ABSTRACT
Balancing water demands with available supplies on a scale that enables sustainable water use constitutes a complex governance challenge. Water managers face the daunting task of balancing water demands amidst variable supplies impacted by climate change and unsustainable extraction leading to drying lakes, depleted rivers, and shrinking aquifers. Traditionally, water managers have focused on bolstering water supplies to meet rising demands. However, this strategy is failing in many regions because of exorbitant costs, alongside environmental, legal, and political obstacles. Despite growing focus, no single solution ensures sustainable water use. Among governance options, capping water extractions holds theoretical promise, but evidence of their efficacy is limited and inconsistent due to shortcomings in their implementation, as detailed in this study. Our paper contributes to the literature on water governance by organizing empirical evidence of different types of caps. We developed a database to analyze 47 cases spanning 14 countries utilizing various types of caps (e.g., volumetric, water level limit, and moratorium) applied to different sources of water (e.g., aquifers, lakes, and rivers). We assess their efficacy in terms of enforceability, adaptability, and performance. We illustrate the strengths and weaknesses of capping as a water management strategy and offer recommendations to enhance its effectiveness.
HIGHLIGHTS
A global review of caps on water use provides the first stocktake of their challenges, opportunities, pathways, and recommendations to ensure sustainable water use.
Caps, found in unexpected regions, indicate broader applicability beyond traditionally water-scarce areas.
Caps often transform over time, initially appearing as one type (e.g., moratorium) before becoming volumetric limits.
INTRODUCTION
Water scarcity emerges when water is being extracted from a water source faster than its replenishment, or when extractions damage freshwater ecosystem health (Steduto et al. 2012; Richter 2014). Reducing water scarcity was identified as a target in the Sustainable Development Goals in 2015; however, with only six years left to meet these goals, many governments are struggling to make progress (Sadoff et al. 2020). Water scarcity imposes significant adverse impacts on economies, communities, and the environment (Pittock & Lankford 2010; Boretti & Rosa 2019; Rosa et al. 2020). Examples worldwide illustrate this challenge, including drying lakes in Africa's Central Rift Valley, Iran, and Chile (Ženko & Menga 2019; Ocampo-Melgar et al. 2022). Overpumping of aquifers is also on the rise in regions spanning the Mediterranean, Central US and Mexico (Hornbeck & Keskin 2014; Reis 2014; Henao Casas et al. 2022), while rivers from Texas to Pakistan and Australia are being heavily depleted or dried up completely, leading to severe ecological repercussions (Dinar 2009; Hoekstra et al. 2012; Liu et al. 2017). Overdrafting of water supplies has contributed to an 84% decline in freshwater vertebrate populations since 1970 (Baker 2020).
Nearly 70% of irrigated croplands face water scarcity at least one month each year (Rosa et al. 2020). Studies indicate that around half of the global population (∼4 billion people) endures severe water scarcity for at least one month each year (Mekonnen & Hoekstra 2016). Furthermore, projections suggest that the urban population facing water scarcity will rise from 933 million in 2016 to 1.7–2.4 billion by 2050 (He et al. 2021). Economic risks posed by water scarcity are significant and further complicated by the reliance on local water supplies to produce goods for a globalized economy. For example, local water scarcity can be transmitted to downstream economies via globalized supply chains (Qu et al. 2018), and major hydrologic basins can experience strongly positive or strongly negative economic impacts due to global trade dynamics and market adaptations to regional scarcity (Dolan et al. 2021). While the effects of water scarcity can cripple an entire region, the most vulnerable and poor people suffer the most severe consequences (Dell'Angelo et al. 2018).
Water scarcity arises from multiple factors affecting supplies and demands of water. For example, climate change is impacting and shifting global water supply patterns (Grafton et al. 2013; Gosling & Arnell 2016), which can be observed in melting glaciers and increasing aridity (Maestre et al. 2015; Immerzeel et al. 2020). On the demand side of the equation, water use globally has shifted due to factors including urbanization, food production for a growing population, and higher standards of living (Marston et al. 2021). Since 1950, urban water demand has increased fivefold (Richter et al. 2013; Padowski & Gorelick 2014). Irrigation for agriculture accounts for 73% of total water withdrawals, and in arid and semi-arid regions, it accounts for more than 95% of total withdrawals, complicating the ability of urban water users to secure sufficient water to support growing populations (Flörke et al. 2018; Garrick et al. 2019a). However, recent findings indicate that water demand can decouple from economic indicators like gross domestic product (GDP), offering the potential to reduce water usage without harming economic productivity (Richter et al. 2020).
Calls to address water scarcity resonate globally from the World Economic Forum to Sustainable Development Goals (Vörösmarty et al. 2018; WEF 2020). Water scarcity is often characterized as a complex issue rooted in water governance (Srinivasan et al. 2012; Vörösmarty et al. 2015). In this context, one governance intervention that has received much attention in water sustainability dialogs is the imposition of ‘caps’ or limits on extractions or withdrawals (Ohlsson 2000; Debaere et al. 2014; Hoekstra 2014; Liu et al. 2017; Richter 2014). The basic premise of capping water use is that extractions can be constrained to stay in balance with water availability.
