LCA Food 2012 in Saint Malo, France! - Manifestations et colloques ...

GROUP 2, SESSION A: CARBON OR WATER FOOTPRINTS, SOIL, BIODIVERSITY 8 th Int. Conference on LCA in the
Agri-Food Sector, 1-4 Oct 2012
63. Green, blue, and grey water use of Canadian wheat and maize
Brian McConkey 1,* , Chao Chen 1 , Dominique Maxime 2 , Suren Kulshrestha 3 , Xavier Vergé 4 , Ray Desjardins 5 ,
Devon Worth 5 , Jim Dyer 6
1 Agriculture and Agri-Food Canada, Swift Current, SK, Canada, 2 Centre interuniversitaire de recherche sur
le cycle de vie des produits, procédés et services, Montréal, QC, Canada, 3 Department: Bioresource Policy,
Business and Economics, University of Saskatchewan, Saskatoon, SK, Canada, 4 Consultant, Ottawa, ON,
Canada, 5 Agriculture and Agri-Food Canada, Ottawa, ON, Canada, 6 Consultant, Cambridge, ON, Canada,
Corresponding author. E-mail: brian.mcconkey@agr.gc.ca
Wheat (Triticum aestivum L.) and maize (Zea mays L.) are the major cereal crops of Canada and their use for
producing bioethanol has heightened interest in their environmental footprint. In semiarid climates of western
Canada, both green and blue water supply are a constant concern. Rainfed agriculture periodically requires
“drought disaster” financial support from government. Irrigation is limited as water in major streams
is fully allocated and water restrictions can occur during drought years. In contrast, water excesses are more
often a concern than deficiencies in the more humid climates of central and eastern Canada.
We used the DSSAT model v4 to estimate 30-yr average green (evapotranspiration of in situ precipitation)
and blue (from water withdrawals) use and yield across a range of important cereal production situations
(Table 1). The model was validated with field data. Estimated blue water use embodied in inputs (pesticide,
fossil fuel, machinery manufacture, fertiliser) was about 1 m 3 (t grain) -1 based on withdrawals. Grey water
use was calculated from P losses from the literature. The emerging Canadian guideline for total P in water to
prevent eutrophication (0.02 mg L -1 ) is 500 times lower than drinking water standard for N (10 mg L -1 as N-
NO3). Since total N losses are about 2 to 10 times greater than those of P, roughly 50-250 times more water
is needed to dilute the P than the N (divide by about 100 to compare with N-based studies).
The total cereal water use in Canada was dominated by grey water (Table 1). Grey water use is higher in
humid climates due to larger P loss in runoff and artificial drainage. Grey water use usually exceeds any
precipitation surplus and this explains why P is the major environmental concern since there is often insufficient
water to dilute the P to a desired concentration in cropland dominated watersheds. Total
(green+blue+grey) water use decreased moving from humid to more arid climate. In contrast and as expected,
the blue + green water use t -1 tended to increase moving to drier climate (e.g. London to Winnipeg,
Lethbridge or Winnipeg to Swift Current). However, the relationship between climate and green+blue water
use was less clear than total water use, probably due to confounding effects of other weather factors (temperature,
rain timing, and sunlight) on production. As expected, maize was more water efficient than wheat
under rainfed conditions but efficiencies between these cereals was similar under irrigation. In the most
semiarid areas, summer fallow is still widely practiced where a crop is purposefully not grown in one year so
as to use the soil-stored precipitation from that year to reduce drought risk for the crop grown the next year.
A systems level calculation that considers the fallow year as an inseparable part of wheat production on
summer fallow has the highest cereal water use t -1 in Canada while the simplistic non-systems analysis that
considers the crop year only would indicate that summer fallow actually decreases water use t -1 ; we believe
only the systems-level analysis is valid. Excluding summer fallow production, green +blue wheat water use
t -1 did not vary much across the diverse climates or with and without irrigation.
If lower green+blue+grey water use t -1 were used as the sole indicator of greater water security for cereal
production in Canada, it would result in the nonsensical conclusion that production should be increased in
the region with regular water shortages and decreased in regions with abundant water. This would exacerbate
the impact of droughts in water-stressed regions to the whole of Canada. Inter-regional comparisons of
green+blue water use were more difficult to interpret although there was an inconsistent trend of higher water
use t -1 as climate became drier. Intra-regional comparisons of water use t -1 help identify crops and production
methods that would, if selected, reduce water requirements for cereal production and lower natural
resource requirements for biofuels produced from those cereals.
747