As water scarcity worsens globally, there is growing interest in finding ways to reduce water consumption, and for reallocating water savings to other uses including environmental restoration. Because irrigated agriculture is responsible for more than 90% of all consumptive water use in water-scarce regions, much attention is being focused on opportunities to save water on irrigated farms. At the same time, many recent journal articles have expressed concern that claims of water-saving potential in irrigation systems lack technical credibility, or are at least exaggerated, due to failures to properly account for key elements of water budgets such as return flows. Critics have also asserted that opportunities for reallocating irrigation savings to other uses are limited because any freed-up water is taken up by other farmers. A comprehensive literature and internet survey was undertaken to identify well-documented studies of water-saving strategies in irrigated agriculture, as well as a review of case studies in which water savings have been successfully transferred to other uses. Our findings suggest that there is in fact considerable potential to reduce consumptive water use in irrigation systems when proper consideration is given to water budget accounting, and those savings can be beneficially reallocated to other purposes.

## Introduction

Water scarcity has been spreading and intensifying around the globe in recent decades, and is now estimated to be affecting water users and ecosystems dependent upon one-third of all rivers, lakes, and aquifers on our planet (Brauman et al., 2016; Richter, 2016). Here we distinguish between natural aridity, in which available water supplies are limited due to relatively low volumes of precipitation and high levels of evapotranspiration, and the existence of scarcity, which results when the sum of consumptive human uses plus environmental water needs begins to approach or exceed the rate of water supply replenishment (United Nations, 2014).

Communities and businesses experience economic or social water scarcity when water supplies are insufficient to meet the demands being placed on them (Richter, 2016). Ecological water scarcity results when freshwater or estuarine ecosystems are depleted of the natural water flows necessary to sustain their species' populations and ecological functions (Poff et al., 1997; Richter et al., 1997; Postel & Richter, 2003). Nearly one-half of all people and cities and three-quarters of all irrigated agriculture now face water shortage risks as available water supplies have become nearly or fully exhausted in water-scarce regions (Brauman et al., 2016), leading the World Economic Forum to identify water shortage crises as being among the top risks to the global economy (WEF, 2016).

Water scarcity and its impacts on people and nature ultimately represent a failure of water governance systems to constrain the consumptive use of water within the limits of sustainable water supply (Postel & Richter, 2003). This shortcoming of water governance is globally ubiquitous even though sustainability principles have been central to integrated water resource management frameworks for decades. These principles include recognition of the need to: ensure that every person has affordable access to clean drinking water; provide adequate water supplies for economically productive uses; and protect ecosystems dependent upon freshwater flows. For instance, recommendations adopted at the 2002 World Summit on Sustainable Development included the statement that ‘Integrated water resources management should be sustainable and optimize water security and human benefit per unit of water while protecting the integrity of ecosystems’ (United Nations, 2001). More recently, the Sustainable Development Goals, unanimously adopted in 2015 by the 193 member states of the United Nations, include a goal stating the intent to ‘Ensure access to water and sanitation for all’ (Goal #6; United Nations, 2015), along with the following commitments:

• By 2020, protect and restore water-related ecosystems, including mountains, forests, wetlands, rivers, aquifers and lakes.

• By 2030, substantially increase water-use efficiency across all sectors and ensure sustainable withdrawals and supply of freshwater to address water scarcity and substantially reduce the number of people suffering from water scarcity.

Despite this wide embrace of sustainability principles, very few governments have been able to successfully avert or lessen the frequency or intensity of water scarcity and shortages (e.g. Figure 1). This is also despite the fact that scarcity has been present in many societies and cultures for more than a century, and its damaging impacts on communities, economies, and ecosystems are well documented (Richter, 2016). Farmers in the Middle East, western Asia and western North America were exhausting many of their local water supplies by the turn of the 20th century. For example, when farmers of European descent first settled in the Gila River valley of central Arizona (USA) in the 1860s, they found extensive marshes and floodplain forests along the Gila River and its tributaries. Their diversions of water to grow water-intensive crops such as corn, barley, and wheat quickly dried the rivers and caused extensive ecological damage. Ironically, much of their irrigation systems had been reconstructed from more than 800 kilometres of irrigation canals originally dug by indigenous Hohokam people, who disappeared from the area around 1450 ad when an extended drought led to the collapse of their irrigated agricultural system (Richter et al., 2013).
Fig. 1.

Typical of rivers and aquifers in water-scarce regions around the globe, consumptive water use in the Murray–Darling Basin of Australia (upper graph) and the Colorado River Basin in the western United States (lower graph) increased gradually over time, eventually reaching and exceeding the limits of water availability. Note that the reduction in consumptive use seen in the Murray–Darling Basin after 1997 was due to the fact that water availability dropped sharply during the Millennium Drought of 1997–2009; by contrast, in the Colorado River Basin the use of stored water in large reservoirs, along with overdraft of groundwater aquifers in the basin, has enabled consumptive water use to exceed the river's supply during recent years (adapted from Richter, 2016; BCM = billion cubic metres, 109).

Fig. 1.

Typical of rivers and aquifers in water-scarce regions around the globe, consumptive water use in the Murray–Darling Basin of Australia (upper graph) and the Colorado River Basin in the western United States (lower graph) increased gradually over time, eventually reaching and exceeding the limits of water availability. Note that the reduction in consumptive use seen in the Murray–Darling Basin after 1997 was due to the fact that water availability dropped sharply during the Millennium Drought of 1997–2009; by contrast, in the Colorado River Basin the use of stored water in large reservoirs, along with overdraft of groundwater aquifers in the basin, has enabled consumptive water use to exceed the river's supply during recent years (adapted from Richter, 2016; BCM = billion cubic metres, 109).

For much of the past century, communities and governments facing water scarcity have placed heavy reliance on their ability to increase available supplies of water, rather than attempting to reduce demands. However, as discussed in Richter (2016), options for increasing water supplies are rapidly waning in many water-scarce regions, for three primary reasons: (1) there is no more surplus water to be found within reach of affordable water importation schemes; (2) the renewable water supply is in many places declining or becoming more variable due to climate change; and (3) the costs to secure more water, such as by using water storage reservoirs, water importation, desalination, or water reuse technologies, are too high. With diminishing opportunities for enhancing water supply, there is an urgent need to give demand management much greater emphasis in efforts to ameliorate water scarcity.

Many cities and even entire countries, such as the United States and Japan, have successfully stabilized their total water withdrawals in recent decades, even while their populations and economies have grown (Richter, 2016). This curtailment of long-term trends of increasing water use has helped to prevent water scarcity from worsening in some regions. However, existing levels of consumptive water use and depletion of water sources remain precariously high in one-third of all water sources evaluated globally by Brauman et al. (2016), and in the 20% of aquifers assessed by Gleeson et al. (2012). Gaining relief from recurring water shortages in these depleted water basins will require concerted effort to reduce consumptive uses.

The greatest opportunities for reducing water consumption will likely be found in irrigated agriculture, where 90% of all consumptive use takes place (Hoekstra & Mekonnen, 2012; Brauman et al., 2016). Irrigation consumes 10 times more water than all other uses combined at the global level (Richter, 2014). As a consequence, there are few places in the world where water scarcity can be alleviated to any meaningful degree without substantially reducing the volume of water being consumptively used in agriculture. This does not necessarily imply the need to reduce agricultural production or revenue generation, however. This paper focuses on the potential water savings that might be realized in irrigated agriculture without loss of crop production, as well as opportunities to redirect saved water toward environmental restoration or to other uses.

## Reducing consumptive water use in irrigated agriculture

Considerable debate has arisen in recent years over the question of how much water – and even whether any water at all – can be realistically and practicably saved in irrigated agriculture, raising concern as to whether or not water-saving strategies in irrigated agriculture can be effectively deployed to alleviate water scarcity (see, for example, Lankford, 2006, 2012; Perry, 2007; Perry et al., 2009; Foster & Perry, 2010; Gleick et al., 2011; Scott et al., 2014). Much of this controversy is centered on two primary issues: the lack of proper water budget accounting in projects purporting to use less water for irrigation; and the feasibility of transferring any water savings to other water users or the environment.

Figure 2 illustrates the primary pathways of water flow into and through a farming area. When irrigation water is withdrawn from a water source and applied to farmland, some portion of the water is evaporated or taken up by crops as consumptive use, some percolates deep underground and becomes non-recoverable, and some portion subsequently returns to accessible surface or sub-surface water sources as return flow, where it may become available for reuse by others or to support environmental flows. Accurate determination of potential water savings in irrigation requires estimation of the volume of water associated with each of the water flow pathways illustrated in Figure 2 so that the net change in ‘water available for subsequent use’ (including environmental uses) can be properly evaluated (Lankford, 2006, 2012; Foster & Perry, 2010).
Fig. 2.