A cap on water extractions can also help ensure that freshwater ecosystems are protected and sustained even while extractions for human needs are occurring (Richter 2010). In dam-regulated rivers, a cap on extractions can be coupled with reservoir operating rules to maintain an appropriate environmental flow regime (Poff 2018). Furthermore, caps are a necessary element for enabling water markets (Debaere et al. 2014; Garrick et al. 2020), which are an incentive-based approach to managing water scarcity (Wight et al. 2021).
This article provides a timely contribution to the literature on water resources management by reviewing nearly 50 examples of caps developed for rivers, aquifers, and lakes globally. We describe the diversity of caps by analyzing their types and the sources of water to which they have been applied, as well as illustrating their geographic distribution. We then assess the performance of the caps in meeting their original intent and identify common implementation obstacles faced by local communities and water managers. We conclude with recommendations for the use of caps as a water policy strategy.
METHODS
For this study, our objective was to uncover and organize examples where water managers and local communities have set a limit or cap on how much water can be removed from a river, lake, or groundwater aquifer as a step toward ensuring the sustainability of their water resources. Defining sustainable water use is challenged by the various values and demands of multiple users that vary over space and time. Sustainable development in the late 20th century was defined as the capacity of ecosystems to consistently provide essential services despite changes in environmental, economic, and social conditions (Becker & Secretariat 1997). In 1998, Gleick defined sustainable water use as ‘the use of water that supports the ability of human society to endure and flourish into the indefinite future without undermining the integrity of the hydrologic cycle or the ecological systems that depend on it’ (Gleick 1998). For a more thorough review of indicator-based water sustainability assessments, see Juwana et al. (2012). For the purposes of this article, we draw on Richter (2010) who frames sustainable water management as recognizing that society benefits significantly from both the extraction of water and the maintenance of adequate flows within freshwater ecosystems (Richter 2010). Therefore, the definition of sustainable water management ‘involves managing water in a manner that ensures that the full array of benefits associated with water, including benefits that derive from adequate water flows and quality remaining within freshwater ecosystems as well as those that require withdrawals of water from freshwater sources, are protected over the long-term, while meeting the basic water needs of all people’ (Richter 2010).
We employed a scoping review methodology, which is designed to provide a narrative or descriptive account of available research without focusing on the strength of the evidence (Arksey & O'Malley 2005; Levac et al. 2010). Scoping reviews have particular strengths for informing policy and practice recommendations that are not currently well addressed in the literature, as is the case with caps for water policy (Bizikova et al. 2020; Piñeiro et al. 2020; Shah 2021). We recognize there are also limitations. Scoping reviews offer broad overviews but often lack depth, failing to provide detailed insights into specific studies. To address this shortcoming, we have provided a Supplementary Material file that provides detailed summaries of each individual case study in our review. These summaries are properly cited and consistently formatted to facilitate further investigation and ensure clarity in sources and interpretations. Scoping reviews can also struggle to assess the quality or risk of bias in the included literature, which can affect the reliability of the findings. To address this, we provide transparency to the readers about the potential biases throughout this manuscript, which are closely related to an English-only search methodology.
This scoping review was prepared following guidelines from the Preferred Reporting Items for Systematic Reviews and Meta-Analyses extension for scoping reviews (PRISMA-ScR) (Tricco et al. 2018). This approach relies on five steps, as described in the following.
1. Identifying the research question. As a first step, our research question focused on taking stock of empirical examples of caps for water extraction. Specifically, we were interested in understanding: can we identify empirical evidence that caps on water use are effective in ensuring sustainable water management? Our research sub-questions were guided by the cases that we uncovered to ask the following six questions:
(a) What type of cap was set? When was it set? How is it formalized or communicated, e.g., in state or federal legislation, administrative policy by a government agency, or other?
(b) Is there any evidence that the cap is working? For instance, have river flows or lake/aquifer levels stabilized?
(c) Who uses the water? (agriculture, urban, industrial, manufacturing, and mining). How much gets used by each sector?
(d) How has water use changed over time? How much water was used historically versus now? How fast has water use increased?
(e) Why was the cap put into place? For example, was the river or wells being dried up, was there a fish kill or other ecological disaster, or other reasons? Is there any information about the economic consequences of the water problems?
(f) How did water users adjust to the cap? Did they need to find ways to use less water? Which water-use sector was the most impacted, or had the greatest difficulty adjusting to the cap?
2. Identifying relevant studies using pre-determined definitions of different types of caps. We have characterized three different strategies for limiting water extractions:
(a) A volumetric limit is set on total water withdrawals, or water consumption, from a river, lake, or aquifer. These limits are usually set as annual volumes but can also vary from month-to-month. For rivers, they can also be set as a percentage of river flow in each month.
(b) A water level limit is set, such as by not allowing a river's flow to drop below a specified flow rate or not allowing groundwater levels to drop below a specified elevation.
(c) A moratorium limit is set, such as by not allowing any more individuals to divert water from a river or by not allowing any more wells to be drilled into an aquifer. This is sometimes called ‘closing’ a river or aquifer.
3. Study selection. A comprehensive search strategy was developed to identify case studies where caps or limits on water extractions have been implemented. Search terms included variations of the key concepts in the research question, such as ‘caps,’ ‘limits on water extraction,’ ‘water overdraft,’ ‘water moratorium,’ using Google Scholar and Scopus. Following best practices, we started with a calibration exercise using standardized forms, accompanied by explanations and elaboration documents, to establish agreement among the team members.
