Markus Andrews

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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

A river created by a melting glacier in Iceland: SWOT will track the world’s water bodies in unprecedented detail.Credit: Nejc Gostincar/E+/Getty

From swirling ocean eddies that help shape the global climate to millions of lakes and rivers, scientists are about to get an unprecedented view of Earth’s water.The US$1.2-billion Surface Water and Ocean Topography satellite (SWOT), which is due to launch on 15 December from the Vandenberg Space Force Base in California, promises to transform research into the global water cycle and provide climate scientists with a fresh lens on a warming world.A joint mission led by NASA and the French National Centre for Space Studies, SWOT will bounce radar off the surface of Earth’s water bodies — including many that are too small to be tracked from space by current methods. The satellite will enable scientists to measure and track the elevation, extent and movement of water across the planet in ground-breaking detail.“It’s a game changer,” says Rosemary Morrow, an oceanographer at the Laboratory of Space, Geophysical and Oceanographic Studies in Toulouse, France and one of the science leads for the mission. “It will be like putting on a pair of glasses when you are short-sighted: things are sort of vague, and then suddenly everything comes into clarity.”Lakes and riversThere are currently publicly available data for just 10,000–20,000 of the roughly 6 million lakes and reservoirs larger than one hectare on the planet today, says Tamlin Pavelsky, a hydrologist at the University of North Carolina at Chapel Hill and another of SWOT’s science leads. SWOT will measure nearly all 6 million every 10 or 11 days. “We’ve never had measurements like this before,” says Pavelsky. “We don’t even have a baseline.”In 2021, a team led by Sarah Cooley, a geographer at Oregon State University in Eugene pieced together existing satellite measurements of surface area and water elevation for some 227,000 lakes1, but Cooley says those are available only every 90 days. “The data that will be provided by SWOT is orders of magnitude beyond what we were able to do,” says Cooley.SWOT has already helped to generate advances in river hydrology. In anticipation of the satellite’s launch, researchers developed new ways to convert measurements of water height, extent and elevation change into flow estimates2. Applying those techniques to existing satellite data, scientists estimated that rivers carried up to 17% more fresh water into the Arctic Ocean between 1984 and 2018 than previously thought3; SWOT is expected to refine this estimate while enabling similar work across the globe.“If SWOT does what we think it’s going to do, it’s going to change the face of hydrology,” says Colin Gleason, a geographer at the University of Massachusetts Amherst and an author on both studies.Ocean eddiesSimilar advances are expected at sea, where SWOT is expected to provide high-resolution measurements that will allow scientists to track currents, swirling eddies and the ebb and flow of tides. These will bolster understanding of water circulation and improve high-resolution models that can track the transfer of heat and carbon dioxide from the warming atmosphere into the depths of the ocean.SWOT will give scientists their first 3D view of eddies, for example, and will be able to detect perturbations around 10 kilometres wide — one-tenth the scale of the best measurements that are currently available, says Morrow. Even these small features are crucial to understanding and predicting the climate, she says.An international consortium involving the United States, France, Australia and others is planning field expeditions at 18 ocean sites around the world next year. These will help to calibrate the SWOT data against on-site measurements under a variety of ocean conditions.“We’re really really excited, but the proof is in the pudding,” Morrow says. “We’re waiting to see what information comes out.” More

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To develop flood risk management strategies, governments need to consider what really matters, namely how and over what period floods affect societal welfare. To do so, we advocate the adoption of a resilience lens in flood risk management. Here, resilience is understood as the ability of a society to cope with flood hazards by resisting, absorbing, accommodating, adapting to, transforming and recovering from the effects of floods on people’s welfare3,4. To analyze and enhance resilience, we need to consider how and over what period floods affect societies and how measures could affect flood impacts and society5. Questions to consider include whether floods will hamper economic activities; whether people can earn sufficient income or their livelihoods are destroyed and whether their health will be affected.Adopting a resilience lens means taking societal welfare as our starting point. From there, the interaction with flood hazards and flood risks can be considered6. For