UT rings alarm bells: rainwater is scarcer than ever

Most people are aware of the shortage of blue water. Think of groundwater and surface water shortages. However, the shortage of green water is relatively unknown. Green water is the overland rainfall directly used by plants before evaporating into the atmosphere. UT researchers Joep Schyns, Arjen Hoekstra and colleagues mapped the shortages of green water in the prestigious scientific journal PNAS. Green water is the most important source of water for the production of food, animal feed, fibres, wood and bioenergy. This is the first study that quantifies the limits to the availability of green water on the basis of the limited annual rainfall.

Across the world, rivers and groundwater reserves are diminishing. Now, a new analysis by the University of Twente shows that the sustainable limit to the direct use of rainwater (green water) has been reached in many regions, and exceeded in various countries in Europe, Central America, the Middle East and South Asia.

Bioeconomy versus biodiversity

Rainfall feeds rivers and groundwater reserves, known as blue water, but part of the rainfall is used by plants directly and returns into the atmosphere through evaporation. This green water is the main source of water for the production of food, fibres (for clothing), animal feed, wood and bioenergy. Furthermore, green water is essential for the biodiversity on land, which fully depends on green water. This causes tensions, as the demand for biomass in the human economy is growing, while the availability of green water is limited. 

Green water scarcity

The water scarcity debate traditionally focuses on blue water. The University of Twente complements the picture of the global scarcity of limited freshwater resources through the quantification of green water scarcity. The researchers estimated the volume of green water used for the production of biomass for the human economy. They compared this with the availability of green water. Annual rainfall is limited and a large part falls in areas that are hard to reach, unproductive or essential to the conservation of nature. 

Reconsidering consumption patterns

In their study, the researchers were able to identify the areas of most significant pressure on yet untouched green water flows, which include tropical rainforests. Schyns: “Due to population growth, the consumption of relatively more animal products and energy from biomass, the human green water footprint will only increase. In order to slow down the growing pressure on limited green water, it is important to reconsider our consumption patterns. Reducing our own footprint will reduce the need to ‘grab' green water for agriculture and forestry at the cost of nature elsewhere.”  

The paper Limits to the world’s green water resources for food, feed, fiber, timber, and bioenergy’  by Joep F. Schyns, Arjen Y. Hoekstra, Martijn J. Booij, Rick J. Hogeboom, and Mesfin M. Mekonnen is published in the Proceedings of the National Academy of Sciences of the USA (PNAS).

drs. J.G.M. van den Elshout (Janneke)
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