4. Extracting and charting the data. We developed and categorized case studies describing the water source, geographic location, and type of cap, as summarized in the Supplementary Material (see example of data entries in Table 1). As part of a larger effort to continuously add to the evidence base, we are committing to maintaining a database on caps, which will be found at the Hydroshare website. To ensure accuracy and consistency in the empirical data collected across different regions and contexts, we provided comprehensive training to our analysts, emphasizing data validation protocols and cross-regional standardization practices. This approach not only enhances the reliability of our findings but also supports ongoing research and adaptation of the database to incorporate new insights.
5. Collating, summarizing, and reporting the results. We developed thematic summaries focusing on the drivers for implementing the caps, the process of implementing the caps, and the effectiveness of the caps as evaluated by three criteria: enforcement, performance, and adaptability (see ‘Results and Discussion’ section). We also wanted to describe the geographic extent, the relationship between the water source (e.g., river) and the type of cap (e.g., volumetric cap), and describe when different types of caps were implemented. For a full summary of all case studies reviewed in this study, see the Supplementary Material.
Example entries from our database of case studies in which extraction limits (caps) have been imposed. This is a snapshot from the database. The numbers in the ‘Enforcement’, ‘Performance’, and ‘Adaptability’ columns are explained later in the text. See the Supplementary Material for full tabular summary including all cases and countries reviewed.
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RESULTS AND DISCUSSION
Geographic distribution of caps
Locations of the case studies reviewed in this study. The map displays 40 points; however, some locations represent multiple sources, which are not distinguishable at this scale.
Locations of the case studies reviewed in this study. The map displays 40 points; however, some locations represent multiple sources, which are not distinguishable at this scale.
We readily acknowledge a potential bias in our search criteria, which has favored peer-reviewed articles published in English. This is related to another bias toward developed economies, which has implications in terms of water governance as well. To address this limitation, future research can enrich this database by incorporating searches that go beyond peer-reviewed papers (e.g., gray literature), perform searches in different languages, and administer surveys to international organizations to uncover additional examples that may not be reflected in the academic literature. Second, while regions, such as the Western United States, Spain, and Australia, are known for water scarcity concerns and provide numerous examples of caps, we have also identified caps in unexpected geographies such as Florida (US), Northern Ireland, and New Zealand. These instances suggest that motivations for caps may extend beyond conventional water scarcity drivers – such as a goal of providing a high level of environmental protection – warranting further exploration of local water governance objectives and capacities.
Types of caps and their sources of water
Alluvial chart showing the source of water and the type of cap imposed.
Bar chart showing the count of different types of caps categorized by their water sources.
Bar chart showing the count of different types of caps categorized by their water sources.
Assessing cap effectiveness
One of the primary motivations for our study was to gain a better understanding of the effectiveness of caps that have been imposed. We assessed effectiveness based on three considerations:
1. Does the cap appear to be enforceable?
2. Have the characteristics of the cap been modified over time, enabling water managers to adaptively improve cap effectiveness?
3. Is there physical evidence that the cap is performing as intended?
In Table S1 (Supplementary Material), we have assigned subjective rankings to each of three categories: enforcement, adaptability, and performance. Enforcement and adaptability are enabling conditions for success. Performance, on the other hand, is an evaluation of the ultimate outcome, i.e., whether the cap appears to be succeeding, or not. Our rankings range from a high score of 3, which implies a high degree of strength or performance in the category, to a low score of 1 which implies weakness or poor performance in the category. Our scoring approach is admittedly simplified and based only on one or two factors in each category due to a general lack of information or data for many case studies that would enable a more in-depth evaluation. To achieve a score of 3 for the ‘Enforcement’ category, the cap would need to be quantitatively defined (resulting in a score of 2); if there is also an ability to enforce the cap limitations on individual water users, the case received a score of 3. A high level for ‘Adaptability’ requires that the cap is quantified and the quantitative nature of the cap can be adjusted as necessary (score of 2), and regulators or water managers have, in fact, made adjustments since the original cap was established (score of 3). Scoring a 3 for ‘Performance’ requires evidence (data) that the cap's purpose appears to be achieved (e.g., the threshold level or volume appears to have been met with stable or improving values).
We were unable to fully evaluate several other important enabling conditions due to insufficient information from the case studies. We selected enforceability and adaptability as our two enabling conditions because we had sufficient data to base our scores on these two conditions. It is also important to note that achieving high scores (3) in these enabling conditions does not necessarily guarantee success (a score of 3 on performance), as other enabling conditions may still be lacking in strength. Scoring a 3 on each of the two enabling conditions is not necessary to score a 3 on performance, as our assessment of performance is based solely on data, suggesting that the intent of the cap appears to be met.
Enforcement
Effective enforcement of rules is crucial for successful common pool resource management, as emphasized by Ostrom (1993). Despite facing challenges such as transaction costs, human resources, and monitoring investments (Garrick et al. 2013; Gaye & Tindimugaya 2018), successful water resource management relies on multiple levels of accountability and enforcement. Enforceability hinges significantly on how the cap is implemented – for instance, whether limits are quantified, and whether they are assigned and monitored individually for each water user or are instead applied universally across the water body. The latter presents immense challenges in enforcement, as identifying noncompliant water users can be quite difficult.