frequent events resistance may be required to allow societies to continue functioning without facing frequent damage. Damage as a result of rare and extreme events may not be avoidable, but such events must be included in our considerations in order to make sure that those events, although damaging, do not turn into disasters. This requires a deep understanding of what makes people vulnerable to floods and how resilience can be improved. We offer four elements linked to this resilience lens to understand what makes a flood disastrous. We aim to enable an informed discussion on how to arrive at appropriate flood risk management strategies (see Fig. 1).Fig. 1: Adopting a resilience lens by operationalizing the four elements into an integrated flood risk management approach.A welfare and recovery capacity (element 1 and 2): Different effects of floods on different areas or societal groups: some have a larger deterioration of welfare or a slower recovery than others. Both the maximum impact and the recovery together determine the impact of a flood disaster. B include beyond-design events (element 3). The grey curve shows the impacts as a function of event extremity. The standard assessment integrates over this curve and uses the resulting expected annual damage as risk measure; this aggregation undermines the role of high-impact but low-probability events. The extreme events must be given attention as well; (C) distributional impacts (element 4). Distributional impacts can be considered spatially or for different social groups. Welfare economics principles can be applied to capture the utility of different communities and vulnerable groups. By aggregating the effects, we may not see how some groups benefit from measures while others pay for them, or still face large risks. Therefore, next to total cost and benefits, also distributed impacts must be used and weighted to enhance equity.Full size imageImpacts on welfare, instead of on asset lossesFloods hit socially vulnerable people harder, because poorer communities often lack the capacity to recover quickly. Vulnerable people or communities have a lower capacity to anticipate, cope with, resist or recover from the impact of hazards7. They may be forced to live in hazardous places, have less access to flood warnings, a less effective network to enhance recovery, and fewer resources to protect their homes or livelihoods. Especially people that already live in poverty may need to shift to destructive strategies such as selling land or cattle or consume seeds to meet other short-term needs. Such strategies can lead to a vicious circle.Using absolute asset-based damages as yardsticks, as is often done in flood risk management, largely underestimates the disproportionally large welfare impact relatively small absolute losses can have on poor people and may lead to biased planning8. As one dollar does not count equally for all people, flood risk planning should move beyond asset-based valuations and put the welfare of people at the core of the assessment9. This can be done, for example, by considering social impacts such as loss of houses (irrespective of their value), deprivation cost, loss of percentage of income, or considering the effect on income generating ability.There are further merits to placing welfare upfront. First, it opens the possibility of better aligning flood risk management with the larger development agenda3, for instance by linking flood risk management to spatial and economic planning. Second, it allows for a better inclusion of non-structural measures in flood risk management strategies, such as adaptive social protection systems that can quickly disburse financial assistance to households when a disaster hits10. Such measures may not reduce asset-based damages but can have significant benefits of increasing recovery rate and dampening welfare losses.Recovery capacityWhen recovery from floods takes longer, the impact of the floods is more disastrous because of the many indirect and cascading effects, which often exceed the direct damage11. Differences in flood impacts across societal groups often link to differences in their ability to recover from flood impacts. To recover, physical damage must be repaired and income generating options must be restored. Accounting for disruption of services of critical infrastructure, cascading impacts12 or addressing people’s recovery capacity are thus crucial to understand the impact of floods on societal welfare. If we consider recovery as part of flood risk management, the effect of recovery enhancing measures can be included to reduce longer-term welfare loss. Measures such as citizen training, micro-credits, affordable insurance to compensate for flood losses and improving critical infrastructure (enhancing its robustness, redundancy, or flexibility) then become relevant.Beyond-design eventsThe July 2021 floods in Europe have shown the devastating impact of beyond-design events, events that exceed the known risks. The flood peak discharge in July 2021 in the Ahr valley was roughly five times higher than the extreme