The primary pathways of water flow in irrigated agriculture. Efforts to determine net water savings from investments in water conservation must account for the volumes of water represented by each of the arrows in this diagram.

Fig. 2.

The primary pathways of water flow in irrigated agriculture. Efforts to determine net water savings from investments in water conservation must account for the volumes of water represented by each of the arrows in this diagram.

Unfortunately, until recently most water conservation efforts on farms, and claims of water-saving potential, have focused solely on changes in the volume of water withdrawn or applied to farm fields (large arrows in Figure 2), neglecting the volume and fate of return flow back into the original water source, creating misleading impressions of water benefits within the overall irrigation network within which water practices have been modified. This insufficient accounting can lead to counter-intuitive outcomes, or a ‘water efficiency paradox’ (Scott et al., 2014), in which seemingly more efficient irrigation application can result in greater net consumptive use, ultimately lessening the volume available for subsequent use.

For example, many farmers have switched from flood (furrow) irrigation – thought to be highly wasteful because of the large volume of water applied to a farm field – to drip irrigation, which uses plastic tubing to place irrigation water directly at the base of the plants, thereby requiring a lesser overall volume of water to be applied, on a specific water duty basis, at the field scale of irrigated systems and farms. However, in many places much of the water thought to be wasted in flood irrigation returns to the original water source after application – via surface run-off or infiltration into shallow groundwater – and becomes available for other users and thus, it is argued, is not truly ‘wasted’ (see return flow arrow in Figure 2). Paradoxically, when more-efficient watering systems such as drip irrigation are installed, the new system may require lower volumes of water withdrawal and application to a farm field, but this reduced volume of withdrawal and farm application (i.e. specific water use, quantified as per hectare water application) can be more than offset by (1) lessened return flows to other users; plus (2) increases in consumptive crop use and productivity due to improved provision of water to the plants (see ‘beneficial consumption by crops’ in Figure 2). This can result in a net reduction of overall water available to subsequent users and increased overall scarcity in the hydrologic system.

A second aspect of the debate relates to the question of whether or not any water savings can be realistically reallocated to other targeted uses, such as to improve environmental flows in a river, or conveyed downstream to urban users. In most accounts of purported water savings in irrigated agriculture, an alleviation of water scarcity is implied, but in the absence of any documentation about the fate of the saved water it is impossible to know whether water scarcity was lessened, or environmental flows improved (Lankford, 2013). In most real-world applications, any water left in water sources due to lessened water applications per hectare by farmers is consumptively used by other farmers sharing the same water source, either through increases in crop production on the same land area, by switching to a more valuable but consumptive crop (e.g. bananas), or by allowing the irrigated land area to expand (Organisation for Economic Co-operation and Development (OECD), 2015). For example, Scott et al. (2014) document that in both the Guadiana River basin of Spain and the Limari River basin of Chile investments in irrigation efficiency from 1995 to 2007 – primarily converting flood irrigation to drip or sprinkler irrigation – substantially reduced the volume of water being applied to the original farmlands. These lessened withdrawals could have had the effect of restoring flows in these rivers, which are regularly dried completely from irrigation use. However, over ensuing decades the irrigated acreage in these basins was allowed to expand twofold in the Limari and threefold in the Guadiana. As a result, overall consumptive use increased substantially, worsening water scarcity in both basins.

Enabling increases in agricultural production is in many instances a highly desirable and intentional outcome of water-saving programs in irrigation farming. However, care must be taken to ensure that such programs do not make farmers more vulnerable to water shortages in the future by causing overall consumptive use to increase. For instance, a severe drought in the Limari basin of Chile during recent years has caused water availability to drop well below the volume needed to meet the consumptive demands that have doubled with expansion of irrigated acreage in the basin and, as a result, farmers have been forced to fallow more than 50% of the irrigated area in the past five years (Pablo Alvarez, Universidad de la Serena, personal communication).

Additionally, it should be explicitly acknowledged that by allowing consumptive water use to grow, river flows or groundwater levels will be further depleted, often to the point of complete drying, with attendant loss of biodiversity and social and economic benefits such as riverine fisheries. In the Limari, excessive drying of the lower section of the river has contributed to a steep decline in the population of freshwater shrimp (Cryphiops caementarius), adversely impacting the local fishing community (Morales & Meruane, 2013).

To assess the feasibility of saving and transferring water in irrigated agriculture, our research group performed a literature and internet review in an effort to identify well-documented field experiments and case studies.

## Literature review of potential water savings in irrigated agriculture

Guided by the depiction of water flow pathways in Figure 2, our research group conducted a literature review for the purpose of identifying technically credible case studies that could help document the water savings attainable by using various strategies or technologies, as described in Box 1. Our findings are summarized in Tables 1 and 2. For each reference reviewed, we evaluated the reduction in consumptive use and the potential increase in the volume of water that could be made available for subsequent use as a result of the strategy or measures applied in the study. In selecting references for inclusion in Tables 1 and 2, we filtered the studies we reviewed using the following questions:

1. Were the flow pathways illustrated in Figure 2 adequately and explicitly accounted for?

2. If all flow pathways were not explicitly accounted for, each of the following questions must be answered in the affirmative:

• (i) If the study documented a reduction in non-beneficial or beneficial consumptive use, could it reasonably be assumed that the water-saving measure applied could result in making an equal volume of water available for subsequent use in many settings?

• (ii) Did the study document, or could it reasonably be assumed, that crop yields would not be decreased (i.e. within 5% of original yield)?

Box 1.
Descriptions of water-saving measures.

No-till farming is a way of growing crops from year to year without disturbing the soil through tillage, thereby leaving crop residues in place. This has the benefit of increasing organic matter in the soil, leading to improved infiltration of precipitation or irrigation water. This practice can also reduce evaporation from the soil by lowering soil temperature due to shading, and reducing wind influences on evaporation.

Mulched ridges are used in furrow (flood) irrigation as a means for incorporating organic or other material into the ridges between tilled furrows, thereby providing many of the benefits described above for no-till farming.

Sprinkler irrigation is a method of applying irrigation water that is similar to natural rainfall. Water is distributed through a system of pipes, usually by using pumping to create pressure that forces water through the pipes. The irrigation water is then sprayed into the air through sprinklers. Various innovations in sprinkler equipment have been used to reduce the loss of irrigation water to evaporation, and to ensure that as much water reaches the plant roots as possible. These innovations include increasing the droplet size emerging from the sprinklers, placing the sprinkler heads close to the ground, changing the angle at which the sprinkler sprays water (i.e. ‘low elevation spray application’ or LESA), and using less pressure in the pipe system to avoid excessive spray (i.e. ‘low energy precision application’ or LEPA).

Drip irrigation refers to the slow application of low pressure water from plastic tubing placed near the plant's root zone, helping to reduce wind or soil evaporation. The plastic tubing can be placed on top of the ground surface, or it can be buried (sub-surface drip irrigation), thereby reducing or nearly eliminating surface evaporation and run-off.

Deficit irrigation is a watering strategy that can be applied using various types of irrigation methods. It involves the application of irrigation water at a rate or frequency lower than the crop's full water requirements. ‘Regulated’ deficit irrigation strategically manages the deficits at developmental stages when water stress will not impact yields negatively. Proper implementation of this strategy requires careful monitoring or modeling of plant stress and soil moisture.

Irrigation scheduling is similar in purpose to deficit irrigation, but does not involve intentionally stressing the plants. Instead, soil moisture – and in some instances, leaf water potential – is monitored to determine when the plant needs more water, thus avoiding application of water when it is not needed. Weather models are commonly used to estimate crop water requirements and optimize the timing and amounts of irrigation applications.

Irrigation timing is a form of irrigation scheduling, but focused on applying irrigation water at times when evaporation can be minimized, such as during night-time when temperatures and wind speeds may be lower.

Alternate wetting and drying (AWD) is a watering strategy primarily applied to rice growing, as an alternative to continual flooding during planting and the growing season. AWD involves flooding the soil and then allowing the field to dry down before being re-flooded, thereby reducing the volume of water consumed.

Source water substitution involves use of an alternate source of water supply, for the purpose of avoiding overuse of traditional freshwater sources such as rivers, lakes, or aquifers. The most common substitute is recycled water, which is provided as wastewater from industries or domestic uses, and may be treated or not treated following the original use. Far less common in irrigated agriculture is the use of desalinated ocean water or salty groundwater.

