Assessing local benefits and impacts from global patterns in agricultural water use

Background/Question/Methods

Agriculture consumes on average 70% of the almost 4,000 cubic kilometers of water extracted worldwide from surface and groundwater for human use. Irrigation is instrumental to worldwide food production, allowing agriculture in arid areas and mitigating the effects of drought.  Water diversion also has environmental impacts, reducing available water for ecosystems downstream. However, global statistics, and even country specific statistics, often do not provide information about the impacts of irrigation on agricultural productivity and on the environment at a scale at which interventions are possible. Using existing global data sets of crop yield, irrigation, and agricultural water use, we identify regions where irrigation has a disproportionate impact on agricultural productivity.  By identifying where and how water is used in irrigation, we can pinpoint places where interventions will have maximum impact, increasing the benefits of agriculture while reducing its harms.

Results/Conclusions

When water extraction exceeds 40% of local water resources, human and natural systems can become stressed and vulnerable to small variations in water availability.  Of all irrigated areas worldwide, 29% exceed the 40% threshold; those areas constitute just 7% of all agricultural land. Where water is scarce, growing crops with higher intrinsic water use efficiency can reduce strain on water resources. However, total water efficiency is low in Africa because of nutrient deficiency: ensuring that irrigated yields are as high as possible by helping farmers engage in good management practices can dramatically increase the value of water applied. A substantial fraction of food production in the developed world is irrigated. In many of these places, however, irrigation is used to subsidize rainfall and counteract dry spells, so only a small fraction of a crops’ total water use comes from irrigation. For example, while China and India both irrigate extensively, multiple cropping in India means crops are grown in the dry season, lowering water efficiency.  Reducing high intensity water use is important not only for environmental protection: Food production is particularly vulnerable to drought and precipitation variability in places where agricultural water use dominates available water resources.