event scenario of the official flood map13 and its return period was estimated to be around 500 years. Such an event was beyond the imagination of people and authorities, which led to high numbers of fatalities and massive destruction.The complexity of flood risk systems, limitations of scientific knowledge but also motivational and cognitive biases in perception and decision making contribute to such surprises14,15. In many regions, climate change and other drivers of change, such as population growth or increasing vulnerability, lead to more frequent situations where current protection systems are overwhelmed. Our third element targets this blind spot of flood risk management: extreme events beyond current design standards to prevent disastrous surprises.This can be done for example by using a storyline approach, narrative scenarios or training exercises and simulation games that stimulate decision-makers to think through the full disaster cycle. Such exercises are known to inspire discussion of potentially long-term unexpected or unintended cascading effects across different systems16. Outliers in ensemble forecasts may be used as a starting point for such scenarios. These explorations guide dialogues towards achieving the desired level of protection and preparedness for extreme events, to reduce the impact to the most crucial objects, locations, or groups of a society, and provide the basis for training of decision-makers.Distributional impacts and equityA resilience lens requires asking the distributional questions of “the five Ws“17: for whom, when, what, where, and why? Most flood risk analyses aggregate risks and flood protection benefits and disregard their distribution across people, space and time. The resilience lens requires unpacking this aggregation by assessing the distributional impacts of alternative measures. Making explicit who wins and who loses can support distributive justice and prevent unintended distributional consequences. Additional measures for compensating worse-off groups can also be prepared. It is one option, for example, to target flood risk protection measures18 at the most socially vulnerable instead of selecting measures based on utilitarian principles. To do so, a risk analysis that shows distributed impacts on a range of social groups and regions must be carried out. These distributional questions also play out between current and future generations (intergenerational justice).The distributional performance of alternative plans can be assessed through a normative analysis. Various ethical principles drawn from theories of distributive justice can be operationalized to evaluate the fairness of alternative measures19. Multiple principles can also be combined. In the Netherlands, the flood protection standard is designed such that every person has at least a minimum level of safety (sufficientarian principle), while additional safety margin is allowed if it is economically sensible (utilitarian principle)20. More

Peru’s water utility companies are protecting peat bogs because of their ability to hold water.Credit: Erica Gies

In just a few months this year, abnormally low water levels in rivers led China to shut down factories and to floods in one-third of Pakistan, killing around 1,500 people and grinding the country to a halt. A dried-up Rhine River threatened to tip Germany’s economy into recession, because cargo ships could not carry standard loads. And the Las Vegas strip turned into a river and flooded casinos, chasing customers away. It seems that such water disasters pepper the news daily now.Many businesses have long lobbied against changing their practices to safeguard the environment, by refusing to implement pollution controls, take climate action or reduce resource use. The costs are too high and would harm economic growth, they argue. Now we are seeing the price of that inaction.With mounting climate-fuelled weather disasters, social inequality, species extinctions and resource scarcity, some corporations have adopted sustainability programmes. One term in this realm is ‘circular economy’, in which practitioners aim to increase the efficiency and reuse of resources, including water — ideally making more goods (and more money) in the process.
Part of Nature Outlook: The circular economy
But the term has its roots in decades of alternative economic theories — known variously as environmental economics, ecological economics, doughnut economics and steady-state economics. These frameworks recognize that the mainstream economics’ goal of eternal growth is impossible on a planet with finite resources.These ideas are beginning to filter into the mainstream, a mark of both the persuasiveness of advocates’ arguments and the declining state of the natural world. But the economists and scientists behind these principles say that some businesses and governments are engaging in greenwashing — claiming their actions to protect the environment are more significant than they really are — rather than making the kinds of fundamental change required to move the global economy onto a truly