Irrigation infrastructure improvement can take many forms, but in Table 1 it is used to refer to improvements in irrigation delivery systems. The most common approach is to line a previously unlined, earthen ditch with plastic or concrete to reduce soil infiltration and loss of water through canal leakage or weed transpiration. Less common is the conversion of an open canal to a piped system for the purpose of reducing both infiltration and surface evaporation.

Temporary or permanent fallowing involves the cessation of crop growing, on either a temporary (e.g. one-year) or permanent basis, for the purpose of reducing consumptive water use. In calculating potential reductions of consumptive use, the water consumed by intentional (i.e. revegetation) or accidental (i.e. weed growth) vegetative regrowth on the farm field must be accounted for.

Crop shifting involves the replacement of a more water-intensive crop with a crop requiring less water consumption. An important consideration is the change in revenue generated; in the ideal case, a lower-value water-intensive crop is replaced by a higher-value crop requiring less water consumption.

Vegetation management is undertaken to reduce undesirable plant growth and associated water consumption, such as the removal of weeds in farm fields or aquatic vegetation in irrigation storage reservoirs.

Table 1.

Water-saving measures in irrigated agriculture that can make more water available for other uses or environmental flow.

Water-Saving MeasurePrimary Flow Pathway EstimatedSaved VolumeWater Savings %Study LocationCitation
Soil Management
No-till farming Reduced non-beneficial consumption 900–1,250 m3/ha/year 33–45% Nebraska, USA van Donk et al. (2010)
No-till farming Reduced non-beneficial consumption 1,020 m3/ha/year 13% California, USA Mitchell et al. (2012)
No-till farming Reduced non-beneficial consumption 591 m3/ha/year 48–54% Kansas, USA Klocke et al. (2009)
Plastic-mulched ridges (furrow irrigation) Reduced water application 1,500 m3/ha/year 50% Shaanxi Province, China Wu et al. (2015)
Irrigation Application
Switch sprinkler to sub-surface drip Reduced non-beneficial consumption 3,000 m3/ha/year 20–25% Kansas, USA Lamm (2005)
Spray heads or low-angle impact sprinklers to LEPA sprinklers Reduced non-beneficial consumption 15–33 m3/ha/irrigation event 6–13% Texas, USA Schneider & Howell (1993), cited in Lamm (2005)
Switch from flood (furrow) to drip Reduced water application 2,000–6,000 m3/ha/year 40–46% at same crop yield Anatolia Region, Turkey Cetin & Bilgel (2002)
Switch from flood (furrow) to drip Reduced water application 3,035 m3/ha/year 56% Umerkot, Pakistan Tagar et al. (2012)
Switch from flood (furrow) to sub-surface drip Reduced water application 1,070–3,180 m3/ha/year 34–47% Texas, USA Enciso et al. (2015)
Switch from flood (furrow) to sub-surface drip Reduced water application 1,060–1,490 m3/ha/year 32–57% California, USA Hanson et al. (1997)
Regulated deficit irrigation (pistachios) Reduced beneficial consumption and reduced water application 1,230 m3/ha/year 33% California, USA Iniesta et al. (2008)
Regulated deficit irrigation (almonds) Reduced beneficial consumption and reduced water application 1,270 m3/ha/year 14% California, USA Stewart et al. (2011)
Regulated deficit irrigation (almonds) Reduced beneficial consumption and reduced water application 1,820–3,112 m3/ha/year 17–29% California, USA Goldhamer et al. (2006)
Regulated deficit irrigation (grapefruit) Reduced beneficial consumption and reduced water application 1,213–1,296 m3/ha/year 28–60% Adana, Turkey Unlu et al. (2014)
Regulated deficit irrigation (strawberries) Reduced beneficial consumption and reduced water application 2,289–2,315 m3/ha/year 19–27% Huelva Province, Spain Lozano et al. (2016)
Regulated deficit irrigation (quinoa) Reduced beneficial consumption and reduced water application 650–1,400 m3/ha/year 32–82% Altiplano, Bolivia Geerts et al. (2008)
Regulated deficit irrigation w/mulching Reduced water application 583 m3/ha/year 76% Shaanxi Province, China Zhou et al. (2011)
Alternate wetting and drying for rice Reduced water application 1,429–2,461 m3/ha/year 18–31% Arkansas, USA Linquist et al. (2015)
Irrigation scheduling (soil moisture monitoring) Reduced water application 1,390 m3/ha each year 27% Colorado, USA Gleason (2013)
Irrigation scheduling (soil moisture) Reduced water application 1,036 m3/ha/year 22% Punjab, India Perveen et al. (2012)
Irrigation scheduling (soil moisture and leaf water potential monitoring) Reduced beneficial consumption and reduced water application 819–1,600 m3/ha/year 42–67% Luancheng, China Zhang (2002)
Irrigation timing (daytime vs night-time) Reduced non-beneficial consumption (evaporation)  12% Salon de Provence, France Molle et al. (2012)
Source water substitution (using desalinated water) Reduced withdrawal of water from source 10,370–25,340 m3/ha/year 100% Jordan Valley, Israel Silber et al. (2015)
Irrigation Infrastructure Improvements
Canal lining, replacing canals w/pipes Reduced non-beneficial consumption (evaporation) 48–1,494 m3/year/metre of canal  Oregon, USA Newton & Perle (2006)
Crop Management
Temporary fallowing of farm land Reduced beneficial consumption 10,085 m3/ha/year 100% California, USA Palo Verde Irrigation District (2015)
Temporary fallowing of farm land Reduced beneficial consumption 16,756 m3/ha/year 100% California, USA Imperial Irrigation District (2014a, 2014b)
Crop shifting (different combinations) See Table 2
Other Vegetation Management
Removal of invasive vegetation Reduced non-beneficial consumption 2,000–4,000 m3/ha/year savings versus native vegetation ∼35% New Mexico, USA Weeks et al. (1987)
Removal of invasive vegetation Reduced non-beneficial consumption 25 m3/ha/year savings versus native vegetation  Cape Province, South Africa Dzikiti et al. (2013)
Aquatic vegetation control Reduced non-beneficial consumption 2,446 m3/ha/year versus open water 27% Nile Delta, Egypt Rashed (2014)
Aquatic vegetation control Reduced non-beneficial consumption 1,300–4,500 m3/ha/year versus open water 17–59% Alabama, USA Boyd (1987)
Water-Saving MeasurePrimary Flow Pathway EstimatedSaved VolumeWater Savings %Study LocationCitation
Soil Management
No-till farming Reduced non-beneficial consumption 900–1,250 m3/ha/year 33–45% Nebraska, USA van Donk et al. (2010)
No-till farming Reduced non-beneficial consumption 1,020 m3/ha/year 13% California, USA Mitchell et al. (2012)
No-till farming Reduced non-beneficial consumption 591 m3/ha/year 48–54% Kansas, USA Klocke et al. (2009)
Plastic-mulched ridges (furrow irrigation) Reduced water application 1,500 m3/ha/year 50% Shaanxi Province, China Wu et al. (2015)
Irrigation Application
Switch sprinkler to sub-surface drip Reduced non-beneficial consumption 3,000 m3/ha/year 20–25% Kansas, USA Lamm (2005)
Spray heads or low-angle impact sprinklers to LEPA sprinklers Reduced non-beneficial consumption 15–33 m3/ha/irrigation event 6–13% Texas, USA Schneider & Howell (1993), cited in Lamm (2005)
Switch from flood (furrow) to drip Reduced water application 2,000–6,000 m3/ha/year 40–46% at same crop yield Anatolia Region, Turkey Cetin & Bilgel (2002)
Switch from flood (furrow) to drip Reduced water application 3,035 m3/ha/year 56% Umerkot, Pakistan Tagar et al. (2012)
Switch from flood (furrow) to sub-surface drip Reduced water application 1,070–3,180 m3/ha/year 34–47% Texas, USA Enciso et al. (2015)
Switch from flood (furrow) to sub-surface drip Reduced water application 1,060–1,490 m3/ha/year 32–57% California, USA Hanson et al. (1997)
Regulated deficit irrigation (pistachios) Reduced beneficial consumption and reduced water application 1,230 m3/ha/year 33% California, USA Iniesta et al. (2008)
Regulated deficit irrigation (almonds) Reduced beneficial consumption and reduced water application 1,270 m3/ha/year 14% California, USA Stewart et al. (2011)
Regulated deficit irrigation (almonds) Reduced beneficial consumption and reduced water application 1,820–3,112 m3/ha/year 17–29% California, USA Goldhamer et al. (2006)
Regulated deficit irrigation (grapefruit) Reduced beneficial consumption and reduced water application 1,213–1,296 m3/ha/year 28–60% Adana, Turkey Unlu et al. (2014)
Regulated deficit irrigation (strawberries) Reduced beneficial consumption and reduced water application 2,289–2,315 m3/ha/year 19–27% Huelva Province, Spain Lozano et al. (2016)
Regulated deficit irrigation (quinoa) Reduced beneficial consumption and reduced water application 650–1,400 m3/ha/year 32–82% Altiplano, Bolivia Geerts et al. (2008)
Regulated deficit irrigation w/mulching Reduced water application 583 m3/ha/year 76% Shaanxi Province, China Zhou et al. (2011)
Alternate wetting and drying for rice Reduced water application 1,429–2,461 m3/ha/year 18–31% Arkansas, USA Linquist et al. (2015)
Irrigation scheduling (soil moisture monitoring) Reduced water application 1,390 m3/ha each year 27% Colorado, USA Gleason (2013)
Irrigation scheduling (soil moisture) Reduced water application 1,036 m3/ha/year 22% Punjab, India Perveen et al. (2012)
Irrigation scheduling (soil moisture and leaf water potential monitoring) Reduced beneficial consumption and reduced water application 819–1,600 m3/ha/year 42–67% Luancheng, China Zhang (2002)
Irrigation timing (daytime vs night-time) Reduced non-beneficial consumption (evaporation)  12% Salon de Provence, France Molle et al. (2012)
Source water substitution (using desalinated water) Reduced withdrawal of water from source 10,370–25,340 m3/ha/year 100% Jordan Valley, Israel Silber et al. (2015)
Irrigation Infrastructure Improvements
Canal lining, replacing canals w/pipes Reduced non-beneficial consumption (evaporation) 48–1,494 m3/year/metre of canal  Oregon, USA Newton & Perle (2006)
Crop Management
Temporary fallowing of farm land Reduced beneficial consumption 10,085 m3/ha/year 100% California, USA Palo Verde Irrigation District (2015)
Temporary fallowing of farm land Reduced beneficial consumption 16,756 m3/ha/year 100% California, USA Imperial Irrigation District (2014a, 2014b)
Crop shifting (different combinations) See Table 2
Other Vegetation Management
Removal of invasive vegetation Reduced non-beneficial consumption 2,000–4,000 m3/ha/year savings versus native vegetation ∼35% New Mexico, USA Weeks et al. (1987)
Removal of invasive vegetation Reduced non-beneficial consumption 25 m3/ha/year savings versus native vegetation  Cape Province, South Africa Dzikiti et al. (2013)
Aquatic vegetation control Reduced non-beneficial consumption 2,446 m3/ha/year versus open water 27% Nile Delta, Egypt Rashed (2014)
Aquatic vegetation control Reduced non-beneficial consumption 1,300–4,500 m3/ha/year versus open water 17–59% Alabama, USA Boyd (1987)
Table 2.

