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Reply to van Noordwijk and Ellison: Moisture recycling: Key to assess hydrological impacts of land cover changes, but not to quantify water allocation to competing demands

Authors:
Joep Schyns at University of Twente
Joep Schyns
Arjen Hoekstra at University of Twente
Arjen Hoekstra
Rick J Hogeboom at University of Twente
Rick J Hogeboom
Martijn J. Booij at University of Twente
Martijn J. Booij
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LETTER
REPLY TO VAN NOORDWIJK AND ELLISON:
Moisture recycling: Key to assess hydrological
impacts of land cover changes, but not to quantify
water allocation to competing demands
Joep F. Schyns
a,1
, Arjen Y. Hoekstra
a,b,1
, Rick J. Hogeboom
a
, and Martijn J. Booij
a
Moisture recycling—the contribution of evapotranspiration
(ET) in a certain area to precipitation in the same area—can
be substantial (1), particularly in forests (2). Land cover
changes can affect local ET and, consequentially, local pre-
cipitation (3, 4). We agree with van Noordwijk and Ellison
(5) that moisture recycling on land is an important process
to consider when assessing the impact of land cover
changes on terrestrial hydrology. However, when we ad-
dress the question in Schyns et al. (6) of how limited green
water resources are being allocated to human use, we are
not interested in changes in hydrology. Rather, we want to
know the extent to which green water flows, as they are,
are made productive for different competing consumption
goods. For this purpose, the rate of moisture recycling is
irrelevant, as we will illustrate by means of an example for
the green water footprint (WF) of forestry products.
Consider a forested area with a precipitation (P) in a
certain time period of 100 water units, which splits up
into 60 units of ET and 40 units of runoff. Suppose an
internal moisture recycling rate of 10%. Suppose that the
forest is a production forest with a wood harvest equal to
the maximum sustainable harvest and no economic
value other than wood production. Thus, according to
standard WF accounting procedures, ET in the basin
(60 units) counts as the green WF of the derived wood
products. The recycled moisture is six units (10% of
60 units). This means that six of the 100 units of P
originate from local ET. Pointing at this moisture recy-
cling, one can argue that the six water units can be used
again and that the green WF should refer only to the part
of the ET that does not come back (7). With such ac-
counting, the green WF would not be 60 water units, it
would be 54. However, one should not count oneself
rich: P remains 100 units since it already includes the
recycled six units; P will not grow to 106 units due to
the recycling. ET, fully used for wood production, re-
mains 60 units. Adjusting WF accounts based on mois-
ture recycling is therefore mistaken. The green WF is not
54 water units, but remains 60 units because the six units
of recycled water are not available for other uses; they
are already used. WF accounting is thus independent of
the rate of evaporation recycling.
Our analysis in Schyns et al. (6) should be inter-
preted not as a call to reduce terrestrial ET, but as a
warning to the increasing human appropriation of lim-
ited ET. To reduce the green WF does not mean to
reduce ET, but rather to reduce the human appropri-
ation of that ET.
1van der Ent RJ, Savenije HHG, Schaefli B, Steele-Dunne SC (2010) Origin and fate of atmospheric moisture over continents. Water
Resour Res 46:W09525.
2Ellison D, et al. (2017) Trees, forests and water: Cool insights for a hot world. Glob Environ Change 43:51–61.
3Bagley JE, Desai AR, Harding KJ, Snyder PK, Foley JA (2014) Drought and deforestation: Has land cover change influenced recent
precipitation extremes in the Amazon? J Clim 27:345–361.
4Spracklen DV, Garcia-Carreras L (2015) The impact of Amazonian deforestation on Amazon basin rainfall. Geophys Res Lett
42:9546–9552.
5van Noordwijk M, Ellison D (2019) Rainfall recycling needs to be considered in defining limits to the world’s green water resources. Proc
Natl Acad Sci USA 116:8102–8103.
6Schyns JF, Hoekstra AY, Booij MJ, Hogeboom RJ, Mekonnen MM (2019) Limits to the world’s green water resources for food, feed,
fiber, timber, and bioenergy. Proc Natl Acad Sci USA 116:4893–4898.
7Berger M, van der Ent R, Eisner S, Bach V, Finkbeiner M (2014) Water accounting and vulnerability evaluation (WAVE): Considering
atmospheric evaporation recycling and the risk of freshwater depletion in water footprinting. Environ Sci Technol 48:4521–4528.
a
Twente Water Centre, University of Twente, 7500 AE Enschede, The Netherlands; and
b
Institute of Water Policy, Lee Kuan Yew School of Public
Policy, National University of Singapore, 259770 Singapore
Author contributions: J.F.S., A.Y.H., R.J.H., and M.J.B. wrote the paper.
The authors declare no conflict of interest.
Published under the PNAS license.
1
To whom correspondence may be addressed. Email: j.f.schyns@utwente.nl or a.y.hoekstra@utwente.nl.
Published online April 9, 2019.
8104
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PNAS
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April 23, 2019
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vol. 116
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no. 17 www.pnas.org/cgi/doi/10.1073/pnas.1903789116
LETTER
... By only looking at water footprints (normalizing water use for crop production), local contexts (e.g. in relation to scarcity or abundance of water) of water are not adequately considered , nor are the downstream and downwind impacts of water use. In a recent discussion, issues around the allocation ('slicing the cake') of green and blue water (assuming unchanged rainfall amounts), were contrasted with land use impacts on the intensity of the hydrological cycle via rainfall recycling ('sizing the cake') (Schyns et al., 2019a, van Noordwijk and Ellison, 2019, Schyns et al., 2019b. Moisture recycling may be key to assessing hydrological impacts of land cover changes, but it is a secondary issue when quantifying water allocation to competing demands. ...
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