sustainable path.Because the dominant culture prioritizes human demands, water is generally viewed as either a commodity or a threat. That perspective inspires single-focus problem solving that ignores the complexity and interconnectedness of water’s relationships with rocks and soil, microbes, plants and animals, including humans, inevitably resulting in unintended consequences.Pumping out groundwater when rivers run low further depletes surface water because the two are linked. Erecting dams to provide water to one group of people deprives other people and ecosystems. Leveeing up rivers and building on wetlands removes space for water to slow, pushing flooding onto neighbouring areas. Paving cities and whisking water away creates localized scarcity.Some corporations are making ‘water neutrality’ or ‘water positive’ pledges, which are a big step forward but not enough, says Michael Kiparsky, director of the Wheeler Water Institute at the University of California, Berkeley’s Center for Law, Energy and the Environment. “If corporations are really serious about water stewardship, they would throw their political and financial heft behind reform of the governance systems that set up this extractive economy around water,” Kiparsky says.More than 11,000 scientists from 153 countries agree that tweaks around the margins are insufficient. In a 2019 letter in the journal BioScience they called for “bold and drastic transformations”, including a “shift from GDP growth and the pursuit of affluence toward sustaining ecosystems and improving human well-being”1. In February, the Intergovernmental Panel on Climate Change, agreed, calling for integrating “natural, social and economic sciences more strongly,” in part by conserving 30–50% of Earth’s ecosystems (see go.nature.com/3sccm6h).A growing group of ecologists, hydrologists, landscape architects, urban planners and environmental engineers — essentially water detectives — are pursuing transformational change, starting from a place of respect for water’s agency and systems. Instead of asking only, ‘What do we want?’ They are also asking, ‘What does water want?’. When filled-in wetlands flood during events such as the torrential 2017 rains in Houston, Texas, researchers realized that, sooner or later, water always wins. Rather than trying to control every molecule, they are instead making space for water along its path, to reduce damage to people’s lives.Broadly speaking, the detectives are discovering that water wants the return of its slow phases — wetlands, floodplains, grasslands, forests and meadows — that human development has eradicated. People have destroyed 87% of the world’s wetlands since 17002, dammed almost two-thirds of the world’s largest rivers3, and doubled the area covered by cities since 19924. All these have drastically altered the water cycle. The water detectives’ projects — part of a global ‘slow water’ movement — all restore space for water to slow on land so it can move underground and repair the crucial surface–groundwater connection.Although the uses of slow-water approaches are unique to each place, they all reflect a willingness to work with local landscapes, climates and cultures rather than try to control or change them. Slow water is distributed throughout the landscape, not centralized. For instance, wetlands and floodplains are scattered across a watershed — an area of land drained by a river and its tributaries — in contrast to a dam and giant reservoir. Around the globe, water detectives are beginning to scale up these projects.Slow waterFor most of California’s state history, groundwater and surface water have been treated as separate resources from both a legal and regulatory perspective. But physically they are linked — by gravity and hydraulic pressure. When river levels run high and spill over into wetlands and floodplains, the flow slows down and seeps underground, raising the water table. Later, that groundwater feeds wetlands, springs and streams from below. “It is hydrologically ridiculous to treat groundwater and surface water differently,” says Kiparsky. “That is as non-circular as you can get.”That legal separation has resulted in overtaxing California’s water supply. The state’s massive water infrastructure — huge dams, levees and long-distance aqueducts — prevents the great rivers of the Central Valley region from occupying their floodplains and naturally recharging groundwater. Plus, when surface water is scarce, people aggressively pump groundwater. But because the two are connected, that further decreases surface water. This depletion means that people have to drill deeper, more expensive wells to reach water. It can also collapse the land, destroying infrastructure. And pumping groundwater near the ocean can allow seawater to push salt inland.Since passage of the 2014 Sustainable Groundwater Management Act (SGMA), California has prioritized recharging groundwater by spreading excess winter water and floodwater on land so it filters underground, or injecting it underground through wells. Various state programmes include incentives for