Opportunities to reduce consumptive water use through crop shifting.

LocationSeasonMost-Intensive CropRequired Water (m3/ha)Least-Intensive CropRequired Water (m3/ha)Potential Water Savings from Substitution (%)Source
Australia 2008–2009 Rice 14,100 Cereal crops not for grain, seed, or hay 2,200 84% ABS (2010)
New South Wales 2008–2009 Rice 14,100 Cereal crops not for grain, seed, or hay 1,900 87% ABS (2010)
Victoria 2008–2009 Fruit and nut trees 4,600 Cereal crops not for grain, seed, or hay 2,100 54% ABS (2010)
Queensland 2008–2009 Cotton 5,800 Cereal crops for hay 2,200 62% ABS (2010)
South Australia 2008–2009 Fruit and nut trees 7,300 Cereal crops for grain or seed 2,100 71% ABS (2010)
Western Australia 2008–2009 Nurseries, cut flowers, and cultivated turf 8,700 Grapevines 1,300 85% ABS (2010)
Tasmania 2008–2009 Vegetables 3,400 Grapevines 1,100 68% ABS (2010)
Northern Territory 2008–2009 Grapevines 8,400 Fruit and nut trees 2,700 68% ABS (2010)
Murray–Darling Basin 2008–2009 Rice 14,100 Cereal crops not for grain, seed, or hay 2,000 86% ABS (2010)
Egypt Winter 2009–2011 Perennial clover 6,945 Fenugreek 2,930 58% Dawoud (2014)
Egypt Summer 2009–2011 Sugar cane 22,793 Sunflower 5,952 74% Dawoud (2014)
California, USA 2013 Rice 13,716 Potatoes 5,181 62% USDA (2013)
Iowa, USA 2013 Potatoes 3,962 Tomatoes 609 85% USDA (2013)
Texas, USA 2013 Rice 7,010 Beans 1,524 78% USDA (2013)
LocationSeasonMost-Intensive CropRequired Water (m3/ha)Least-Intensive CropRequired Water (m3/ha)Potential Water Savings from Substitution (%)Source
Australia 2008–2009 Rice 14,100 Cereal crops not for grain, seed, or hay 2,200 84% ABS (2010)
New South Wales 2008–2009 Rice 14,100 Cereal crops not for grain, seed, or hay 1,900 87% ABS (2010)
Victoria 2008–2009 Fruit and nut trees 4,600 Cereal crops not for grain, seed, or hay 2,100 54% ABS (2010)
Queensland 2008–2009 Cotton 5,800 Cereal crops for hay 2,200 62% ABS (2010)
South Australia 2008–2009 Fruit and nut trees 7,300 Cereal crops for grain or seed 2,100 71% ABS (2010)
Western Australia 2008–2009 Nurseries, cut flowers, and cultivated turf 8,700 Grapevines 1,300 85% ABS (2010)
Tasmania 2008–2009 Vegetables 3,400 Grapevines 1,100 68% ABS (2010)
Northern Territory 2008–2009 Grapevines 8,400 Fruit and nut trees 2,700 68% ABS (2010)
Murray–Darling Basin 2008–2009 Rice 14,100 Cereal crops not for grain, seed, or hay 2,000 86% ABS (2010)
Egypt Winter 2009–2011 Perennial clover 6,945 Fenugreek 2,930 58% Dawoud (2014)
Egypt Summer 2009–2011 Sugar cane 22,793 Sunflower 5,952 74% Dawoud (2014)
California, USA 2013 Rice 13,716 Potatoes 5,181 62% USDA (2013)
Iowa, USA 2013 Potatoes 3,962 Tomatoes 609 85% USDA (2013)
Texas, USA 2013 Rice 7,010 Beans 1,524 78% USDA (2013)

ABS = Australian Bureau of Statistics, USDA = United States Department of Agriculture.

As an illustration of our evaluation of question 2(i) above, we found many studies involving infrastructure improvements in irrigation delivery systems, such as lining earthen canals with concrete or replacing ditches with pipes for the purpose of reducing leakage. However, very few of these studies addressed the fate of leaked water before the improvements were made, leaving open the question of whether the leaked water infiltrated into soils and became available for subsequent use, or was instead evaporated or taken up by weeds or other vegetation. The fact that these studies did not document return flows was not taken as a cause for rejecting the study, because it is reasonable to assume that in many settings leaked water would not be recoverable for subsequent use, or could be returned to a river or other water source to support environmental benefits.

The water-saving potential of most of the measures listed in Tables 1 and 2 will vary with climatic region (e.g. evaporation potential) or with type of crop or other vegetation. Our objective was not to comprehensively project the volume or percentage of water savings that can be expected in every setting; instead, we have sought to illustrate the likelihood and the relative magnitudes of water-saving potential available using various types of strategies or technologies. We also did not attempt to quantitatively rank the ease, reliability, or cost of applying each of these measures due to very large differences in skills, labor costs, and economies across the geographies in which these measures could be applied.