Clarity in expectations for water users is another crucial factor. Clear communication regarding the specific volumes each user is allowed helps reduce ambiguity and enhances compliance. When rules are vague or inconsistently applied, enforcement is weakened, leading to potential disputes and undermining trust in the system.
Additionally, the capacity of the enforcing entity – encompassing technical expertise, adequate funding, and clear legal authority – is vital. Entities that lack these resources struggle to monitor compliance effectively or to punish noncompliance, leading to gaps in enforcement. For instance, limited funding can restrict the ability to conduct regular inspections, while insufficient legal authority may prevent the imposition of penalties for noncompliance.
Finally, the social and political context, including community support and stakeholder engagement, plays a role in enforcement success. High levels of local engagement can enhance compliance through community-led monitoring and peer pressure, while a lack of stakeholder buy-in can result in resistance and non-cooperation. Without well-defined, adequately supported, and clearly communicated enforcement mechanisms, tracking and ensuring adherence to caps become unmanageable, ultimately compromising the effectiveness of resource management.
Next, we illustrate both successes and challenges by summarizing selected case studies in Boxes 1 and 2.
Eastern La Mancha Aquifer, Spain
Enforcement: 3 Adaptability: 3. Performance: 3
The Eastern La Mancha Aquifer lies in the headwaters of the Jucar River basin in eastern Spain. It is the largest aquifer in the Iberian Peninsula and is used to supply around 400 million cubic meters (Mm3) of water for irrigated crops each year, while also providing 8 Mm3/year of water supply for the town of Albacete (275,000 inhabitants). Intensive development of irrigated agriculture during the latter decades of the 20th century caused significant depletion of the aquifer. The main crops grown include barley, wheat, corn, alfalfa, onion, garlic, and vineyards.
The intensive use of groundwater for agriculture in La Mancha has caused considerable damage to the aquatic ecosystems of the upper Jucar River but also to human uses downstream because of the reduction of groundwater contributions to river flows (Esteban & Albiac 2012). The aquifer historically would discharge water into the Jucar River, but lowering of the water table caused a reversal of groundwater flow, and the river began draining into the aquifer, leading to severe low flows and water quality degradation.
The motivation for improved management of the aquifer came from growing awareness within the farming community of the aquifer's depletion, with associated increases in pumping costs, as well as the town of Albacete's desire to extract more water from the Jucar River. The farmers formed a water user association to jointly manage the aquifer in 1995, setting controls on water use through an agreement between farmers and the aquifer irrigation association, the state government, and the water basin authority of the Jucar River Basin (Esteban & Albiac 2012). Enforcement of water-use quotas is based on monitoring by remote sensing and individual cultivation plans provided by each farmer.
The efforts of the water user association have resulted in substantially reduced extractions since 2000, from 400 to 300 Mm3/year (Esteban & Albiac 2012). Their success is attributed to the fact that the farmers themselves are directly involved in the enforcement and control process (Esteban & Albiac 2012). This success stands in sharp contrast to the failure of a top-down approach in the neighboring Western La Mancha Aquifer, in which the river basin authority set the water-use quotas and held the responsibility for enforcement. This ‘top-down’ management regime was largely ignored by farmers, and the basin authority was unable to enforce it, both due to a lack of resources and a lack of political will (Esteban & Albiac 2012). Illegal well drilling and overuse of groundwater continued.
Mara River, Tanzania
Enforcement: 1 Adaptability: 1 Performance: 1
The Mara River begins in the highlands of Kenya's Mau Forest and feeds into the Tanzanian side of Lake Victoria. The river plays an essential role in maintaining the biodiversity of East Africa's flora and fauna (Sustainable Water Partnership 2020). Most notably, the river feeds through the Maasai Mara National Reserve and Serengeti National Park – a UNESCO World Heritage Site (WWF 2003). Lying in the center of the Great Rift Valley, the basin supports many endemic species, a booming agro-tourism industry, and ancient agricultural practices. However, the vitality of the Mara-Serengeti ecosystem has been threatened by unsustainable water abstractions and unregulated agricultural expansion (WWF 2021).
A volumetric cap on total water extractions was set in 2018 to prevent water use beyond the ‘allocatable yield.’ This yield is determined as the total volume of water in the river minus a ‘reserve’ of water that is set aside to provide water for basic human needs and ecosystem protection (Sustainable Water Partnership 2020).
However, only 1% of water abstractors have obtained permits, and 3% are in the application process, making enforcement nearly impossible (Sustainable Water Partnership 2020). Additionally, effective enforcement will require the allocation of the allocatable yield to each individual permit holder, which cannot be achieved until all users obtain permits. Unrestricted access to the river over decades has depleted water supplies, impacting farmers' ability to irrigate crops and depleting ecologically critical river flows. These challenges are expected to grow in the future, as water demands are expected to grow exponentially in the decades to come (Sustainable Water Partnership 2020).
Performance
We sought to evaluate the success or failure of each case by collecting relevant trend data on water levels or volumes of use over time. However, collecting such data is challenging because of a paucity of water monitoring stations, or difficulty in accessing data records for many of the cases. Additionally, many of the case studies we surveyed either did not state their capping mechanism in a measurable or enforceable manner, or they did not collect and publish relevant data needed to assess performance. Despite these challenges, we managed to evaluate the performance of 39 cases, with illustrative examples discussed in Boxes 3–5.
