1 Introduction

Farming has always been a betting game. The risks are always there. A late frost. A plague of insects. Stem-cracking storms. The creek needed for irrigation that ran dry too early.

The decision to start farming more than 10,000 years ago may in fact have been the biggest bet ever placed by humankind because it implicitly assumed that we could stay in one place and grow enough food to sustain ourselves and our communities. As migratory hunters and gatherers we managed against the risk of insufficient food or water by walking our way out of trouble. We could head for places shown to us by elders and ancestors, places where edible plants or wild game could be found, and where streams and springs ran perennially or where rain pooled in tinajas.

Once we became agrarian our relationship with water changed in a profound way. No longer would we go to the water. We started betting it would come to us.

For thousands of years farmers worldwide have been winning that bet enough of the time to feed their families and communities and—for the past century—feeding others around the globe. Since the early twentieth century governments have also hedged farmer’s bets with massive investments in water infrastructure: storage reservoirs that hold and release water when farmers need it, and canals that bring irrigation water to farms often located far away from water sources. This water infrastructure has enabled farmers to persist through climatic fluctuations and unreliable precipitation and runoff. In fact, farmers have become so capable of accessing irrigation water that today, nearly 90% of all anthropogenic water consumption on the planet goes to irrigating farms [1].

2 Facing nature’s water limits

Ironically, these efforts to improve access to water enabled us to consume the entirety of renewable supplies in many water basins. We are now bumping up against the hard limits of water supplies that are physically and affordably available. Human consumption of fresh water is now approaching or exceeding renewable rates of replenishment in at least one-third of all freshwater sources, including rivers, lakes, and aquifers supplying nearly three-quarters of the world’s irrigated farmland (e.g., Fig. 1)[2].

Fig. 1
figure 1

During the past century, consumptive uses of the Colorado River’s water increased rapidly until total consumption began to approach, and regularly exceed, the river’s annual flow volumes. Large reservoirs built on the river—including Lakes Powell and Mead—helped support extensive farmland and rapidly growing cities, facilitating maximal use of the river’s flow. Since consumption began to exceed annual river flows beginning in the late 1970s, the deficit has been accommodated by extracting water stored in reservoirs and aquifers, causing water levels to drop precipitously since 2000. Consumption has exceeded replenishment in three of every four years in recent decades (note that Lakes Powell and Mead were not filled completely until the 1980s)

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Over the past century, global water extractions increased six-fold [3] to the point where even major rivers including the Colorado, the Rio Grande-Bravo, the Indus, the Ganges, and the Yellow are being completely dried up before reaching their deltas [4], and water levels in massive lakes and aquifers are plummeting as well [4, 5]. It has been estimated that at least 30% of global water consumed today is being supplied from over-exploited, non-sustainable water sources, meaning our extractions are exceeding ecologically safe or sustainable levels; this percentage is projected to increase to 50% in coming decades with climate change [6]. At the same time, irrigation has expanded by?~?11% in recent decades, with half of the expansion occurring in regions already experiencing water stress [7]. More than 40% of food production relies on unsustainable groundwater use, including regions such as the North China Plain Aquifer, where groundwater levels have dropped by as much as two meters per year for six decades [8]. This aquifer supports 40% of China’s grain production, including two-thirds of the country’s wheat.

3 The bite of the shark

In the opening speech of a global water and climate conference convened in Mexico City in 2012, the Director General of Mexico’s water ministry opened with this statement: “Climate change is the shark in our future and water is its teeth.”

Climate change is substantially worsening the odds in irrigated farming because water supplies are diminishing [9], and crop water needs are increasing [10, 11] in many already-stressed regions as temperatures warm, exacerbating imbalances between water consumption and availability and threatening to expand water scarcity into many additional water basins. Snowfields are increasingly evaporating directly into the atmosphere instead of melting to fill rivers and reservoirs [12]. Hotter temperatures suck the moisture out of soils [13], creating empty pore space that must be refilled before subsequent rains or snowmelt will run off the land into rivers or lakes, meaning the soils “take the first drink” and reduce the volume reaching water sources.