However, our literature review did lead to a general conclusion that strategies for reducing beneficial consumptive use through crop management – i.e. by temporary fallowing or shifting to a new crop type – stand out in terms of reliability in water savings, and may be some of the easiest and cheapest strategies to pursue. It is also clear from our cursory review of crop prices and trends across many different regions that possibilities exist for farmers to shift to alternate crops, thereby saving substantial volumes of water while enabling them to sustain or even increase agricultural revenues (see Tables 2 and 3). For example, a shift from rice to cereal grains in the Murray–Darling Basin of Australia could be expected to earn 50% more revenue per unit of water consumed (Table 3) while at the same time consuming 86% less water (Table 2). Even greater revenue and water-saving benefits could be realized from other crop-shifting transitions, particularly when converting to crops with very high water productivity. Such crop shifting will in many instances entail upfront capital costs in converting farm fields from one crop type to another, and may in some instances also require expenditures in new farm machinery or irrigation infrastructure to enable more-efficient irrigation of the new crop type. However, such conversions can be expected to yield attractive revenue and water benefits in many cases.

Table 3.

Comparison of water productivity for various crops, Australia and California.

Australiaa
Californiab
Crop Type(AU$/gigalitre of water)Crop TypeUS$/acre-foot of water
Nurseries 15.5 Vegetables, horticulture 14,318
Vegetables 4.6 Cucurbits 6,343
Fruit and nuts 2.9 Fresh tomatoes 5,621
Grapes 1.7 Fruits 3,281
Cotton 0.6 Vine grapes 3,129
Pasture (hay) 0.3 Onions, garlic 2,046
Cereals 0.3 Potatoes 2,046
Other broadacre crops 0.3 Almonds, pistachios 1,452
Rice 0.2 Cotton 692
Sugar beets 685
Grains 628
Corn 609
Rice 524
Safflower 460
Alfalfa 357
Irrigated pasture 91
Australiaa
Californiab
Crop Type(AU$/gigalitre of water)Crop TypeUS$/acre-foot of water
Nurseries 15.5 Vegetables, horticulture 14,318
Vegetables 4.6 Cucurbits 6,343
Fruit and nuts 2.9 Fresh tomatoes 5,621
Grapes 1.7 Fruits 3,281
Cotton 0.6 Vine grapes 3,129
Pasture (hay) 0.3 Onions, garlic 2,046
Cereals 0.3 Potatoes 2,046
Other broadacre crops 0.3 Almonds, pistachios 1,452
Rice 0.2 Cotton 692
Sugar beets 685
Grains 628
Corn 609
Rice 524
Safflower 460
Alfalfa 357
Irrigated pasture 91

aAverage of 2005–2008 data from Australian Bureau of Statistics, Gross Value of Irrigated Agricultural Production.

## Successful efforts to transfer saved water to other uses

Our research also revealed a large number of programs in which water savings in irrigated agriculture have been successfully reallocated for environmental restoration or other uses (Table 4). Most of these transfers of saved water have been executed within the context of formal water rights systems, in which farmers have ceded their right to irrigate on either a temporary or permanent basis and sold some or all of their water rights to other water users or conservation interests. Three examples illustrate the variety of actors involved in these transactions as well as the funding sources that have enabled water transfers from irrigation to other uses. In each of these three cases, overall agricultural revenues have increased even while consumptive water use was reduced (more details are available in Richter (2016)).

• San Diego County and Imperial Irrigation District, USA – The San Diego County Water Authority has entered into an agreement with the Imperial Irrigation District that compensates farmers willing to temporarily fallow their farm land, or to implement other water-saving measures such as canal lining. The transfer of the saved water to San Diego County presently accounts for more than one-third of the water authority's supply, at a volume of more than 200 million cubic metres each year (Richter, 2016). Total compensation paid by the water authority to the irrigation district presently exceeds US$60 million each year. • Murray–Darling Basin, Australia – The Murray–Darling Basin Plan, adopted in 2012, calls for a consumptive use reduction of 2,750 million cubic metres, representing nearly 20% of water rights used for irrigation within the basin (Richter, 2014). The goals of this program include recovering water for the environment and enhancing overall water security. The Australian Commonwealth government has appropriated AU$8.9 billion1 for this purpose, with AU$3.1 billion for direct purchase of water rights from farmers and another AU$5.8 billion for implementation of water-saving measures on farms. As of June 2016, a total of 2,432 million cubic metres of water rights had been secured for the environment (Australian Government, 2016).

To supplement these public efforts, The Nature Conservancy, a non-governmental conservation organization, in 2015 launched a Murray–Darling Basin Balanced Water Fund using private impact investment funds and philanthropic contributions to enable purchases of water rights from farmers (The Nature Conservancy, 2015). As of June 2016, 8.3 million cubic metres had been acquired by the fund. On average, 20% of the fund's water allocations will be dedicated to environmental purposes.

• Chaobai River Basin, China – In response to a persistent decade-long drought that began in 1999 and the loss of more than half of its municipal reservoir storage capacity, the Beijing Municipal People's Government entered into an agreement with upstream farmers in the Chaobai River basin to shift from paddy rice irrigation to dryland crops (primarily corn), in an effort to increase water flows into Miyun Reservoir. Nearly 7,000 hectares were enrolled in the program, which has paid farmers 8,250 yuan (∼US\$1,244) per hectare per year, resulting in a net profit of more than 33% for farmers, and reduced their time spent in farming by 137 days per hectare (Wu et al., 2013). The project resulted in increased flows to the Miyun municipal reservoir of 29 million cubic metres per year.

Table 4.

Examples of programs that have transferred water savings to the environment or other users.