Lower Colorado River, California/Arizona/Nevada, USA
Enforcement: 3 Adaptability: 3 Performance: 1
Consumptive use of Colorado River water increased gradually over the last century, reaching a peak in the early 2000s. River flow has decreased by ∼20% since 2000, but total consumptive use has decreased by only 15% on average. With consumption regularly exceeding supply in recent decades, the water stored in Lake Mead and Lake Powell has been heavily depleted (∼75% empty at the end of 2022). Note that the two reservoirs did not fill completely until the early 1980s. BCM, billion cubic meters. (Reprinted with permission).
Consumptive use of Colorado River water increased gradually over the last century, reaching a peak in the early 2000s. River flow has decreased by ∼20% since 2000, but total consumptive use has decreased by only 15% on average. With consumption regularly exceeding supply in recent decades, the water stored in Lake Mead and Lake Powell has been heavily depleted (∼75% empty at the end of 2022). Note that the two reservoirs did not fill completely until the early 1980s. BCM, billion cubic meters. (Reprinted with permission).
During a particularly dry period from 2002 to 2004, the volume of water stored in Lake Powell and Lake Mead – the two largest reservoirs in the United States – fell by nearly half, triggering water and energy security concerns (hydropower is generated at both lakes). In response, the US Department of the Interior negotiated with the states dependent upon the river to develop a set of ‘Interim Guidelines’ in 2007 that included mandatory, fluctuating caps on water use as Lake Mead reaches critically low levels (Castle & Fleck 2019). However, water storage in both Lake Mead and Lake Powell continued to decline, leading to a new set of agreements among the states and federal governments in the US and Mexico known collectively as the ‘Drought Contingency Plan’ of 2019 that includes increasingly severe water-use reductions as Lake Mead levels continue to fall (WRA 2022). However, reservoirs have continued to decline following the implementation of the Drought Contingency Plan. It is evident that the caps on water consumption have not brought water use down to a sustainable level, and a much lower volumetric cap needs to be imposed, particularly as climate warming continues to reduce the flow of the Colorado River. Water use takes place both upstream of these reservoirs as well as downstream through the delivery of water from these reservoirs. The reservoir levels are, therefore, a reflection of the balance of supply and demand in the overall river basin. For a more detailed analysis see Udall & Overpeck (2017). This case illustrates that even when the effectiveness and adaptability of management strategies are high, performance can still be low, influenced by factors beyond the scope of this paper. This underscores the complexity of balancing water supply and demand in large-scale hydrological systems, especially in the context of prolonged drought, increasing consumption, and the impacts of climate change.
Arbuckle–Simpson Aquifer, Oklahoma, USA
Enforcement: 3 Adaptability: 2 Performance: 3
In 2002, a consortium of towns in central Oklahoma proposed to build a 142-km pipeline to import water from the Arbuckle–Simpson Aquifer. A citizen's group protested the proposal, fearing that increased pumping of groundwater would degrade ‘public supply, farms, mining, wildlife conservation, recreation, and the scenic beauty of springs, streams, and waterfalls.’ Their protest led to a legislated moratorium in 2003 on any new well permits and a mandate for the state's water agency to determine the ‘maximum allowable yield’ of the aquifer. The maximum allowable yield was quantified and limits on extraction began to be implemented in 2013.
The water level of monitoring wells in the Arbuckle–Simpson Aquifer appears to have remained dynamically stable since a volumetric cap was installed in 2013.
The water level of monitoring wells in the Arbuckle–Simpson Aquifer appears to have remained dynamically stable since a volumetric cap was installed in 2013.
Eastern Snake Plain Aquifer, Idaho, USAEnforcement: 2 Adaptability: 2 Performance: 2
The Eastern Snake Plain Aquifer (ESPA) is predominantly used for agriculture, with a current irrigation coverage of 809,000 hectares (2 million acres). The area produces more than 180 different crops; milk and cattle are the leading agricultural commodities.
During the first half of the 20th century, large water diversions from the Snake River were used to flood irrigate farm fields, and much of this irrigation water seeped into the ESPA, rapidly increasing its volume. However, in the mid-20th century, a surge in groundwater development occurred, while at the same time irrigators using river water shifted from flood irrigation to sprinklers, resulting in greatly reduced aquifer recharge. The ESPA experienced a 60% volume loss due to an overdraft of 234 Mm3/year. Drying wells and higher pumping costs affected both agricultural and urban groundwater users. Lower aquifer levels also impacted downstream river users because of lessened groundwater discharge, violating many surface water rights.
An Eastern Snake River Plain Comprehensive Aquifer Management Plan was finalized in 2009, setting a long-term (20-year) goal of recovering aquifer levels (Idaho Water Resource Board 2009). The aquifer management plan outlined key strategies, including a cap on aquifer withdrawals and an increase in managed aquifer recharge. The aquifer's volume has been increasing since 2016, largely in response to early success in fully meeting the goal for managed aquifer recharge (Hipke et al. 2022). The remaining aquifer recovery is expected to be achieved through continued efforts to reduce groundwater withdrawals by 13% by 2026.