The most recent World Meteorological Organization report on the “State of Global Water Resources” documents that 36% of rivers monitored have experienced below-normal flows over the past decade, impacting farm production as well as global trade [14, 15]. The most recent report from the Intergovernmental Panel on Climate Change (IPCC) documents streamflow declines in parts of western and central Africa, eastern Asia, southern Europe, western North America and eastern Australia [9]. The Mississippi, Amazon, and Rhine rivers have experienced record-breaking low flows in recent years, leading to severe disruption of the barge transportation system vital for the shipment of grains and farm inputs [16,17,18]. Recent climate projections suggest that the greatest future declines in river flow are expected to occur in the same temperate and sub-tropical regions that are already experiencing water scarcity, and where most of the world’s irrigated farmland is located, including the western US, western Europe, the Middle East, southern South America, and northwestern India [9, 19].

Farmers have been able to persevere through many droughts since the early days of agrarianism. But many now realize that climate-driven changes in water availability present a fundamentally different challenge, and the drying of irrigation supplies witnessed in recent decades isn’t going to go away. We may see brief interludes with wetter years, but the long-term outlook is chronic aridification [9, 20,21,22].

4 An illustrative example: Upper Rio Grande Basin, US

The farmland trends described above are very well illustrated by the evolution of farming during the past 140 years along the Upper Rio Grande in the US. In the 1880s, European settlers from the eastern US brought immense herds of cattle into the San Luis Valley in the river’s headwaters in Colorado [23]. These cattle were fed from the valley’s extensive irrigated hayfields and then exported to Chicago and other distant markets by newly established rail lines. The production of cattle-feed crops and other food needed to support a rapidly growing population depleted the river’s flow in its entirety during the irrigation season, so farmers began supplementing their water supply with groundwater pumping. Today, the river and more than 10,000 wells are used to irrigate barley, wheat, potatoes, and alfalfa [24]. However, river flows have been 13% below average in recent decades, lessening the water available for crop production as well as groundwater recharge. Groundwater levels have plummeted (Fig. 2), driving up pumping costs. Nearly 40% of farmland in the valley has gone out of production in recent decades [25]. The future of these Colorado farms looks ominous, as the state engineer has mandated that if groundwater levels are not recovered by 2030, thousands of wells will be shut down permanently, driving many more farms out of business [23].

Fig. 2
figure 2

Climate change and chronic overuse of available water supplies have created deficit water balances in many of the planet’s farming regions. These two graphs tell that story for the Upper Rio Grande in the southwestern US. (Left) In the headwaters of the river, excessive groundwater pumping in the San Luis Valley has caused aquifer levels to plummet in recent decades. (Right) Storage reservoirs in the Rio Grande basin have also been severely depleted downstream in New Mexico. Sources: Davis Engineering Service, Inc and US Bureau of Reclamation

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Even though farmers in the San Luis Valley are struggling with water shortages, the Rio Grande Compact of 1938 requires the state of Colorado to allow a small portion of the river’s runoff to pass downstream into New Mexico, and New Mexico in turn must share water with Texas [26]. However, the water flowing into New Mexico—which is supplemented to some degree by summer monsoon rainfall—has been insufficient to meet farming needs in recent decades. Half of irrigated farmland along the river in New Mexico has gone out of production [25], and irrigation supplies for remaining farmers are now commonly curtailed early in the growing season. Farmers have increasingly turned to groundwater pumping to meet shortfalls, causing aquifer declines. Even with substantially reduced irrigation from the river, reservoirs along the river and its tributaries in New Mexico have been depleted by more than 80% since 2000 (Fig. 2). Additionally, New Mexico is now in serious water debt to Texas, which has triggered a US Supreme Court lawsuit among the states. The odds for New Mexican farmers look quite bleak at this time, because the lawsuit could force substantial additional curtailment of irrigation in New Mexico, and the river is expected to decline by 16–28% in coming decades due to climate change [27].

