being the production of additional desalinated water. Given current energy prices, however, the cost of using desalinated water appears to be prohibitively expensive for most crops25,34. Israel would then face a choice between continuing to support local agricultural production, despite the high costs, or moving to import required food supplies, which could be expensive and present potential national security risks. Second, Israel would have to find an environmentally acceptable method of discharging the large quantities of wastewater previously allocated to agriculture. If reusing treated effluent for agriculture is no longer viable, Israel will need to adjust its water treatment infrastructure.One plausible scenario for coping with increased quantities of domestic wastewater is the possibility of treating this water to a higher level so that it can be re-used as drinking water, as has been done in potable reuse programs for years in American states like California, Virginia, and Colorado35. Expanded potable reuse could also lead to a decrease in Israel’s demand for desalinated water, lowering energy costs and greenhouse gas emissions while ameliorating pressure on Israel’s coastal landscape and ecosystems. Treating wastewater to a higher level could also enable continued use by farmers, albeit at a higher cost.Effect of climate change minimal compared to population riseOur analysis shows that the expected effects of climate change on Israel’s water supply are likely to be minimal compared to those of population growth. An assumed 20% decline in production from natural water resources by 2065 (“Methods””), represents a decrease of 245 million m3 per year in comparison to 2020 levels. Even if we consider a larger decline in natural sources due to climate change, the lost capacity pales in comparison to the increased demand from population growth, which is an order of magnitude larger. That is, our projections show that Israel’s water supply will remain precarious even if the worst consequences of global climate change do not materialize. Of course, even if climate change’s effects on Israel’s drinking water may be small compared to that caused by population growth, any change in precipitation patterns also has the potential to raise the risk of forest fires, cause increased flooding, and affect the region’s wildlife.Security concerns and regional cooperationIn past military conflicts, Israel’s coastal desalination facilities have been a target for both rocket and cyber-attacks. Thus far, Israel’s Iron Dome and other defense systems have withstood these challenges. Nonetheless, should a desalination plant be forced offline for a prolonged period of time, it could potentially disrupt water supply.It is also important to note Israel’s obligations to provide fixed quantities of water to the Palestinian Authority and Jordan, pursuant to the Oslo II Accords and the 1994 peace treaty with Jordan. While it is beyond the scope of this analysis, Israel’s neighbors are themselves under intense pressure to meet the water demands of growing populations. Unlike Israel, Palestine and Jordan are already suffering from major deficits in supply, with access severely limited. Moreover, Israel’s neighbors are less well positioned to increase desalination capacity. Water scarcity in Jordan, Palestine, and other countries in the region has the potential to cause significant unrest, representing a major security concern for Israel and its neighbors. The possibility of Israel supplying desalinated water to its neighbors has often been suggested as a possible component of regional peace building36. In fact, in 2021 Israel agreed to double its annual water supply to Jordan to 100 MCM37. Any additional steps to the export of water to Jordan or Palestine would, however, add an additional component of stress to an Israeli water system that will already be facing unprecedented demand driven by population growth.The sustainability of any future plan to address Israeli water scarcity could be bolstered by steps to increase cooperation between Israel and its neighbors. At present, for example, significant quantities of untreated wastewater flow from the West Bank into Israel38,39,40. Likewise, sewage discharge from Gaza into the Mediterranean has in the past caused fouling of membranes at Israel’s Ashkelon desalination plant, even forcing the plant to go offline41. Capture and treatment of wastewater within Palestine would have the dual benefit of increasing potential irrigation supplies for Palestinian farmers while reducing pollution of transboundary water resources42. Increased water access, of course, also has the potential to decrease water-driven security risks in the region.Ecological concernsThe projections presented here only consider how an increase