Program NameLocationYear StartedYear EndedPermanent or Temporary Transfers?Total Volume Transferred (MCM)**Original Water UseNew Water UseCitation
USA
Colorado Instream Flow Program State of Colorado 1973 ongoing permanent 369 various environmental flow CWCB (2016)
Colorado Instream Flow Program State of Colorado 1973 ongoing temporary 71 various environmental flow CWCB (2016)
Colorado Water Trust State of Colorado 2001 ongoing both 23 primarily agricultural or municipal environmental flow Colorado Water Trust (2016)
Columbia Basin Water Transaction Program Columbia River Basin (many States) 2002 ongoing permanent 99 various environmental flow National Fish & Wildlife Foundation (2016a)
Columbia Basin Water Transaction Program Columbia River Basin (many States) 2002 ongoing temporary 1,300 various environmental flow National Fish & Wildlife Foundation (2016a)
Great Basin Land and Water Trust, Truckee River Program Truckee River Basin (States of Nevada, Utah, and California) 1998 ongoing permanent 10 irrigation environmental flow Great Basin Land & Water (2016)
Idaho Water Transaction Program Upper Salmon River Basin (State of Idaho) 2003 ongoing temporary 24 various environmental flow Idaho Department of Water Resources (2016)
Imperial Irrigation District Quantification Settlement Agreement Imperial Valley (State of California) 2003 ongoing temporary 1,479 irrigation municipal Imperial Irrigation District (2014b)
Juniper Ridge Irrigation Hydroelectric Pipeline Project Deschutes River Basin (State of Oregon) 2009 2010 permanent 18 canal leakage environmental flow LifeLast (2016)
New Mexico Strategic Water Reserve State of New Mexico 2005 ongoing temporary various environmental flow New Mexico Interstate Stream Commission (2015)
Palo Verde Irrigation District & Metropolitan Water District Agreement State of California 2004 ongoing temporary 37 irrigation municipal Palo Verde Irrigation District (2015)
Scott River Water Trust Scott River Basin (State of California) 2007 ongoing temporary irrigation environmental flow Scott River Water Trust (2016)
Shasta Water Transactions Program Shasta River Basin (State of California) 2012 ongoing temporary irrigation environmental flow The Nature Conservancy (2016)
Swalley Irrigation District Piping Project Deschutes River Basin (State of Oregon) 2005 2007 permanent irrigation environmental flow Aylward (2013)
The Freshwater Trust's Flow Restoration Program Deschutes River Basin (State of Oregon) 1996 ongoing permanent 29 irrigation environmental flow National Fish & Wildlife Foundation (2016a)
Trans-Pecos Water and Land Trust Rio Grande River Basin (State of Texas) 2005 ongoing temporary irrigation environmental flow Trans-Pecos Water & Land Trust (2016)
Tumalo Irrigation District Bend Feed Canal Piping Project State of Oregon 1999 2002 permanent irrigation environmental flow Aylward (2013)
Walker Basin Restoration Walker Lake Basin (State of Nevada) 2010 ongoing permanent 22 irrigation environmental flow National Fish & Wildlife Foundation (2016b)
Australia
On-Farm Irrigation Efficiency Program, Rounds 1–3 Murray–Darling Basin (many States) 2010 2013 permanent 20 irrigation environmental flow and irrigation Cutting (2013)
Private Irrigation Infrastructure Program for South Australia, Round 2 South Australia 2012 ongoing permanent irrigation environmental flow and irrigation Cutting (2013)
Murray–Darling Basin Balanced Water Fund Murray–Darling Basin (multiple States) 2016 ongoing temporary 2 per year on average irrigation environmental watering of wetlands Richter (2016)
China
Paddy Rice to Dryland Crop Project (PPRDC) Miyun Reservoir, Chaobi River 2006 ongoing permanent 29 irrigation municipal Wu et al. (2013)
Zhangye Water Reallocation Heihe River Basin 2000 ongoing permanent 70 irrigation industrial Wang et al. (2015)
Japan
Agricultural Water Reorganization Measures (AWRM) Tone River Basin 1968 2003 permanent 347 irrigation municipal Matsuno et al. (2007)
Taiwan
Changhwa and Yunlin Irrigation Association & Formosa Petrochemical Corporation Taiwan 1997 2003 temporary 341 irrigation industrial Huang et al. (2007)
Program NameLocationYear StartedYear EndedPermanent or Temporary Transfers?Total Volume Transferred (MCM)**Original Water UseNew Water UseCitation
USA
Colorado Instream Flow Program State of Colorado 1973 ongoing permanent 369 various environmental flow CWCB (2016)
Colorado Instream Flow Program State of Colorado 1973 ongoing temporary 71 various environmental flow CWCB (2016)
Colorado Water Trust State of Colorado 2001 ongoing both 23 primarily agricultural or municipal environmental flow Colorado Water Trust (2016)
Columbia Basin Water Transaction Program Columbia River Basin (many States) 2002 ongoing permanent 99 various environmental flow National Fish & Wildlife Foundation (2016a)
Columbia Basin Water Transaction Program Columbia River Basin (many States) 2002 ongoing temporary 1,300 various environmental flow National Fish & Wildlife Foundation (2016a)
Great Basin Land and Water Trust, Truckee River Program Truckee River Basin (States of Nevada, Utah, and California) 1998 ongoing permanent 10 irrigation environmental flow Great Basin Land & Water (2016)
Idaho Water Transaction Program Upper Salmon River Basin (State of Idaho) 2003 ongoing temporary 24 various environmental flow Idaho Department of Water Resources (2016)
Imperial Irrigation District Quantification Settlement Agreement Imperial Valley (State of California) 2003 ongoing temporary 1,479 irrigation municipal Imperial Irrigation District (2014b)
Juniper Ridge Irrigation Hydroelectric Pipeline Project Deschutes River Basin (State of Oregon) 2009 2010 permanent 18 canal leakage environmental flow LifeLast (2016)
New Mexico Strategic Water Reserve State of New Mexico 2005 ongoing temporary various environmental flow New Mexico Interstate Stream Commission (2015)
Palo Verde Irrigation District & Metropolitan Water District Agreement State of California 2004 ongoing temporary 37 irrigation municipal Palo Verde Irrigation District (2015)
Scott River Water Trust Scott River Basin (State of California) 2007 ongoing temporary irrigation environmental flow Scott River Water Trust (2016)
Shasta Water Transactions Program Shasta River Basin (State of California) 2012 ongoing temporary irrigation environmental flow The Nature Conservancy (2016)
Swalley Irrigation District Piping Project Deschutes River Basin (State of Oregon) 2005 2007 permanent irrigation environmental flow Aylward (2013)
The Freshwater Trust's Flow Restoration Program Deschutes River Basin (State of Oregon) 1996 ongoing permanent 29 irrigation environmental flow National Fish & Wildlife Foundation (2016a)
Trans-Pecos Water and Land Trust Rio Grande River Basin (State of Texas) 2005 ongoing temporary irrigation environmental flow Trans-Pecos Water & Land Trust (2016)
Tumalo Irrigation District Bend Feed Canal Piping Project State of Oregon 1999 2002 permanent irrigation environmental flow Aylward (2013)
Walker Basin Restoration Walker Lake Basin (State of Nevada) 2010 ongoing permanent 22 irrigation environmental flow National Fish & Wildlife Foundation (2016b)
Australia
On-Farm Irrigation Efficiency Program, Rounds 1–3 Murray–Darling Basin (many States) 2010 2013 permanent 20 irrigation environmental flow and irrigation Cutting (2013)
Private Irrigation Infrastructure Program for South Australia, Round 2 South Australia 2012 ongoing permanent irrigation environmental flow and irrigation Cutting (2013)
Murray–Darling Basin Balanced Water Fund Murray–Darling Basin (multiple States) 2016 ongoing temporary 2 per year on average irrigation environmental watering of wetlands Richter (2016)
China
Paddy Rice to Dryland Crop Project (PPRDC) Miyun Reservoir, Chaobi River 2006 ongoing permanent 29 irrigation municipal Wu et al. (2013)
Zhangye Water Reallocation Heihe River Basin 2000 ongoing permanent 70 irrigation industrial Wang et al. (2015)
Japan
Agricultural Water Reorganization Measures (AWRM) Tone River Basin 1968 2003 permanent 347 irrigation municipal Matsuno et al. (2007)
Taiwan
Changhwa and Yunlin Irrigation Association & Formosa Petrochemical Corporation Taiwan 1997 2003 temporary 341 irrigation industrial Huang et al. (2007)

**MCM = million cubic metres, CWCB = Colorado Water Conservation Board.

## Essential policies and regulations to enable successful water savings and transfers

The criticisms and skepticism of water savings in irrigated agriculture as reported in recent technical literature are well founded. However, our research group concluded that the inadequacies found in implementation and reporting of water savings are attributable to readily identifiable shortcomings in water governance that can be addressed through reform in water policy and water-use regulations. Some of the specific enabling conditions and reporting requirements that will be essential to successful implementation include the following.

• Establishing rights-based water allocation systems: Abundant evidence from around the globe suggests that in the absence of a governance mechanism – such as issuance of surface water rights or well permits or other communal arrangements – to regulate allowable extractions, a water resource is likely to be depleted to exhaustion (Wada et al., 2012; Richter, 2014, 2016; OECD, 2015; Brauman et al., 2016). The issuance of well-defined rights, supported by proper monitoring and enforcement, is also essential to the process of transferring water use rights to other users or the environment, and accounting for the cumulative consumptive use of a water source. It is particularly important to base the volume of the water right on allowable consumptive use – rather than on allowable withdrawal volumes – so that the net impact of the use on the hydrologic system can be readily ascertained. The total volume of consumptive use authorized through water rights should be explicitly tied to water availability, with consideration of the volume of water that needs to be left in freshwater ecosystems at any time to support their ecological health. The Australian system of issuing water entitlements – defining the maximum volume of water that could be consumed in a year by an entitlement holder – combined with a process for issuing seasonally- or annually-adjusted, percentage-based allocation shares (i.e. as x% of entitlement) – has proven very successful for this purpose (Richter, 2014; Young, 2015).

• Capping consumptive water use: Placing a limit on the total volume of water that can be consumed from each water source is an essential condition for success, as it enables governmental authorities or irrigation districts to prevent users from overusing a water resource (Richter, 2014, 2016). Such a cap can be constructed through limiting the total volume of rights or permits that are issued, but it should include a mechanism for adjusting consumptive use according to the seasonal or annual variability in water supply, as discussed in the bullet above. The implementation of a cap can be greatly aided by infrastructural mechanisms, such as control gates on irrigation canals, that enable regulators to limit, monitor, and enforce the allowable flow of water into an area.

• Allowing for transfers of water-use rights: The issuance of well-defined water rights not only enables regulation of total consumptive use in accordance with capped limits, but it also provides a means of accounting for transfers or trading of water-use rights among different users or sectors. Each of the water transfer programs listed in Table 4 and the three case studies highlighted in the previous section are based upon the temporary or permanent trading of water-use rights. These transfers can be greatly aided by the establishment of a well-functioning water market, in which the holders of water-use rights can be incentivized to reduce their consumptive use because of their ability to sell the portion of their rights that is no longer needed (Debaere et al., 2014; Richter, 2016).

## Summary and conclusions

Given that one-third of all freshwater sources on the planet are now being heavily depleted by excessive water consumption, and given that irrigated agriculture accounts for more than 90% of that consumption in water-scarce basins, it is essential to identify and implement strategies that can reduce consumptive irrigation use while maintaining agricultural production and livelihoods. Additionally, given the dire state of freshwater species and ecosystems in water-stressed regions, much of the saved water needs to be used to augment freshwater systems that have been excessively depleted.