farmers to percolate water on fallow fields, flood management that sets back levees, allowing floodplains to once again serve their purpose, and a search for palaeo valleys — special geological features that could rapidly move heavy water flows underground.But key hurdles remain to seize the bounty of winter floods, says Kiparsky. The main problem is that, despite the SGMA, legal legacies of the artificial divide between surface water and groundwater linger. Colorado is managing this better, he says, because it has integrated the rights systems for groundwater and surface water. Connecting them legally facilitates multipurpose projects such as routing winter water to recharge ponds, which provides habitats for birds and human recreation. The water infiltrates the ground and rejoins the river, effectively making that same water available to farmers later in the year.Peru is also focused on the connection between surface water and groundwater. Almost two-thirds of its population live on a desert coastal plain that receives less than 2.5 centimetres of rain per year and relies on water from the Andes, including from melting glaciers. In 2019, the World Bank predicted that drought-management systems in Lima — dams, reservoirs and under-city storage — would be inadequate by 20305. Over the past decade, Peru has passed a series of laws that recognize nature as part of water infrastructure and require water utilities to invest a percentage of user fees in wetlands, grasslands and groundwater systems.One type of investment is the protection of rare high-altitude wetlands called bofedales, or cushion bogs, which slow water runoff that might otherwise cause flooding or landslides, and hold onto wet-season water, releasing it in the dry season. Bofedales are peatlands, which cover just 3% of global land area but store 10% of freshwater and 30% of land-based carbon6. Unfortunately, these bogs have been subject to peat thievery for the nursery trade. Utility investments are introducing surveillance to protect bofedales and restoring damaged wetlands. Scientists have also studied a local practice of carving out more space for water in the landscape to expand the bofedales, and found that these expansions can store similar quantities of water as the original bogs7.Peru’s water utilities are also investing in a practice innovated by the Wari people 1,400 years ago. In a few Andean villages, Wari descendants still build hand-cobbled canals called amunas. The amunas route wet-season flows from mountain creeks to natural infiltration basins, where the water sinks underground and moves downslope much more slowly than it would on the surface. It emerges weeks to months later from lower-altitude springs, where farmers tap it to irrigate crops.“If we plant the water, we can harvest the water,” says Lucila Castillo Flores, a communal farmer in the Andes village of Huamantanga above the Chillón River valley in Peru. Their culture of reciprocity, with the landscape and with each other, governs how communal farmers care for the water and share the bounty. Because much of the water they use for irrigation seeps back underground, it eventually returns to rivers that supply Lima. Hydrological engineer Boris Ochoa-Tocachi, chief executive of the Ecuador-based environmental consultancy firm ATUK, and his co-researchers used dye tracers, weirs and surveys of traditional knowledge to calculate the impact of restoring amunas throughout the highlands. Lima already has 5% less water than its consumers need. The researchers showed that restoring amunas throughout the largest watershed that supplies Lima could make up that water deficit and give the capital an extra 5%, extending availability into the dry season by an average of 45 days8.Working with wildlifeTaking a holistic approach is also paying off in Washington state and in the United Kingdom, where people are allowing beavers space for their water needs. The rodents in turn protect people from droughts, wildfires and floods. Before people killed the majority of beavers, North America and Europe were much boggier, thanks to beaver dams that slowed water on the land, which gave the animals a wider area to travel, safe from land predators. Before the arrival of the Europeans, 10% of North America was covered in beaver-created, ecologically diverse wetlands.Environmental scientist Benjamin Dittbrenner, at Northeastern University in Boston, Massachusetts, studied the work of beavers that were relocated from human-settled areas into wilder locations in Washington state. In the first year after relocation, beaver ponds created an average of 75 times more surface and groundwater storage per 100 metres of stream than did the control site9. As snowfall decreases with climate change, such beaver-enabled water storage will become more important. Dittbrenner found that the beaver’s work would increase summer water availability by 5% in historically snowy basins. That’s about 15 million cubic metres in just one basin, he estimates — almost one-quarter of the capacity of the Tolt Reservoir that serves Seattle, Washington.

