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. Significance Precipitation over land partitions into runoff via surface water and groundwater (blue water) and evapotranspiration (green water). We expand the traditional debate on water scarcity, which solely focuses on blue water, by assessing green water scarcity. The current debate on water scarcity is heavily skewed, since it leaves unnoticed the bulk of water availability––which is green––and the bulk of water use––which is also green. Green water is the main source of water to produce food, feed, fiber, timber, and bioenergy. Thus, to understand how freshwater scarcity constrains the production of these vital goods, explicating and including (limits to) green water use is imperative.

Research on water scarcity has mainly focussed on blue water (ground- and surface water), but green water (soil moisture returning to the atmosphere through evaporation) is also scarce, because its availability is limited and there are competing demands for green water. Crop production, grazing lands, forestry and terrestrial ecosystems are all sustained by green water. The implicit distribution or explicit allocation of limited green water resources over competitive demands determines which economic and environmental goods and services will be produced and may affect food security and nature conservation. We need to better understand green water scarcity to be able to measure, model, predict and handle it. This paper reviews and classifies around 80 indicators of green water availability and scarcity, and discusses the way forward to develop operational green water scarcity indicators that can broaden the scope of water scarcity assessments.

National water use accounts are generally limited to statistics on water withdrawals in the different sectors of economy. They are restricted to "blue water accounts" related to production, thus excluding (a) "green" and "grey water accounts", (b) accounts of internal and international virtual water flows and (c) water accounts related to consumption. This paper shows how national water-use accounts can be extended through an example for Indonesia. The study quantifies interprovincial virtual water flows related to trade in crop products and assesses the green, blue and grey water footprint related to the consumption of crop products per Indonesian province. The study shows that the average water footprint in Indonesia insofar related to consumption of crop products is 1131 m³/cap/yr, but provincial water footprints vary between 859 and 1895 m³/cap/yr. Java, the most water-scarce island, has a net virtual water import and the most significant external water footprint. This large external water footprint is relieving the water scarcity on this island. Trade will remain necessary to supply food to the most densely populated areas where water scarcity is highest (Java).