Reference: Schyns, J.F., Hoekstra, A.Y., Booij, M.J., Hogeboom, R.J., and Mekonnen, M.M. (2019). Limits to the world’s green water resources for food, feed, fiber, timber, and bioenergy. Proceedings of the National Academy of Sciences. 116:11:4893-4898. https://doi.org/10.1073/pnas.1817380116
Blue vs. Green
The hydrologic cycle is taught in elementary school. Young students learn how water moves into the atmosphere by evaporation and transpiration and then precipitates back to the earth as rain or snow (see figure 1). By sixth grade, most understand that though most water evaporates directly from lakes, oceans, and other water bodies, about 10% of the moisture present in our atmosphere comes directly from plants (liquid water is taken up by their roots and later released as vapor through leaves). This understanding, however, seems lost in the minds of many policy makers. Water research and policy have traditionally focused on blue water, which includes surface and groundwater. Blue water, however, as many sixth graders have learned, is only part of the hydrological equation.
Within the last couple of decades, researchers have begun to call attention to green water. Green water refers to water that falls to land via precipitation and is then taken up by plants to be returned to the atmosphere through evapotranspiration (the other side of the hydrological cycle – see figure 2). Interestingly, green water is the primary class of water used in food, timber, bioenergy, feed, and fiber production. This means that though blue water makes up more of the available water supply, green water is still immensely important to our quality of life. In this paper, Joep Schyns and colleagues investigate the use and availability of green water globally. In response to their findings, they advocate for the inclusion of green water in water scarcity, bioenergy, and food security assessments.
To understand water scarcity more holistically, the authors determined the green water footprint (amount used) of each geographic region, then compared it to the maximum sustainable amount. To do this, they calculated the amount of green water used for livestock grazing, urban areas, and crop and wood production at a 5×5 arc minute resolution, roughly 102 km (or 6.22miles). After determining the amount of green-water being used for human activities, the researchers then quantified the maximum sustainable footprint based on the total green water flow available minus that which is to be reserved for nature.
Maximum sustainable green water flow was calculated based on land accessibility and use, land suitability based on agricultural and ecological factors, and requirements for biodiversity conservation. Understanding how much green water is necessary for biodiversity conservation required the use of a spatially explicit map highlighting areas selected to meet the Aichi Biodiversity Target 11 (this aims to conserve 17% of terrestrial/inland water, as well as 10% of marine/coastal areas, by 2020). Once the maximum sustainable footprint was calculated, it was compared to the actual amount of green water being used for human purposes. The amount of green water scarcity was then determined for each country by looking at the ratio of the green water footprint to the maximum sustainable footprint as a national aggregate.
Feeling blue about the green
The authors found that, globally, 56% of the green water flow that is sustainably available is already being used for human activities, though this percentage varies a great deal by country. Green water use beyond sustainable levels is mostly driven by grazing and crop production. More than half of this unsustainable water use is occurring in only 10 countries (including the United States, Brazil, and India, to name a few – see figure 3). Water scarcity (based on the national aggregate ratio), was found to be most severe in countries within the Middle East, South Asia, Europe, and Central America.
Though there are many variables and uncertainties involved in these types of calculations, the authors consider their findings conservative. Because our world has a limited amount of green water flow, it is important that we recognize that there are limits to the amount we as humans can sustainably use. Already, these limits are being exceeded in many areas. As the human population continues to grow, so will the demand for green water. Thus, inclusion of green water is vital to proper assessment of water scarcity.
For more information on water footprints, or to calculate your own, check out www.waterfootprint.org . Arjen Hoekstra, one of the authors of this paper, is on their supervisory board, and he explains water footprints in a video on the “what is a water footprint” page.