Agriculture, Ecosystems and Environment 82 2000 105–119
Increasing agricultural water use efficiency to meet future food production
J.S. Wallace
∗
Centre for Ecology and Hydrology, Wallingford, Oxfordshire OX10 8BB, UK
Abstract
With the world’s population set to increase by 65 3.7 billion by ∼2050, the additional food required to feed future generations will put further enormous pressure on freshwater resources. This is because agriculture is the largest single user of
fresh water, accounting for ∼75 of current human water use. At present ∼7 of the world’s population live in areas where water is scarce. This is predicted to rise to a staggering 67 of the world’s population by 2050. Because of this water scarcity
and because new arable land is also limited, future increases in production will have to come mainly by growing more food on existing land and water. This paper looks at how this might be achieved by examining the efficiency with which water is
used in agriculture. Globally, in both irrigated and rain fed agriculture only about 10–30 of the available water as rainfall, surface or groundwater is used by plants as transpiration. In arid and semi-arid areas, where water is scarce and population
growth is high, this figure is nearer 5 in rain fed crops. There is, therefore, great potential for improving water use efficiency in agriculture, particularly, in those areas where the need is greatest. The technical basis for improving agricultural water use
efficiency is illustrated. This may be achieved by increasing the total amount of the water resource that is made available to plants for transpiration andor by increasing the efficiency with which transpired water produces biomass. It is concluded
that there is much scope for improvement, particularly, in the former and that future global change research should shift its emphasis to addressing this real and immediate challenge. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Global change; Population increase; Agriculture; Water use efficiency
1. Introduction
To date the global change community has had a strong focus on the causes and impacts of increased at-
mospheric CO
2
level, i.e. global warming and climate change. This focus has drawn together an impressive
body of international research effort and has made very significant progress in studying the physical and
physiological processes which are central to under- standing the effects of atmospheric CO
2
increase on vegetation. This research underpins the design of ef-
fective mitigation strategies. However, one disadvan-
∗
Fax: +44-1491-692424. E-mail address: jswceh.ac.uk J.S. Wallace.
tage of this strong ‘carbon’ focus is that other im- portant global issues have, by default rather than by
design, received much less attention. This paper ar- gues that the Earth is facing another important global
change, which is both more important and more cer- tain than changing atmospheric CO
2
concentration. This change is the massive increase in world popula-
tion which will occur within the next 50 years. This has many far reaching implications for both science
and society, but one central issue is the challenge of growing enough food for this increased population
when water resources are limited and already highly exploited, particularly, in those areas of the world
where the population increase is greatest. The techni- cal research agenda which evolves from these water
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106 J.S. Wallace Agriculture, Ecosystems and Environment 82 2000 105–119
resource implications is identified as one of agricul- tural water use efficiency. The current levels of water
use efficiency are outlined and used to demonstrate the scope for improvement. Examples of the technical ba-
sis for improved water use efficiency are given; these show how runoff, soil evaporation and drainage may
be reduced in order to optimise the amount of water re- tained in the soil to support plant growth. The issue of
water use efficiency and scale is also discussed, show- ing how the large scale e.g. catchment water use ef-
ficiency may be increased by recycling run off andor drainage. Finally, there is a plea to the global change
community to apply some of its intellectual capac- ity to addressing the technical agenda which evolves
from the agricultural water use efficiency problem. This agenda is not only scientifically and technically
challenging, but is also of major importance in con- tributing to the solutions to what is arguably one of the
most pressing global issues currently facing mankind.
2. The most important global change