Adaptability
The prominence of adaptive water management has grown over the decades, acknowledging the dynamic nature of water supplies and demands (Georgakakos et al. 2012; Zeff et al. 2016; Pahl-Wostl 2007; Grafton et al. 2013). Adaptation within capped systems involves revising curtailments (e.g., Upper Clark Fork River) or adjusting cap limits (e.g., Eastern Snake River Plain Aquifer). The commonality among these case studies is the need for adaptation to fulfill the intended objective in the face of evolving hydrological conditions or water uses. The ability of a cap to adapt to changing contexts, where water supplies and demands differ from its inception, provides a crucial safety net for sustainability. Boxes 6 and 7 provide illustrations from the database.
Santa Fe River, Florida (USA)Enforcement: 3 Adaptability: 3 Performance: 3
A minimum river flow level was set in 2013 to protect a suite of water resource values that include the provision of physical habitat suitable for the passage of species such as the Suwannee bass over shallow shoals that exist in some areas of the river; the importance of detrital material for detritivores to obtain nutrients and maintain riverine food webs (Southwest Florida Water Management District 2021); and adequate water quality, particularly as suitable for the Oval Pigtoe mussel (Pleurobema pyriforme) that is listed as a federally endangered species (Southwest Florida Water Management District 2021). This freshwater mussel species has seen significant declines due to habitat loss caused by water pollution.
A water supply assessment conducted in 2010 determined that water resources were likely to be adversely impacted before 2030, creating the need to set minimum flow levels and water withdrawal limits using permits (Southwest Florida Water Management District 2013). The permits limit how much water can be withdrawn from the river or from groundwater to prevent violation of the minimum flow level. The initial flow level set in 2013 was modified in 2021 after additional data and groundwater modeling results became available, demonstrating the ability of water managers to modify the caps as more information becomes available (Southwest Florida Water Management District 2021). Water managers are able to adjust permitted withdrawals every five years (Sustainable Water Partnership 2020). New permits are allowed to be issued only if they are not expected to cause the minimum flow level to be violated.
An assessment of the program in 2021 concluded that the current conditions of the Santa Fe River comply with the minimum flow levels at each of the reference gauging stations, but hydrologic models suggest that by 2040 the minimum flow conditions will not likely be met (Southwest Florida Water Management District 2021). Importantly, the state of Florida requires water managers to prepare a recovery and prevention strategy if the water body is projected to fall below the minimum flow threshold within 20 years, providing additional ability to adapt permits and implement other strategies as needed (Southwest Florida Water Management District 2021).
Edwards Aquifer, Texas (USA)Enforcement: 3 Adaptability: 3 Performance: 3
The Edwards Aquifer supports both human uses, such as urban and agricultural purposes, and unique species residing within the karst limestone aquifer or in springs discharging from it (USGS 2016). Rapid population growth in the San Antonio (Texas) metropolitan area from 1950 to 1990 led to greatly increased groundwater pumping, causing aquifer levels to drop and reducing natural spring flows.
In 1991, the Sierra Club (a conservation organization) filed a lawsuit against the US Fish and Wildlife Service (USFWS), expressing concerns about insufficient protection for aquifer-dependent species listed as threatened or endangered under the US Endangered Species Act (Babbitt et al. 2018). The lawsuit aimed to enforce a minimum spring flow level in Comal and San Marcos Springs, crucial habitats for imperiled species. A federal judge ruled in favor of the Sierra Club in 1993, requiring the USFWS to determine ecologically critical spring flow levels. Simultaneously, the Texas Water Commission was tasked with devising a plan to maintain critical levels in San Marcos and Comal Springs. The Texas State Legislature was ordered to establish a governing system limiting groundwater withdrawals.
In response, the Texas legislature enacted a bill in 1993, establishing the Edwards Aquifer Authority to manage the aquifer and capping pumping at 555 Mm3/year (450,000 AF/year) by 2004 and 493 Mm3/year (400,000 AF/year) by 2008. The bill also mandated the establishment of quantified water rights for all groundwater users, such that the volume of pumping could be quantified and enabling water trading among holders of aquifer rights (Texas Senate Bill 1477; Edwards Aquifer Authority n.d.).
The Edwards Aquifer Authority struggled to constrain pumping within the capped limits, causing the agency to work with the USFWS to design a new approach that maximized water availability while sustaining endangered species. A new ‘Critical Period Management’ plan, initiated in 2007, increased the volumetric limit to 705 Mm3/year (572,000 AF/year) but sharply reduced allowable pumping in stages during droughts, linked to falling aquifer levels (Debaere et al. 2014). Importantly, the 2007 legislation also called for the creation of an ‘Edwards Aquifer Recovery Implementation Program’ to further examine additional drought and habitat management measures that may be necessary to protect the endangered species.
The water caps for the Edwards Aquifer appear to be working well. The endangered species populations are steady or increasing, total well pumping has decreased, and springflows have remained stable (Votteler 2023).
DISCUSSION
While scoping reviews have limitations, such as providing broad overviews without delving deeply into specific studies, they still contribute significantly by mapping the extent and nature of evidence in a field. To partly address these limitations, we provide in the Supplementary Material detailed case study reviews so that researchers interested in understanding more nuanced aspects of water policy, enabling conditions, etc. can have access to specific insights. Additionally, future researchers could consider adopting a living review approach – one that is continuously updated to incorporate new evidence as it becomes available. This approach not only keeps the review current but also strengthens the connection between evidence and practice, enhancing its utility for decision-makers (Simmonds et al. 2022).
