Limits to the world's green water resources for food, feed, fiber, timber, and bioenergy

Abstract

Green water--rainfall over land that eventually flows back to the atmosphere as evapotranspiration--is the main source of water to produce food, feed, fiber, timber, and bioenergy. To understand how freshwater scarcity constrains production of these goods, we need to consider limits to the green water footprint (WF_g), the green water flow allocated to human society. However, research traditionally focuses on scarcity of blue water--groundwater and surface water. Here we expand the debate on water scarcity by considering green water scarcity (WS_g). At 5 × 5 arc-minute spatial resolution, we quantify WF_g and the maximum sustainable level to this footprint (WF_g,m), while accounting for green water requirements to support biodiversity. We then estimate WS_g per country as the ratio of the national aggregate WF_g to the national aggregate WF_g,m We find that globally WF_g amounts to 56% of WF_g,m, and overshoots it in several places, for example in countries in Europe, Central America, the Middle East, and South Asia. The sustainably available green water flows in these countries are mostly or fully allocated to human activities (predominately agriculture and forestry), occasionally at the cost of green water flows earmarked for nature. By ignoring limits to the growing human WF_g, we risk further loss of ecosystem values that depend on the remaining untouched green water flows. We emphasize that green water is a critical and limited resource that should explicitly be part of any assessment of water scarcity, food security, or bioenergy potential.

Keywords: green water; water consumption; water footprint; water scarcity; water sustainability.

Fig. 1.

Allocation of the total green water flow from the terrestrial Earth surface (72 × 10³ km³ y⁻¹). Values are in 1,000 km³ y⁻¹. Arrows represent green water flows from different sorts of land, as indicated by the labels. Overshoot amounts to 1.8 × 10³ km³ y⁻¹ and relates to overuse of green water resources in crop production (0.9 × 10³ km³ y⁻¹), grazing (0.6 × 10³ km³ y⁻¹), wood production (0.2 × 10³ km³ y⁻¹), and urban areas (0.1 × 10³ km³ y⁻¹).

Fig. 2.

The degree of human appropriation of the WF_g,m, at 5 × 5 arc-minute grid cell resolution, expressed as the ratio of the total WF_g to the total WF_g,m. Overshoot occurs where WF_g is located in biodiversity conservation areas to effectively achieve the ABT 11.

Fig. 3.

WS_g per country, expressed as the ratio of the national aggregate WF_g to the national aggregate WF_g,m. Countries with WS_g = 1 have fully allocated their sustainably available green water flow to human activities (or overshoot is canceled out by remaining potential in another part of the country). Country-specific estimates of WF_g, WF_g,m, WS_g, and overshoot as percentage of WF_g are included in SI Appendix, Table S1.