5 Impacts of farm water shortages

In 2018, more than 13,000 irrigated farms encompassing nearly 623,000 hectares in the United States reported interruptions in irrigation supplies that impacted crop yields [28]. Due to water shortages in 2021 and 2022, farmers in the Central Valley of California (US)—one of the most productive agricultural regions in the world—had their water deliveries cut by 43%, resulting in the fallowing of more than 304,000 hectares (10% of farmland), direct economic losses of US$1.7 billion and the loss of 12,000 farm jobs [29]. Many other countries are experiencing similar widespread impacts of water shortfalls in agriculture. The global NGO Sustainable Waters maintains a map database documenting water-shortage impacts in agriculture, providing many site-specific examples of losses in food productivity and farm revenues, and increased crop prices [30].

Water scarcity is affecting rainfed agriculture as well as irrigated farms [9]. An estimated three-quarters of the global harvested area (~?454 million hectares) experienced drought induced yield losses between 1983 and 2009, and the cumulative production losses corresponded to USD $166 billion [31]. Between 2006 and 2016, droughts contributed to food insecurity and malnutrition in northern, eastern and southern Africa, Asia and the Pacific [32]. Increasing variability and uncertainty of rainfall is driving many previously rainfed farmers to begin irrigation, placing greater pressure on limited water supplies [33]. Irrigation water requirements are projected to increase two- to three-fold by the end of the century, with an estimated 14% of this increase directly attributable to climate change [34].

Water-shortage events are becoming costly not just to farmers but also consumers, both in terms of rising food prices as well governmental subsidy and compensation programs supported by taxation. An estimated US$7 billion every year goes to global agricultural subsidies that help enable over-consumption of water supplies [35, 36]. From 1995 to 2020, the US Federal Crop Insurance Program paid farmers an average of US$2.28 billion per year in drought-related indemnity payments or subsidies on crop insurance premiums [37]. The program’s indemnity payouts across all categories of crop damage have increased nearly sixfold over the past two decades, largely due to the increasing frequency of climate-related disasters [38]. Critics have argued that crop support programs dissuade farmers from adapting their farming practices, for example, switching to different crops better suited to changing environmental conditions such as climate change and water scarcity [36, 39].

Certain crop types are more vulnerable to water shortage risks than others [40]. Of great concern is the water risk exposure of wheat, maize (corn), and rice, which provide more than half the world’s food calories. More than 70% of all wheat production, 60% of maize, and a third of rice is situated in areas with extremely high levels of water scarcity [41]. Recurring water shortages and associated impacts on crop production is affecting food security[42, 43] and disrupting national and global supply chains [44].

6 Collateral impacts on cities and freshwater ecosystems

During recent decades, the rise in irrigation consumption—presently accounting for 90% of all anthropogenic consumption globally—has slowed considerably, while urban water use is increasing rapidly [3, 45]. However, water scarcity is beginning to constrain urban growth, creating mounting tension and competition between agricultural and urban water users [46]. In some basins such as the Colorado River in the western US—where agriculture accounts for nearly three-fourths of anthropogenic consumption—falling reservoir (Fig. 1) and aquifer levels [47] could trigger legal mandates to substantially curtail water use on both farms and in cities [48].

In response to water scarcity in the western US, many cities have aggressively implemented urban water conservation measures that have succeeded in reducing water use by 19% even while urban populations grew by 21% [49, 50]. The water constraints on urban growth can also be substantially reduced when irrigated farms reduce their consumption and transfer the saved water to urban use; a savings of 10% on irrigated farms could help nearly 80% of cities around the globe to overcome future water deficits [46, 51]. Given that the economic productivity of water use for urban uses is typically an order of magnitude greater than agricultural use [52], many cities have the financial capability to enter into water trades with agricultural producers [51].

The depletion of the world’s water sources is also having devastating impacts on freshwater ecosystems and species. Freshwater species populations have declined by 85% over the past half-century, and water depletions have been identified as a leading cause [53]. For this reason, water modelers and planners are increasingly including environmental flow requirements in their assessments of future water availability for farming or urban use [9, 54].