in water demand could impact future demand from desalination. We do not examine how rising population levels might limit access to water resources for recreational purposes. Nonetheless, we can expect that a larger population will put increased strain on access to Israel’s streams, rivers, and lakes43. Likewise, reduced natural flows are liable to stress the flora and fauna in the country’s national parks and nature reserves43. A recent report by Israel’s State Comptroller revealed that the country’s compliance with the UN Convention on Biodiversity is woefully inadequate, with the country failing to meet 74% of the convention’s measurable objectives44. Providing nature with reliable and reasonable water flows will be critical to preserving the country’s unique ecosystems, but increasingly difficult given the anticipated growth in anthropogenic demands.Technological ImprovementsThe trends discussed here are robust even if dramatic technological improvements allow Israel to greatly reduce per capita water consumption. For instance, if we assume a 30% decline in per capita consumption, a truly dramatic change considering historical values and Israel’s already impressive water conservation practices (Methods), Israel would still need to produce 2.3 billion m3 of desalinated water in 2065 for the high-growth scenario. This constitutes a 350% increase in capacity compared to 2020 levels and would require significant infrastructure investment.Global BellwetherThe extent to which Israel is able to meet the water demands of a growing population in the face of increasingly insufficient natural supplies could provide valuable insight for regions and governments facing similar pressures. The population growth rate in the American Southwest, for instance, has far outpaced that of the U.S. as a whole, with water resources in the region already extremely stressed. In contrast to Israel, the American Southwest lacks the advantages of a centralized water authority. Additionally, many of the population centers in the American Southwest are far removed from potential sources of desalinated water, making the challenge of water delivery even greater and the value of efficiency and wastewater treatment and reuse higher. Likewise, middle-income countries facing acute water scarcity (e.g., Brazil, South Africa) may look to Israel’s experience as they seek to increase water supplies for growing populations.Hydrological stability is typically considered a prerequisite for sustainability. In water-scarce regions, projected climate change-driven precipitation decreases matter. But the anticipated shortages caused by population growth appear to matter far more. Desalination offers a possible way-out of such conundrums. But for the foreseeable future, the absence of low-carbon electricity to power this energy-intensive process means that relying on desalination technology will contribute to increased greenhouse gas emissions. Should Israel struggle in its effort to meet growing water demand, or be unable to do so without significantly increasing carbon emissions, it will provide a stark warning of the challenges ahead.Water in the context of other constraints on israeli population growthWhile many technologically-optimistic managers perceive desalination as a panacea for providing water supply under conditions of steady population growth, in other areas of life, solutions are more elusive. This is particularly true in designing infrastructure that utilizes land resources, such as housing, agriculture, and the production of raw materials for construction.To meet projected demand for residential housing between 2020–2030, Israel will need to add an additional 560,000 housing units to present stock. Due to the nature of exponential growth functions, however, demand will grow to over 1.05 million housing units between 2050–2060. Supplying the corresponding housing and infrastructure is expected to put further pressure on Israel’s open spaces, which are already disappearing at a rate of 30 km2 a year8. The depletion of open spaces, including agricultural lands, could also pose a threat to Israel’s food security in the future. Already, official figures cite current Israeli food imports at around 64% of total calories consumed by the population45 with some experts calculating even greater dependence on food imports46. Besides expanding the carbon footprint of Israel’s food supply, such significant reliance on imported crops increases the country’s vulnerability and exposure to global shocks in the food markets during times of international turbulence or military conflict. More