Our research has documented reliable, credible ways to reduce consumptive water use in irrigated agriculture, and we have also identified many programs – both governmental and private – that have shown how to transfer water savings to other uses, including environmental restoration. However, success in these endeavors will require strict attention to proper water budget accounting in farm applications, as well as strong governance over water resources to reverse the pervasive trend of increasing depletion of the planet's freshwater sources.

1

All references to billion in this paper represent 109.

## References

References
Australian Bureau of Statistics (ABS)
(
2010
).
Water Use on Australian Farms 2008–09
.
ABS
,
Canberra
.
Australian Government
(
2016
).
Environmental Water Holdings. Available at https://www.environment.gov.au/water/cewo/about/water-holdings (accessed 16 August 2016)
.
Aylward
B.
(
2013
).
Environmental water transactions: water management
.
Chapter 6
in
Water Transactions Handbook: A Practitioner's Handbook
.
Aylward
B.
(ed.).
Ecosystem Economics
,
Bend, Oregon
.
Boyd
C. E.
(
1987
).
Evapotranspiration/evaporation (E/Eo) ratios for aquatic plants
.
Journal of Aquatic Plant Management
25
,
1
3
.
Brauman
K.
Richter
B. D.
Postel
S.
Malsy
M.
Florke
M.
(
2016
).
Water depletion: an improved metric for incorporating seasonal and dry-year water scarcity into water risk assessments
.
Elementa
doi: 10.12952/journal.elementa.000083
.
Cetin
O.
Bilgel
L.
(
2002
).
Effects of different irrigation methods on shedding and yield of cotton
.
Agricultural Water Management
54
,
1
15
.
(
2016
).
Completed transactions. Available at: http://cwcb.state.co.us/environment/instream-flow-program/Pages/CompletedTransactions.aspx (accessed 16 August 2016)
.
(
2016
).
Available at: http://www.coloradowatertrust.org/ (accessed 16 August 2016)
.
Cutting
M.
(
2013
).
Irrigation efficiency investment in the SA Murray-Darling basin region
. In:
Presentation at Irrigation Australia Regional Conference
,
28–30 May 2013
,
Griffith, New South Wales
,
Australia
. .
Dawoud
S. D. Z.
(
2014
).
Economic optimal allocation of irrigation water in Egypt
.
Journal of Development and Agricultural Economics
6
,
472
480
.
Debaere
P.
Richter
B. D.
Davis
K. F.
Duvall
M. S.
Gephart
J. A.
O'Bannon
C. E.
Pelnik
C.
Powell
E. M.
Smith
T. W.
(
2014
).
Water markets as a response to scarcity
.
Water Policy
16
,
625
649
.
Dzikiti
S.
Schachtschneider
K.
Naiken
V.
Gush
M.
Moses
G.
Le Maitre
D. C.
(
2013
).
Water relations and the effects of clearing invasive Prosopis trees on groundwater in an arid environment in the Northern Cape, South Africa
.
Journal of Arid Environments
90
,
103
113
.
Enciso
J.
Jifon
J.
Anciso
J.
Ribera
L.
(
2015
).
Productivity of onions using subsurface drip irrigation versus furrow irrigation systems with an Internet based irrigation scheduling program
.
International Journal of Agronomy
.
http://dx.doi.org/10.1155/2015/178180
.
Geerts
S.
Raes
D.
Garcia
M.
Vacher
J.
Mamani
R.
Mendoza
J.
Huanca
R.
Morales
B.
Miranda
R.
Cusicanqui
J.
C.
(
2008
).
Introducing deficit irrigation to stabilize yields of quinoa (Chenopodium quinoa Willd)
.
European Journal of Agronomy
28
,
427
436
.
Gleason
D. J.
(
2013
).
Evapotranspiration-based Irrigation Scheduling Tools for Use in Eastern Colorado
.
MS Thesis
.
,
Fort Collins, CO
, p.
225
.
Gleeson
T.
Y.
Bierkens
M. F. P.
van Beek
L. P. H.
(
2012
).
Water balance of global aquifers revealed by groundwater footprint
.
Nature
488
,
197
200
.
Gleick
P. H.
Christian-Smith
J.
Cooley
H.
(
2011
).
Water-use efficiency and productivity: rethinking the basin approach
.
Water International
36
,
784
798
.
Great Basin Land & Water
(
2016
).
Available at: http://www.greatbasinlandandwater.org/ (accessed 16 August 2016)
.
Hanson
B. R.
Schwankl
L. J.
Schulbach
K. F.
Pettygrove
G. S.
(
1997
).
A comparison of furrow, surface drip, and subsurface drip irrigation on lettuce yield and applied water
.
Agricultural Water Management
33
,
139
157
.
Hoekstra
A. Y.
Mekonnen
M. M.
(
2012
).
The water footprint of humanity
.
Proceedings of the National Academy of Sciences
109
,
3232
3237
.
Huang
C. C.
Tsai
M. H.
Lin
W. T.
Ho
Y. F.
Tan
C. H.
Sung
Y. L.
(
2007
).
Experiences of water transfer from the agricultural to the non-agricultural sector in Taiwan
.
5
,
271
277
.
Idaho Department of Water Resources
(
2016
).
Transferring a water right. Available at: https://www.idwr.idaho.gov/water-rights/transfers/search.html (accessed 16 August 2016)
.
Imperial Irrigation District
(
2014a
).
Imperial Irrigation District Fallowing Program Status Report
.
IID
,
Imperial
,
California
.
Available at: http://www.iid.com/home/showdocument?id=9292 (accessed 28 July 2016)
.
Imperial Irrigation District
(
2014b
).
Quantification Settlement Agreement Implementation Report 2010–2013
.
Imperial
,
California
.
Iniesta
F.
Testi
L.
Goldhamer
D. A.
Fereres
E.
(
2008
).
Quantifying reductions in consumptive water use under regulated deficit irrigation in pistachio (Pistacia vera L.)
.
Agricultural Water Management
95
,
877
886
.
Klocke
N. L.
Currie
R. S.
Aiken
R. M.
(
2009
).
Soil water evaporation and crop residues
.
Transactions of the American Society of Agricultural and Biological Engineers
52
,
103
110
.
Lamm
F. R.
(
2005
).
SDI for conserving water in corn production
.
Impacts of Global Climate Change
,
doi: 10.1061/40792(173)557
.
Lankford
B. A.
(
2006
).
Localising irrigation efficiency
.
Irrigation and Drainage Systems
55
,
345
362
.
Lankford
B. A.
(
2012
).
Fictions, fractions, factorials and fractures; on the framing of irrigation efficiency
.
Agricultural Water Management
108
,
27
38
.
Lankford
B. A.
(
2013
).
Resource Efficiency Complexity and the Commons: The Paracommons and Paradoxes of Natural Resource Losses, Wastes and Wastages
.
Routledge
,
London
.
LifeLast
(
2016
).
Case study: Juniper Ridge Irrigation Hydroelectric Pipeline. Available at: http://www.lifelast.com/case-studies/juniper-ridge-irrigation-hydroelectric-pipeline/ (accessed 16 August 2016)
.
Linquist
B. A.
Anders
M. M.
M. A. A.
Chaney
R. L.
Nalley
L. L.
Da Rosa
E. F. F.
Van Kessel
C.
(
2015
).
Reducing greenhouse gas emissions, water use, and grain arsenic levels in rice systems
.
Global Change Biology
21
,
407
417
.
Lozano
D.
Ruiz
N.
Gavilán
P.
(
2016
).
Consumptive water use and irrigation performance of strawberries
.
Agricultural Water Management
169
,
44
51
.
Matsuno
Y.
Hatcho
N.
Shindo
S.
(
2007
).
Water transfer from agriculture to urban domestic users: a case study of the Tone River Basin, Japan
.
5
,
239
246
.
Medellin-Azuara
J.
Lund
J.
Howitt
R.
(
2015
).
Jobs per Drop Irrigating California Crops
.
California WaterBlog, University of California at Davis, Center for Watershed Sciences
,
28 April 2015
.
Mitchell
J. P.
Singh
P. N.
Wallender
W. W.
Munk
D. S.
Wroble
J. F.
Horwath
W. R.
Hogan
P.
Roy
R.
Hanson
B. R.
(
2012
).
No-tillage and high-residue practices reduce soil water evaporation
.
California Agriculture
66
,
55
61
.
Molle
B.
Tomas
S.
Hendawi
