Forests need water. Without water, there is no vegetation. When it rains over a forest, the rainwater wets the leaves and the soil surface. The water that temporarily stays on top of the leaves will evaporate when the sun comes out again. The water in the soil will be taken up by the trees and transpire through the leaves, which contributes to the growth of the tree.
In both cases, the water is sent back into the atmosphere and – although it will rain again someday – this water will no longer be available at that specific place and moment in time. The forest has used that water. It has a water footprint.
We need forests. Forests provide a whole range of so-called ecosystem services. Forests regulate temperature and rainfall patterns across the Earth, provide habitat for wildlife, prevent erosion of soils, and they are popular for humans to recreate. Amongst these and many other services, forests also provide wood.
The water use by a forest indirectly serves the ecosystem services the forest generates. In this study, we have estimated which part of forest water use is related to the production of wood and wood-derived products such as lumber, pulp, paper, fuel, and firewood. In other words, we have estimated the water footprint of wood and derived products.
We first estimated the water use of production forests worldwide at a high spatial resolution for each year in the period 1961-2010. Subsequently, we attributed only part of the total forest water use to wood production. We did this based on the relative value of wood production compared to the total value of ecosystem services generated by forests. In the end, we estimated how much wood – and thus indirectly, water – you would need to produce several end products made of wood (see figure).
The figure shows how the global-average water footprint of wood and several derived products has been estimated. The full article contains much larger list of products with their water footprint as well as maps showing the spatial variation in the water footprint of roundwood (wood in the rough). Figure from Schyns et al., 2017.
The value of a forest? Yes, all ecosystem services generated by a forest have a certain value. Wood has a market price. The other ecosystem services also have a value, though these are often not captured in market prices. It is possible to estimate the value of for example climate regulation, habitat and prevention of soil erosion in monetary terms as well. There is a whole field of science devoted to estimating the value of nature. Scholars in that field have estimated the value of ecosystem services in temperate/boreal forests and (sub)tropical forests, which we have used for this study.
We have found that the water footprint of wood increased by 25% over 50 years to 961 billion meters cube per year in 2001-2010. With this, the forestry sector adds about 12% to the total water footprint of humanity, which also includes the agricultural, industrial, and domestic sectors. Our estimate of the water footprint of global wood production contributes to a more complete picture of the human appropriation of water and feeds the debate on water for food, energy, and wood.
We have also learned that intensification of wood production (getting more wood of one hectare of forest) – which is seemingly more efficient – does not necessarily reduce the water footprint per unit of wood. The thing is that although you get more wood value per hectare, other ecosystem services such as habitat and recreation reduce in value once you start exploiting the forest more intensively. The most effective way to reduce the water footprint of the forestry sector is to recycle wood products. By using less water to produce new wood we can leave more water available for the generation of other ecosystem services.
These findings are described in the article entitled The water footprint of wood for lumber, pulp, paper, fuel and firewood, published in the journal Advances in Water Resources. This work was conducted by Joep F. Schyns, Martijn J. Booij, and Arjen Y. Hoekstra from the University of Twente.