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Groundwater storage variations by integrating GRACE/FO-derived TWS with other terrestrial water storage components for the past two decadesGRACE/FO TWS anomalies for the combined Sacramento, San Joaquin and Tulare basins (Fig. 1, Fig. 2a) were used to calculate groundwater storage anomalies in California’s Central Valley. The GRACE/FO time series (Fig. 2a) for the combined basins is indicative of a region that has experienced successive droughts, punctuated by brief wet periods, resulting in significant cumulative water loss during the study period.Fig. 2: Datasets used for groundwater storage anomaly calculation and GRACE/FO data evaluation in the Central Valley.a GRACE/FO observed monthly total water storage (TWS) anomalies. Red arrow indicates the driest winter in TWS for the past two decades at the begining of 2021. b Three water balance fluxes of precipitation (P), evapotranspiration (ET), and streamflow (Q). c Comparison of monthly change in TWS (dS/dt) between that derived from GRACE/FO and from an observed water balance. d Anomalies of three TWS components of soil moisture (SM), surface water (SW), and snow water equivalent (SWE). All variables are represented in equivalent water height in millimeters for the study region.Full size imageBefore estimating groundwater storage changes, GRACE/FO TWS were first evaluated by comparing its monthly changes to those from an observed water balance calculation (see Eq.(1) in Methods). Figure 2b shows the observed water flux components including precipitation (P), evapotranspiration (ET) and streamflow discharge (Q) for the combined river basins, while Fig. 2c shows a close correspondence between dS/dt derived from GRACE/FO, and that computed using P–ET–Q in Eq.(1). The Root Mean Squared Difference between the two is 26.4 mm/month, and is within the range of the mean uncertainty using GRACE/FO measurements (43.6 mm/month). Such a 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 storage change estimates described below12.Groundwater storage anomalies were estimated by subtracting the anomalies of soil moisture, surface water, and SWE (Fig. 2d) from GRACE/FO TWS anomalies (Fig. 2a) following Eq.(2) as detailed in Methods. The SWE, soil moisture and surface water datasets were obtained from operational, publicly available sources, including the National Oceanic Atmosphere Administration’s Snow Data Assimilation System (SNODAS)43, NASA’s North American Land Data Assimilation System (NLDAS)44, and the California Data Exchange Center45, respectively, ensuring data accessibility for potential routine monitoring following this approach.Figure 3a shows the monthly groundwater storage anomalies derived from GRACE/FO and the datasets shown in Figs. 2a, d in the Central Valley between September 2003 and December 2021. Three notable periods of groundwater recharge and loss were identified in the past 18 years. Groundwater recharge occurred during wet periods from October 2003 to July 2006, March 2011 to July 2011, and October 2018 to August 2019, shown as blue arrows in Fig. 3a. Groundwater loss phases correspond to the well-known droughts that occurred during that time period, namely August 2006–February 2011, August 2011–March 2017, and since September 2019, shown as red arrows in Fig. 3a. A pattern of short phases of recharge followed by longer phases of groundwater loss emerges, resulting in longer-term groundwater depletion over the last two decades. Estimated rates and the total volumes of groundwater gains and losses are summarized in Table 1.Fig. 3: Groundwater storage variations in California’s Central Valley.a GRACE/FO-derived groundwater storage anomalies from September 2003–December 2021 in the Central Valley. The green shaded margin is the uncertainty of groundwater storage. Red arrows represent groundwater loss trends during the droughts of 2006–2011, 2011–2017, and since 2019. Blue arrows represent the three short recharge periods. The black line shows the groundwater depletion trend from 2003–2021. b comparison of deseasonalized anomalies of GRACE/FO derived groundwater and water table depth anomalies from monitoring wells in the Central Valley.Full size imageTable 1 Groundwater change rates and total groundwater volume changes in the Central ValleyFull size tableA groundwater recharge phase (22.7 ± 16.0 mm/yr; 3.49 ±  2.5 km3/yr) in the Central Valley was observed at the beginning of the GRACE mission during 2003–2006 (1st recharge in Fig. 3a and Table 1), when the precipitation amounts were close to or slightly higher than the 20-year average. The NOAA National Weather Service report46 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 ± 4.2 km3of groundwater was replenished during this phase of the analysis.This period of groundwater increase was followed by the 4.5-year drought that began in August 2006. During the 2006–2011 drought (1st drought in Fig. 3a and Table 1), 