Examining case studies of caps reveals the intricate nature of their design and implementation, highlighting the need for meticulous coordination between disciplines in both the physical and social sciences. On one front, caps must account for both short- and long-term water availability, integrating strategies to mitigate the impacts of climate change that disproportionately affect environmental flows (Horne et al. 2017). These considerations often rely on modeling various interconnected hydrological systems. On another front, the effectiveness of caps hinges upon a social agreement regarding water usage behavior and the management of a shared resource pool. For instance, cap design must anticipate unintended consequences such as shifting extraction pressures from one source to another, especially amidst escalating local water demands (Marston & Cai 2016). While the physical and social calculations are essential components, they alone are inadequate for achieving the objectives of a cap and ensuring its enforcement, performance, and adaptability.
An important consideration in the design, implementation, and monitoring of water caps is the potential for unintended consequences. These include increased illegal water use when caps are imposed (e.g., Azraq, Beauce, Guadalquivir, Guadiana, La Mancha, Murray-Darling, Rio Conchos, and Tarim) and setting caps that are too lenient, leading to continued resource decline (e.g., Colorado River and San Luis Valley). Other issues include untruthful reporting of water use (e.g., Republican River) and delays in cap enforcement, as seen in California's aquifers under the Sustainable Groundwater Management Act, where caps will not be enforced until 2040, prompting increased pumping in anticipation of future restrictions. Further examples are detailed in the Supplementary Material.
Our assessment centered on enforcement, adaptability, and performance due to their significant impact on the efficacy of water scarcity mitigation at a large scale, benefiting both humans and the environment. Our discussion aims to extract insights from the case studies, shedding light on shared implementation hurdles. Subsequently, we propose precise policy recommendations to overcome these challenges, with a special emphasis on enhancing enforcement, optimizing performance, and fostering adaptability.
Common implementation obstacles
We were able to identify a variety of implementation obstacles as discussed for individual case studies included in the Supplementary Material. We were unable to find documentation of obstacles for all of our case studies, and because much of the information found was of an anecdotal nature, we could not provide a statistical evaluation of obstacles. However, these are some of the obstacles noted in multiple case studies:
1. Lack of sufficient staff and budget undermines coordination – The effectiveness of caps in addressing water scarcity is hindered by various challenges, including insufficient staffing and budget allocations, which undermine coordination across all phases of cap establishment from design to monitoring. Garrick & Aylward (2012) offer a comprehensive analysis of adaptive efficiency, highlighting the critical role of adequate funding and personnel in modeling, stakeholder engagement, and ongoing monitoring.
For instance, in the Edwards Aquifer case study, initial cap strategies had to be modified due to inadequate funding. The Texas legislature's inability to secure sufficient funding led to a relaxation of the cap to 705 Mm3 (572,000 AF), necessitating the adoption of a ‘Critical Period Management’ plan to mitigate the impact of reduced withdrawals during droughts (Debaere et al. 2014).
Similarly, in the Mara River case, a lack of hydrogeological information has impeded performance assessment. Despite groundwater sources constituting 83% of all abstraction points, basin authorities have been constrained to focus on surface water analysis due to data limitations (Sustainable Water Partnership 2020), resulting in challenges in measuring the cap's effectiveness.
Moreover, in the Beauce Aquifer cap in France, enforcement was hampered by inadequate funding during the 1990s. Administrative departments with varying approaches to limiting extraction lacked the necessary financial and human resources, leading to continued aquifer depletion (Rinaudo et al. 2020).
Addressing these challenges requires concerted efforts to allocate sufficient resources for cap implementation, including robust funding for modeling, stakeholder engagement, and monitoring infrastructure, as highlighted by Garrick & Aylward (2012). Only through adequate resourcing can caps fulfill their potential to mitigate water scarcity and ensure sustainable water management.
2. Lack of support from water users – Challenges in shifting water-use behavior from independent to interdependent present significant obstacles in managing water resources, exemplifying the common pool resource dilemma (Schlüter & Pahl-Wostl 2007; Bernstein et al. 2019). This challenge is compounded by the multifaceted nature of water, serving both as a production input (e.g., for agriculture) and holding varying values for different user groups (Garrick et al. 2017).
In the case of the Beauce Aquifer, the French government transitioned from centralized to decentralized management, entrusting water users' associations with collective withdrawals (Girard 2016). While this approach benefits farmers with reduced withdrawal fees, some perceive it as undermining their control over private property, hindering collective action.
Similarly, the Indus River cap confronts the challenge of aligning economic incentives with cap design. Pakistan faces impending food scarcity due to water security issues, disproportionately impacting lower-class farmers who rely on irrigated agriculture for livelihoods (Janjua et al. 2021). Distrust in water distribution and corruption further complicates the introduction of water rights and permits, exacerbated by conflicting governance approaches within the Indus River basin (Qureshi et al. 2010; Yang et al. 2014). As a consequence, surface water supplies are diminishing under climate change, driving increased groundwater pumping and subsequent storage declines measured from 2005 to 2015 using GRACE satellite data (Rodell & Bailing 2023). Overcoming these challenges necessitates holistic strategies that address socioeconomic disparities, foster trust, and align economic incentives with sustainable water management goals.