7 Policies that can drive transformational change

With three-quarters of irrigated farmland currently vulnerable to water shortages on a periodic or chronic basis, and with climate change reducing water supplies in many water-stressed regions, it is quite clear that a fundamental transformation in the use of water for irrigation farming will be needed to sustain current levels of agricultural production. Even more daunting is the challenge of feeding a projected global population of nearly 10 billion in 2050 [55].

After working on water scarcity challenges with communities and governments for four decades, I have come to understand that an essential precursor to enduring and effective social change is an admission that the present course—the ‘status quo’—is untenable. Farming communities will need to accept the reality that a transformation in irrigation use is needed, i.e., the existing irrigation footprint must be modified to adapt to this new water reality. In many communities this may include the need to dispel the notion that the dryness of recent decades is not just another cyclical drought—although variability between drier and wetter will always exist—but that the trend is toward increasing aridification under climate change [9, 22].

If consensus around the need for change can be attained, future options can be productively explored. Promising early results are emerging in many regions [4]. Farmer surveys are being used to gain understanding of farmer beliefs, perspectives, concerns, ideas, and receptivity for different change strategies [56]. Scenario planning is bringing community members together to discuss the future they want to build [57].

The story of the farming community dependent upon the East La Mancha aquifer in Spain provides a hopeful example of what can be accomplished when farmers acknowledge the need for changes in water use and work together to resolve their water crisis [58]. During the latter half of the past century, groundwater pumping for irrigation increased five-fold, substantially exceeding the rate of aquifer recharge. Declining aquifer levels increased pumping costs and reduced groundwater discharge into the Jucar River, leading to water pollution and ecological degradation. The farming community formed a water user association and began collaborating with the state government and the Jucar River basin authority to design and implement volumetric limits on groundwater use, resulting in a 25% reduction in pumping. This case study is one of 47 reviewed by Wight and others [59], which includes numerous other examples of successful efforts to rebalance over-drafted water sources.

Agricultural strategies for coping with water scarcity, including approaches for reducing irrigation consumption to a sustainable level—have been extensively studied and documented by many researchers, scientists, and policymakers [51]. These strategies include crop switching, irrigation efficiency improvements, deficit irrigation, canal lining, temporary fallowing or farmland retirement, and regenerative approaches for building soil organic matter and water-holding capacity [51]. However, farmer adoption of these strategies has been slow to non-existent in many farming regions owing to a variety of financial, legal, and cultural hindrances [60]. For instance, even when shifting to another crop type may be more profitable, farmers may face financial challenges in converting to new irrigation equipment, lack access to sufficient farm labor, or remain uncertain about markets for the alternate crop [23]. Given that aversion to governmental regulation is common around the globe, incentive-based, collaborative programs that provide opportunities for farmers to design their own future have proven to be most successful.

However, as many have noted, any investments in such water-saving measures must be accompanied by careful measurement to ascertain whether water consumption is actually lowered. In many instances, water consumption unexpectedly increases due to the fact that higher levels of crop growth result from more careful application of irrigation water, and water savings are taken up by other farmers in the area [61].

In addition to on-farm approaches for reducing water consumption, policy initiatives—either regulatory or incentive focused or both—are needed to mobilize water conservation approaches at a scale necessary to rebalance basin water budgets. A few examples are highlighted here.

7.1 Instating limits or ‘caps’ on water use

The basic concept of a cap is to limit water use in one of three ways [59]: (1) by setting volumetric limits (usually annual or monthly) on each water user to maintain total water use at a sustainable level; (2) by establishing a minimum level at which a river, lake, or aquifer must be maintained, and adjusting water use accordingly; and (3) setting moratoriums on withdrawal or pumping permits or rights to preemptively avert over-extraction. Wight and others [59] reviewed 47 case studies from around the world in their evaluation of the efficacy of caps in constraining water use. These investigators found that more than 40% of cases have been successful in constraining water use to the targeted level, and the failures of the remaining cases could be explained by shortcomings in enforceability or adaptability to adjust targeted water reductions when necessary. The concept of setting limits on water consumption as a means for fostering sustainable use underpins many regional initiatives, including the European Union’s “Water Framework Directive” [62], California’s “Sustainable Groundwater Management Act” [63], and Australia’s “Murray-Darling Basin Plan” [64].