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 Ref. 12, an additional year of data was included here, and represented the complete drought phase through 2011, rather than through 2010, as in Ref. 12. Although the groundwater loss rate is slightly higher than the 38.9 ± 9.5 mm/yr reported in Ref. 12, the difference falls within the 95% confidence interval, confirming the consistency between the two analyses.Prior to the second drought, a short, rapid recharge phase (March–July 2011, 2nd recharge in Fig. 3a and Table 1) replenished 29.6 ± 15.7 km3 of groundwater (462.5 ± 157.8 mm/yr; 71.07 ± 24.25 km3/yr), as a result of the strong El Niño in 2010 that brought abundant precipitation in early 201147.The groundwater loss rate for the second phase of drought in the GRACE/FO record (2011–2017, 2nd drought in Fig. 3a and Table 1) was 42.7 ± 5.8 mm/yr (6.56 ± 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 (32.2 km3) at full capacity. The GRACE/FO-based groundwater 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. This second phase of drought ended with atmospheric river events that brought heavy precipitation to California in early 201748.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 period23,34 suggest that groundwater recharge occurred during this data gap. We estimate that during the lifetime of original GRACE mission (2003–2017), 41.8 ± 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 (3rd recharge in Fig. 3a) we estimated that groundwater storage increased by 26.6 ± 16.0 km3 (188.8 ± 108.9 mm/yr; 29.02 ± 16.73 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, major water inputs in the region, including precipitation and SWE, significantly decreased in the winters of 2019 and 2020 (Figs. 2b and d). These two winters rank the years 2019 and 2020 as fourth driest consecutive 2-year period on record49. In particular, precipitation reached an 18 year low in the winter of 2020–2021 (Fig. 2b), and TWS (Fig. 2a) shows this same time period as the driest wet season in the GRACE/FO record. Between September 2019 and December 2021 (Present drought in Fig. 3a), total groundwater losses in the Central Valley were 20.0 ± 5.1 km3 (55.8 ± 21.8 mm/yr; 8.58 ± 3.35 km3/yr), which is roughly 31% faster than the previous two droughts.During the present megadrought in southwestern North America (2003–2021), groundwater anomalies 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 44.3 ± 0.9 km3, an amount that is nearly than 1.4 times the full capacity of Lake Mead.Longer-term trends and comparison to observationsThe GRACE/FO groundwater estimates were compared with water table depth anomalies observed from groundwater wells, as shown in Fig. 3b. A valley-wide water table depth was obtained by averaging measurements from available wells located within Central Valley, managed by California’s DWR and USGS23 (see Methods). Seasonal variations of GRACE/FO derived groundwater storage changes and the observed water table depth were removed by subtracting their climatologies, i.e. deseasonalized groundwater storage and water table anomalies, to avoid seasonal inconsistencies between the two measurements, and to only examine their long term trends. Overall, 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. 3b 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 well observations than in the GRACE/FO estimates. Discrepancies may be attributed to the irregular availability of groundwater well data, and to a major decline in available well observations since late 2018 (see Methods, Supporting Information, and Fig. S3). Both of these factors underscore the challenges of estimating large-area groundwater dynamics from well data alone, and of validating groundwater models and satellite observations.Figure 4 shows cumulative groundwater losses from 1962–2021 using the CVHM13 and GRACE/FO. From 2003 to 2014 when both CVHM and GRACE data were available, the groundwater depletion rate for the CVHM was 16.3 ± 6.3 mm/yr (2.51 ± 0.97 km3), matching that from GRACE, 14.7 ± 6.0 mm/yr (2.25 ± 0.92 km3), indicating that the two methods are compatible and may be combined for the further analysis. The combined CVHM-GRACE/FO groundwater depletion rate was calculated by using both CVHM estimations from 1962–2014 and GRACE-derived groundwater storage changes from 2003–2021 through linear regression analysis. The result shows that the groundwater depletion rate from 1962 to 2021 was 12.1 ± 0.8 mm/yr (1.86 ± 0.12 km3/yr), shown as the black line in Fig. 4, resulting in a total groundwater loss of 111.5 ± 0.9 km3. In addition, Fig. 4 shows that the periods for groundwater recovery were shorter, and mostly driven by extreme weather events46,47,48,50 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 consequences46,47,48,50. 