Beavers help to protect people from floods.Credit: Troy Harrison/Getty Images

Beavers have fire-fighting skills too, says Emily Fairfax, an ecohydrologist at California State University Channel Islands in Camarillo. When beavers are allowed to repopulate stretches of stream, the widened wet zone can create an important fire break. Their ponds raise the water table beyond the stream itself, making plants less flammable because they have increased access to water.And beavers can actually help to prevent flooding. Their dams slow water, so it trickles out over an extended period of time, reducing peak flows that have been increasingly inundating streamside towns in England. Researchers from the University of Exeter, UK, found that during storms, peak flows were on average 30% lower in water leaving beaver dams than in sites without beaver dams10. These benefits held even in saturated, midwinter conditions.Beaver ponds also help to scrub pollutants from the water and create habitats for other animals. The value for these services is around US$69,000 per square kilometre annually, says Fairfax. “If you let them just go bananas”, a beaver couple and their kits can engineer a mile of stream in a year, she says. Because beavers typically live 10 to 12 years, the value of a lifetime of work for two beavers would be $1.7 million, she says. And if we returned to having 100 million to 400 million beavers in North America, she adds, “then the numbers really start blowing up”.System changeFor the most part, mainstream economics doesn’t take into account the many crucial services provided by healthy, intact ecosystems: water generation, pollution mitigation, food production, crop pollination, flood protection and more.Value calculations such as Fairfax’s are increasingly tabulated by scientists but usually ignored by the market. One early effort to put a monetary value on those services was a landmark report11 in Nature in 1997, co-authored by Robert Costanza, an ecological economist at the Institute for Global Prosperity at University College London. At the time, global ecosystem services were worth tens of trillions of dollars, more than global gross domestic product (GDP). In an updated paper published in 2014, the global economy had grown but ecosystem services were still worth considerably more12.Another problem: the degradation of those services is typically not counted against profits; instead, those costs are paid by the environment and people. Hannah Druckenmiller, an environmental economist and data scientist at the non-profit organization Resources for the Future in Washington DC, has calculated that permitting development on one hectare of wetlands incurs property damages of more than $12,000 per year13. That’s because water that has been displaced from an area that used to absorb it floods surrounding communities. Druckenmiller estimates the value of wetlands nationwide, just for flood absorption, to be $1.2 trillion to 2.9 trillion. And that is a conservative estimate, based on flood damage data covering just around 30% of households in floodplains.The overarching problem is that the main measure of economic health, GDP, has a narrow focus on market-based production and consumption and does not accurately measure human well-being, Costanza asserts. “A circular economy that similarly limits itself to production will also fall short,” he says. If the goal is well-being, “the question becomes: should you be producing and consuming all those things in the first place?”. Protecting and restoring natural resources and rebuilding social capital, he says, are more likely to achieve well-being.
More from Nature Outlooks
One way to do that is to put more natural ecosystems into a common asset trust, or ‘the commons’. Creating state or local parks, hunting reserves, or wildlife refuges can restrict development and provide significant benefits to the community, says Druckenmiller. Communities that invest in protecting a wetland to prevent flood damages will see the benefit of avoided costs quickly, she says, often with a payback period of less than five years.Another strategy to protect the commons, says Costanza, is the ‘rights of nature movement’, which began in the early 1970s and has gained ground over the past 15 years. It includes enshrinements in the constitutions of Bolivia and Ecuador, local government changes across the United States, and personhood for the Whanganui River in New Zealand, the Ganges River in India and the Magpie River in Canada. That might sound unusual to some people, but in the United States, some corporations have personhood. Granting personhood to a river enables people to argue in court on behalf of its rights. A river’s rights can include freedom from pollution, protection of its cycles and evolution, and space to fulfil its ecosystem functions. The rights of nature movement recognizes that healthy ecosystems make everything work, and “people are part of that system and not separate from it”, says Costanza.States reforming century-old water rights, utilities investing in wetlands and Indigenous techniques and scientists deploying beavers for their engineering prowess are definitive shifts from business as usual. “We’ve made a lot of progress integrating [natural capital] into the system, where it doesn’t get pushed aside because other things are higher priority,” says Druckenmiller.But Costanza thinks much deeper change is needed. “A lot of the things that we’re talking about with the circular economy — regenerating wetlands, planting forests, dealing with climate change — are difficult to implement because the underlying goal is still GDP growth, and these things get in the way of that,” he says.People applying slow-water approaches are doing what they can in the dominant economy. But Costanza says that people can better protect social capital and environmental systems by switching from GDP to metrics such as the Genuine Progress Indicator or one of “literally hundreds” of alternatives, he says.Society’s fundamental goals might seem like a high bar to set, but some of these metrics have already been adopted by governments in Maryland, Vermont, Bhutan and New Zealand. Such shifts move beyond greenwashed versions of a circular economy and help to facilitate water detectives’ work in caring for water systems so that they can sustain human and other life. More

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