The paradox of irrigation efficiency

Abstract

![][1] Sprinkler irrigation supports grape vines in the Okanagan Basin, British Columbia. PHOTO: GRANT FAINT/GETTY IMAGES Reconciling higher freshwater demands with finite freshwater resources remains one of the great policy dilemmas. Given that crop irrigation constitutes 70% of global water extractions, which contributes up to 40% of globally available calories ([ 1 ][2]), governments often support increases in irrigation efficiency (IE), promoting advanced technologies to improve the “crop per drop.” This provides private benefits to irrigators and is justified, in part, on the premise that increases in IE “save” water for reallocation to other sectors, including cities and the environment. Yet substantial scientific evidence ([ 2 ][3]) has long shown that increased IE rarely delivers the presumed public-good benefits of increased water availability. Decision-makers typically have not known or understood the importance of basin-scale water accounting or of the behavioral responses of irrigators to subsidies to increase IE. We show that to mitigate global water scarcity, increases in IE must be accompanied by robust water accounting and measurements, a cap on extractions, an assessment of uncertainties, the valuation of trade-offs, and a better understanding of the incentives and behavior of irrigators. Field IE is the ratio of the volume of all irrigation water beneficially used on a farmer's field [predominantly, evapotranspiration (ET) by crops and salt removal to maintain soil productivity] to the total volume of irrigation water applied (adjusted for changes in water stored for irrigation in the soil) ([ 2 ][3]). Annually, governments spend billions of dollars subsidizing advanced irrigation technologies, such as sprinklers or drip systems ([ 3 ][4]). Sometimes their goal is to increase IE on the understanding that this will allow water to be reallocated from irrigation to cities ([ 4 ][5]), industry, or the environment, while maintaining or even increasing agricultural production. But water saved at a farm scale typically does not reduce water consumption at a watershed or basin scale. Increases in IE for field crops are rarely associated with increased water availability at a larger scale ([ 5 ][6]), and an increase in IE that reduces water extractions may have a negligible effect on water consumption. This paradox, that an increase in IE at a farm scale fails to increase the water availability at a watershed and basin scale, is explained by the fact that previously nonconsumed water “losses” at a farm scale (for example, runoff) are frequently recovered and reused at a watershed and basin scale. Advanced irrigation technologies that increase IE may even increase on-farm water consumption, groundwater extractions ([ 6 ][7]), and water consumption per hectare ([ 5 ][6]). At a farm scale, this can arise from a switch to more water-intensive crops and, with the same crop, may occur when there is a strong marginal yield response from additional water. Moreover, the absence of an increase in water consumption per hectare because of a higher IE does not necessarily mean that the water potentially available for reallocation and reuse (see supplementary materials) at a watershed or basin scale increases. Subsidies for drip irrigation may reduce the water applied per hectare and increase water extractions because a higher IE can induce increases in the irrigated area, as shown for the Lower Rio Grande, New Mexico ([ 7 ][8]). Although the hydrology related to IE has been known for decades, it is often overlooked or ignored. For example, the United Nations (UN) High-Level Panel on Water, comprising 11 sitting heads of state or government, recommends “…incentives for water users, including irrigators, to use water efficiently” ([ 8 ][9]) but fails to explicitly recognize that this may increase, rather than decrease, water consumption. Similar to IE, there is also confusion in policy circles about the effects of an increase in efficiency or water productivity (the biophysical or monetary output per volume of water inputs) on basin-scale water availability (see supplementary materials). The UN Sustainable Development Goal (SDG) 6.4, for instance, seeks to increase water use efficiency, but this does not necessarily mean reduced water extractions. There are reasons why this evidence may be overlooked by policy-makers: Evidence resides in a specialized literature; subsidies for IE can promote rent-seeking behavior by beneficiaries who lobby to continue subsidies; and comprehensive water accounting from the scale of the field to that of the watershed or basin is necessary but frequently absent. Such accounting quantifies field water applications; ET by crops and weeds; evaporation from soil and water surfaces; and, particularly, surface and subsurface water flows returned to the environment or utilized elsewhere at the watershed or basin scale. We respond to the paradox ([ 2 ][3], [ 9 ][10]) with two key insights and a research and policy agenda to deliver on SDG 6 (“ensure availability and sustainable management of water and sanitation for all”). First, irrigation systems are frequently managed to maximize irrigated crop production. This provides benefits but means more water is transpired locally and lost for other uses. Second, locally extracted, but not consumed, water flows to surface supplies and groundwater. Such volumes, perceived as losses to farmers and the irrigation system, do not disappear. They frequently have value and are typically recovered and reused elsewhere in a watershed or basin. The figure visualizes the paradox within a watershed, showing three types of irrigation with different IEs: drip, sprinkler, and surface. Inflows are precipitation and interbasin transfers. Outflows are (i) beneficial water consumption from transpiration by crops; (ii) nonbeneficial water consumption through transpiration by...