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.Fig. 4: Yearly cumulative groundwater losses in the Central Valley.Groundwater losses combining the USGS’s Central Valley Hydrologic Model (CVHM)13 and the GRACE/FO estimates since 1962. The black line represents the overall groundwater depletion from 1962 to 2021 calculated by combining the CVHM and GRACE estimates.Full size imageFigures 3a and 4, along with 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 (12.1 ± 0.8 mm/yr; 1.86 ± 0.12 km3/yr). The most recent phase of groundwater loss, between September 2019 and August 2021 (55.8 ± 21.8 mm/yr; 8.58 ± 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 five times faster than the long-term depletion rate.Relationship between surface water allocations and estimated groundwater storage changesFigure 5a compares GRACE/FO estimated monthly groundwater storage variations to annual surface water allocations (in % of annual maximum) via the two primary aqueducts in the Central Valley, the California State Water Project (SWP)51 and the federal Central Valley Water Project (CVP)52. The two aqueducts transport surface water from northern California to the south. Figure 5b compares the annual groundwater storage changes (net fluxes) to the total surface water deliveries from both the CVP and SWP (in km3). The annual groundwater change was calculated as the difference of the mean annual groundwater anomalies between two consecutive years. Figure 5a, b show that when surface water is abundant, greater allocations are made to farmers, relieving stress on groundwater and allowing for recovery, and vice versa.Fig. 5: Groundwater and surface water management in Central Valley.a Comparison between annual surface water allocations in the aqueducts of the California State Water Project (SWP) and the federal Central Valley Water Project (CVP) and GRACE/FO-derived groundwater storage anomalies. b Comparison between annual surface water deliveries (dark blue bars) of SWP and CVP to the GRACE/FO derived groundwater changes (red and green bars) in Central Valley. The groundwater changes in 2003, 2017, and 2018 are not included because GRACE/FO-derived data do not have complete coverage over the year.Full size imageBetween 2003 and 2007, surface water storage was increasing (Fig. 2d), allowing for larger allocations ( >60%) from both aqueducts, less reliance on groundwater, and hence increasing groundwater storage. Surface water deliveries in Central Valley reached a high for the study period in 2016, resulting in about 5 km3 recharge (Fig. 5b). Surface water storage, and hence allocations, decreased between 2007 and 2009, resulting in significant groundwater storage decline. Surface water deliveries decreased to 2.30 km3 in 2009, corresponding to the highest annual groundwater storage loss by 7.86 km3 during the 1st drought period.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, and 5% in 2016, while the SWP reached its lowest allocation for the study period, 5% in 2014. The low surface water delivery volumes in 2014 and 2015 drove corresponding annual groundwater losses of 9.66 and 7.64 km3, respectively, and led to intensified groundwater pumping through 2016 (Fig. 5b).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 5% in 2021 for the CVP and SWP, explaining in part the increased rate of groundwater loss during this time period. In 2021, the annual groundwater loss was 9.22 km3, matching the greatest annual loss during the study period, which occurred in 2014.Demonstration of GRACE/FO-derived groundwater storage changes to support regional groundwater managementGRACE/FO-derived groundwater storage changes were also estimated in the Sacramento, San Joaquin, and Tulare basins, as shown in Fig. 6 and Table 2. The same periods of groundwater recharge and loss in the Central Valley are used to calculate the gains and losses for the three basins, including longer-term depletion rates. Overall, the individual basin follows similar trends, i.e. three short recharge phases, followed by three longer droughts, as was presented for the entire Central Valley. During the 1st recharge phase, similar rates of groundwater recharge can be observed in the Sacramento and Tulare basins, with increasing rates of 39.0 ± 20.0 and 27.5 ± 15.8 mm/yr (2.81 ± 1.44 and 1.17 ± 0.67 km3/yr (Fig. 6a, c and Table 2)), resulting in groundwater increases of 8.0 ± 2.4 km3 and 3.3 ± 1.1 km3 in the two basins, respectively. Although a slight groundwater loss of 0.7 ± 2.0 km3 (6.4 ± 29.6 mm/yr; 0.26 ± 1.21 km3/yr) in the San Joaquin basin is observed for this period (Fig. 6b and Table 2), the loss rate is not statistically significant (within an uncertainty of 95% confidence interval), indicating that groundwater supply and consumption were nearly balanced in the basin.Fig. 6: Groundwater storage