3. Lack of authority to ensure enforcement – Insufficient authority to ensure enforcement presents a critical challenge in the effective implementation of caps. While a significant number of caps scored high on enforcement in our assessment, there remains substantial room for improvement, with only 62% of all case studies meeting this criterion.
For instance, in the Guadiana River cap, regulations such as water permits and consumption monitoring were undermined by lax enforcement, leading to widespread breaches of the law and a lack of accountability for damages incurred (Timmerman & Doze 2005). Similarly, in the Azraq Basin in Jordan, despite measures such as metering and pumping quotas, regulatory and enforcement efforts have failed to halt the decline in groundwater levels. Overpumping persists, with groundwater levels declining across the basin, as evidenced by a 2013 analysis revealing continuous decreases ranging from −0.14 to −2.3 m/year (Goode & Subah 2013).
Addressing this challenge requires bolstering regulatory frameworks and enhancing enforcement mechanisms to ensure compliance and accountability. Strengthening legal provisions and investing in monitoring and enforcement capacity are essential to safeguarding water resources and achieving sustainable management goals.
Recommendations for use of caps in water policy
In reviewing both the success of many caps as well as implementation obstacles or failures in others, we offer the following summary recommendations for improving water policy strategy for designing, implementing, and monitoring caps:
1. Clearly state the purpose of the cap and its intended outcomes. This step is critical to address the lack of support from water users obstacle that caps face. There is an opportunity for future research on the role of communication dynamics in managing common pool resources (Lopez & Villamayor-Tomas 2017; Herne et al. 2023; Hoffmann et al. 2023).
2. Ensure that the cap is defined in easily understood quantitative terms and can be applied to individual water users. This step builds from step one and focuses on improving buy-in from water users and requires sufficient staff and budget. Research focused on understanding the translation of scientific knowledge into water policy (Chappells et al. 2015) could improve cap effectiveness for all water users (Lavallee et al. 2021).
3. Acknowledge that if water use already exceeds available water supplies, implementing a moratorium on new uses will not be adequate to achieve sustainable levels of water use. This recommendation highlights a challenging yet essential step: recognizing the need for cutbacks in water usage to restore a sustainable water balance. However, there are encouraging signs that restoring water balance and promoting reallocation can contribute to sustainable water use (Marston & Cai 2016; Aguilar-Barajas & Garrick 2019).
4. Prioritize transparent and honest communication regarding the anticipated impacts of the cap, especially if water-use reductions are required. While many impacts of caps are thoroughly considered in the design and implementation phases, predicting unintended consequences of water policies can be challenging (Grafton et al. 2017; Garrick et al. 2019b). However, transparent and honest communication fosters trust among water users (Muro & Jeffrey 2012), laying a solid foundation for effective cooperation and understanding. This can include providing annual reporting of cap implementation/success as well as transparent communication of shortcomings/obstacles.
5. Plainly define how caps will be implemented and how success will be evaluated by ensuring that necessary monitoring mechanisms are in place. As stated by Higgins et al. (2021) ‘Monitoring and evaluation are needed to illuminate successes and failures, trends, and guide adaptive responses from knowledge gained and to respond to dynamic situations.’ Monitoring requires sufficient budgeting and staff over the long term (Nel et al. 2009) and, therefore, it is imperative to ensure that all necessary implementation costs are carefully budgeted and needed funds are secured. Despite these associated costs, these investments not only facilitate informed decision-making but also foster trust among water users, usually yielding a high return on investment.
6. Verify the implementing entity has the legal authority to enforce the cap. This step is critical to provide the necessary framework for compliance and accountability. Without proper legal backing, the effectiveness of the cap may be compromised, leading to potential loopholes or challenges in enforcement.
CONCLUSION
Water scarcity poses a significant threat to human health, freshwater ecosystems, and economies worldwide. Decades of experience has shown that there is no one-size-fits-all solution for water management in the face of scarce water resources. Amidst this complexity, caps designed to limit water withdrawals offer an enticing solution for water managers grappling with scarcity. However, empirical evidence detailing the feasibility, outcomes, and suitability of such caps remains limited. Our paper contributes to the literature on water scarcity governance by examining empirical evidence from 47 cases of caps across 14 countries, utilizing various types of caps applied to different water sources. While our analysis underscores several obstacles that have hindered caps' enforceability, adaptability, and performance, we also highlight instances where caps have shown promising performance under challenging conditions. Drawing on insights from our study, we offer six recommendations to enhance water policies supporting caps, spanning design, implementation, and monitoring phases. We emphasize transparent communication, clear evaluation criteria, and adequate resources for monitoring and enforcement. Moreover, our analysis suggests that caps offer a valuable competitive advantage in promoting sustainable water use across diverse sources. It is crucial to widely publicize cap performance so water users understand the impacts of their actions on achieving intended outcomes. Additionally, gaining understanding and support from water users for instating a cap is paramount. In conclusion, while cap enforcement, adaptability, and performance face significant challenges and require sustained investment and stakeholder engagement, there are promising examples that highlight their potential. By advancing research to address these obstacles, caps could add to the toolbox of water scarcity governance strategies, supporting the resilience of freshwater ecosystems and the economies that depend on them.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.