7.2 Science-based targets

More than 80 non-governmental organizations have formed a consortium known as the “Science-Based Targets Network” to develop a suite of science-based standards for companies and cities, so they can comprehensively address their environmental impacts across biodiversity, land, freshwaters, oceans, and climate change [65]. The guidance produced by the network helps companies to comprehensively quantify their environmental impacts across their operations and value chains and then implement restorative actions at the landscape or basin level. Freshwater targets center on an objective of constraining overall water consumption within a basin at a level that protects environmental flows [66]; in this way, the approach is compatible with cap-setting as discussed above. When total consumption levels within a basin are higher than desired, individual companies or cities can voluntarily commit to reducing their water consumption by a percentage based on the degree of basin overconsumption.

7.3 Codex Planetarius

An initiative led by the World Wildlife Fund is developing standards for countries and food producers participating in global trade [67]. In contrast to the voluntary, science-based targets approach described above, Codex Planetarius is intended to become a mandatory system to sustain the health of renewable environmental resources used to produce globally traded food by establishing and requiring minimum performance levels for agricultural producers to enter global markets. The proposed standard for freshwater quantity requires that the water sources being used in food production are being managed for ecological sustainability, i.e., not contributing to the depletion of the water source(s) and protecting environmental flows in rivers.

8 Conclusion

It is very hard to be optimistic about the future of irrigated agriculture, given current rates of increasing water consumption and decreasing water availability under climate change. However, there is tremendous latent potential for rebalancing the water budgets of water sources used for irrigation. It is possible to imagine a global-scale rearrangement of crop production that is optimally suited to growing each crop in the places best suited, with particular attention to each crop’s water productivity (i.e., most crop per drop). However, it is currently unimaginable that the world’s governments could pull off the degree of coordination and collaboration necessary to make this happen.

A somewhat less ambitious goal could be founded on improving crop water productivity within each nation to attain benchmark levels. National governments could provide assistance and incentives to push the minimum water productivity of each crop upward to the current 25th global percentile, for instance. A recent evaluation of potential reductions in water consumption that might be achieved with productivity improvements conducted for the US estimated an average of 11% savings in agricultural water consumption at the 25th percentile of each region [68]. There is great need for more attention and investment in field demonstrations and data collection to evaluate whether water productivity can realistically be improved, and to what degree and by what means. However, given assessments such as the recent US benchmark study, improving water productivity alone cannot be expected to fully rebalance all basin water budgets, particularly if environmental flow needs are considered.

The push toward sustainable irrigation management could begin with the compelling notion suggested by the medical first principle of “stopping the bleeding,” meaning adopting a rational goal of preventing any further depletion of water sources as suggested by the World Wildlife Fund’s Codex Planetarius effort, described previously. This achievement alone would serve to rebalance the water budgets of rivers, lakes, and aquifers in which current levels of consumption are greater than replenishment on a periodic or chronic basis. Once the ongoing depletion is arrested, hydrologic restoration of already-depleted basins could begin.

It is important to acknowledge that the stimulus for reducing water consumption in agriculture—and for rebalancing water sources for ecological sustainability—does not need to rely solely on farmers. Consumers can also play a powerful role through their dietary and other commodity choices [69, 70]. Meat-based diets are substantially more water consumptive than vegetarian or other plant-based diets.

Ultimately, our greatest hope may lie with the farmers themselves, acting in their own self-interest. They are continually making decisions about what to grow, and how much to grow, given the cost and availability of farm inputs such as labor, fertilizer, and water, and the profits they can realize from the variety of crops they could grow. If the water’s not there—due to physical or regulatory limits—or lowered demand for certain crops or foods results in lower profits, they adjust. After all, they have been betting on themselves and staying in the game for more than 10,000 years.