variations in the three Central Valley sub-basins.GRACE/FO-derived groundwater anomalies during September 2003–December 2021 in the (a) Sacramento, (b) San Joaquin, and (c) Tulare basins. The green shaded margins are the uncertainty of groundwater storage estimates. Red arrows represent groundwater loss trends during the droughts of 2006–2011, 2011–2017, and since 2019. Blue arrows represent the three short recharge periods. The black line shows the overall groundwater depletion trend from 2003–2021. Comparison of deseasonalized anomalies of GRACE/FO derived groundwater and water table depth anomalies from monitoring wells for the (d) Sacramento, e San Joaquin, and (f) Tulare basins.Full size imageTable 2 Groundwater change rates and total groundwater volume changes in the three sub-basins in the study regionFull size tableWhen entering to the 1st drought phase, results show that the Sacramento, San Joaquin, and Tulare basins all experienced similar groundwater loss rates of ~42 mm/yr (40–44 mm/yr) (Fig.6a–c and Table 2). The drought ended with the strong El Niño in 201047.During the 2nd drought, all three basins experienced significant losing trends. Figure 6a–c, and Table 2 show that the Tulare basin suffered more severe groundwater losses than the other basins, with a loss rate of 62.9 ± 4.4 mm/yr (−2.67 ± 0.19 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 groundwater storage changes during the 18 year study period show that the depletion rates in the Sacramento, San Joaquin, and Tulare basins, were 12.9 ± 1.8, 16.2 ± 1.9, and 20.6 ± 1.5 mm/yr (0.93 ± 0.13, 0.67 ± 0.08, and 0.88 ± 0.06 km3/yr) (Fig. 6a–c and Table 2), respectively, indicating that the southern Central Valley (combined San Joaquin and Tulare) lost more groundwater than the north, similar to the findings of earlier studies23,30. However, the situation was reversed in the drought that began in September 2019 (present drought in Fig. 6a–c), 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 (1.56 ± 1.03 and 2.55 ± 0.60 km3/yr) for the San Joaquin and Tulare basins, respectively.The deseasonalized GRACE/FO-derived groundwater storage and observed water table anomalies are compared for each of the three basins. Similar to the approach for the whole Central Valley, wells with available measurements within a particular basin boundary were averaged to represent the water table depth variation for the basin (see Methods and Supplementary Information). The two measurements show similar trends and variations for the Sacramento and Tulare basins, except for a strong water table rise in the winter of 2019 for the Tulare basin. As discussed earlier for the entire Central Valley, a dramatic decrease in the number of available well observations after late 2018 may have resulted in an inconsistent record of water table depth.While the Sacramento and Tulare basins showed generally good agreement between GRACE/FO-derived groundwater storage changes and observed well measurements, less correspondence was observed in the San Joaquin basin, particularly during the 1st drought period. However, the two drought phases from 2011–2017 and after 2019 are clearly recognizable, with water table observations falling in response to increased groundwater pumping.Figure 6 highlights both strengths and weaknesses of using the GRACE/FO approach at the sub-basin scale of the individual Sacramento, San Joaquin, and Tulare basins. On the one hand, sub-basin analyses provide important insights into groundwater storage variations across the Valley, in particular, sub-basin trends, which could ultimately inform SGMA performance and provide early warning (in the case of the Sacramento basin) for those regions where groundwater losses are unexpected. On the other, the sub-basins are considerably smaller than the ~154,000 km2 area of the Central Valley, which corresponds the lower area limit for an acceptable level of error for monthly TWSA detection36,53,54,55. (Note that the longer time period associated with the trend calculations mitigates this issue somewhat, resulting in greater confidence in the sub-basin trends than the monthly variations). Hence the GRACE/FO-derived groundwater storage variations at these sub-basin scales should be used judiciously.As with the whole-valley comparisons to observations, the sub-basin analyses are faced with the same challenges as described above, i.e. the difficulties in assembling larger-area water table depth averages from unevenly distributed well observations collected at disparate times and for varying periods of time. In spite of these challenges, the regional groundwater analyses for the sub-basins demonstrates the potential utility of GRACE/FO-derived groundwater storage changes for supporting regional groundwater management efforts. More