M.
Granier
J.
(
2012
).
Evaporation and wind drift losses during sprinkler irrigation influenced by droplet size distribution
.
Irrigation and Drainage
61
,
240
250
.
National Fish & Wildlife Foundation
(
2016a
).
Columbia Basin Water Transactions Program. Available at: http://www.cbwtp.org/jsp/cbwtp/index.jsp (accessed 16 August 2016)
.
National Fish & Wildlife Foundation
(
2016b
).
Walker Basin Restoration Program: 2010–2015 Program Report
.
NFWF
,
Washington DC
.
New Mexico Interstate Stream Commission
(
2015
).
Interstate Stream Commission's Annual Legislative Report for 2015 on the Strategic Water Reserve
. .
Newton
D.
Perle
M.
(
2006
).
Irrigation District Water Efficiency Cost Analysis and Prioritization
.
Deschutes Water Alliance
,
Bend
,
Oregon
.
Organisation for Economic Co-operation and Development (OECD)
(
2015
).
Drying Wells, Rising Stakes: Towards Sustainable Agricultural Groundwater Use
.
OECD Studies on Water, OECD Publishing
,
Paris
.
Palo Verde Irrigation District
(
2015
).
Calendar Year 2014 Fallowed Land Verification Report
.
PVID
,
Blythe, California
, p.
17
.
Perry
C.
(
2007
).
Efficient irrigation; inefficient communication; flawed recommendations
.
Irrigation and Drainage
56
,
367
378
.
Perry
C.
Steduto
P.
Allen
R. G.
Burt
C. M.
(
2009
).
Increasing productivity in irrigated agriculture: agronomic constraints and hydrological realities
.
Agricultural Water Management
96
,
1517
1524
.
Perveen
S.
Krishnamurthy
C. K.
Sidhu
R. S.
Vatta
K.
Kaur
B.
Modi
V.
Fishman
R.
Polycarpou
L.
Lall
U.
(
2012
).
Restoring Groundwater in Punjab, India's Breadbasket: Finding Agricultural Solutions for Water Sustainability
.
Columbia Water Center White Paper, Columbia University
,
New York
, p.
28
.
Poff
N. L.
Allan
J. D.
Bain
M. B.
Karr
J. R.
Prestegaard
K. L.
Richter
B. D.
Sparks
R. E.
Stromberg
J. C.
(
1997
).
The natural flow regime: a paradigm for river conservation and restoration
.
BioScience
47
,
769
784
.
Postel
S.
Richter
B.
(
2003
).
Rivers for Life: Managing Water for People and Nature
.
Island Press
,
Washington DC
.
Richter
B.
(
2014
).
Chasing Water: A Guide for Moving from Scarcity to Sustainability
.
Island Press
,
Washington DC
.
Richter
B.
(
2016
).
Water Share: Using Water Markets and Impact Investment to Drive Sustainability
.
The Nature Conservancy
,
Washington DC
.
Richter
B. D.
Baumgartner
J. V.
Wigington
R.
Braun
D. P.
(
1997
).
How much water does a river need?
Freshwater Biology
37
,
231
249
.
Richter
B. D.
Abell
D.
Bacha
E.
Brauman
K.
Calos
S.
Cohn
A.
Disla
C.
Friedlander O'Brien
S.
Hodges
D.
Kaiser
S.
Loughran
M.
Mestre
C.
Reardon
M.
Siegfried
E.
(
2013
).
Tapped out: how can cities secure their water future?
Water Policy
15
,
335
363
.
Schneider
A. D.
Howell
T. A.
(
1993
).
Reducing sprinkler losses
. In:
Proceedings of the Central Plains Irrigation Shortcourse
,
Sterling, CO
,
2–3 February 1993
, pp.
43
46
.
Scott
C. A.
Vicuña
S.
Blanco-Gutiérrez
I.
Meza
F.
Varela-Ortega
C.
(
2014
).
Irrigation efficiency and water-policy implications for river basin resilience
.
Hydrology and Earth System Sciences
18
,
1339
1348
.
Scott River Water Trust
(
2016
).
Annual Reports, 2009–2012
. Available at: http://www.scottwatertrust.org/about.html (accessed 16 August 2016)
.
Silber
A.
Israeli
Y.
Elingold
I.
Levi
M.
Levkovitch
I.
Russo
D.
Assouline
S.
(
2015
).
Irrigation with desalinated water: a step toward increasing water saving and crop yields
.
Water Resources Research
51
,
450
464
.
Stewart
W. L.
Fulton
A. E.
Krueger
W. H.
Lampinen
B. D.
Shackel
K. A.
(
2011
).
Regulated deficit irrigation reduces water use of almonds without affecting yield
.
California Agriculture
65
,
90
95
.
Tagar
A.
Chandio
F. A.
Mari
I. A.
Wagan
B.
(
2012
).
Comparative study of drip and furrow irrigation methods at farmer's field in Umarkot
.
World Academy of Science, Engineering and Technology
69
,
863
867
.
The Nature Conservancy
(
2015
).
Information Memorandum: The Murray-Darling Basin Balanced Water Fund
. .
The Nature Conservancy
(
2016
).
Shasta Water Transactions Program
.
Available at: http://www.casalmon.org/Shasta-Water-Transaction-Program (accessed 21 September 2016)
.
Trans-Pecos Water and Land Trust
(
2016
).
Water Rights Acquisition Project
. (
accessed 16 August 2016)
.
United Nations
(
2001
).
Water – a key to sustainable development: recommendations for action
. In:
International Conference on Freshwater
,
Bonn
,
Germany
,
3–7 December 2001
.
United Nations
(
2014
).
Driving Harmonization of Water-Related Terminology. CEO Water Mandate, in collaboration with Alliance for Water Stewardship, CDP, Ceres, The Nature Conservancy, Water Footprint Network, World Resources Institute, and WWF
.
United Nations
(
2015
).
Transforming our World: the 2030 Agenda for Sustainable Development, Resolution 70/1 adopted by the General Assembly on 25 September 2015
.
Unlu
M.
Kanber
R.
Koc
D.
Ozekici
B.
Kekec
U.
Yesiloglu
T.
Ortas
I.
Unlu
F.
Kapur
B.
Tekin
S.
Kathner
J.
Gebbers
R.
Zude
M.
Peeters
A.
Ben-Gal
A.
(
2014
).
Irrigation scheduling of grapefruit trees in a Mediterranean environment throughout evaluation of plant water status and evapotranspiration
.
Turkish Journal of Agriculture and Forestry
38
,
908
915
.
US Department of Agriculture (USDA)
(
2013
).
2012 Census of Agriculture. Crops Grown in the Open and Pasture Data
.
USDA
,
Washington DC
.
van Donk
S. J.
Martin
D. L.
Irmak
S.
Melvin
S. R.
Petersen
J. L.
Davison
D. R.
(
2010
).
Crop residue cover effects on evaporation, soil water content, and yield of deficit-irrigated corn in west-central Nebraska
.
Transactions of the American Society of Agricultural and Biological Engineers
53
,
1787
1797
.
Y.
van Beek
L. P. H.
Bierkens
M. F. P.
(
2012
).
Nonsustainable groundwater sustaining irrigation: a global assessment
.
Water Resources Research
48
,
W00L06
,
doi:10.1029/2011WR010562
.
Wang
X.
Yang
H.
Shi
M.
Zhou
D.
Zhang
Z.
(
2015
).
Managing stakeholder's conflicts for water reallocation from agriculture to industry in the Heihe River Basin in Northwest China
.
Science of the Total Environment
505
,
823
832
.
Weeks
E. P.
Weaver
H. L.
Campbell
G. S.
Tanner
B. D.
(
1987
).
Water Use by Saltcedar and by Replacement Vegetation in the Pecos River Floodplain Between Acme and Artesia, New Mexico. USGS Professional Paper 491-G
,
United States Geological Survey
,
Washington DC
, p.
33
.
WEF (World Economic Forum)
(
2016
).
The Global Risks Report
.
World Economic Forum
,
Geneva
.
Young
M.
(
2015
).
Unbundling Water Rights: A Blueprint for Development of Robust Water Allocation Systems in the Western United States
.
Nicholas Institute, Duke University
,
Durham, North Carolina
.
Zhang
X.
(
2002
).
Linking water balance to irrigation scheduling: a case study in the piedmont of Mount Taihang
. In:
Regional Water and Soil Assessment for Managing Sustainable Agriculture in China and Australia
,
McVicar
T. R.
Li
R.
Walker
J.
Fitzpatrick
R. W.
Liu
C.
(eds).
ACIAR Monograph No. 84
, pp.
57
69
,
Australian Centre for International Agricultural Research
,
Canberra, Australia
.