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
Fig. 1 is based on United Nations Population Di- vision data and shows that the total population of the
world has increased by 125 since 1950 Fischer and
Fig. 1. Historic and future world population. Source: UN Population Division 1997 Fischer and Heilig, 1997.
Heilig, 1997. This figure also shows the best available estimates of how the world population will increase in
the next 50 years. As these are estimates, three scenari- os are presented based on different assumptions about
fertility rates in women. The most optimistic scenario is for a low fertility rate where the world population
will increase by ∼2 billion or 35. The median pro- jection is for an increase of 65 or an extra 3.7 billion
people. If female fertility rates remain high, then the world population would double by 2050.
Two other factors need to be pointed out. Firstly, most of the world population increase will occur
within the next 25–30 years. This is a very short time to prepare for the huge impacts which will be
described later. Secondly, almost all of the world’s population increase will be in developing countries.
Fischer and Heilig 1997 have pointed out that this developing world ‘population explosion’ is a histori-
cally unique phenomenon. No European country has ever experienced such high population growth rates or
absolute numbers, even during their fastest growth pe- riods. In one sense, Fig. 1 is the equivalent to the time
trend in atmospheric CO
2
concentration measured at the Mauna Loa observatory in Hawaii. More than
any other single illustration of the atmospheric CO
2
problem, that oscillating, but constantly escalating
J.S. Wallace Agriculture, Ecosystems and Environment 82 2000 105–119 107
line conveyed the nature and urgency of the CO
2
is- sue. Although it does not oscillate, the massive and
inexorable increase in the number of human beings in the world should be recognised for what it is — the
most important global change facing mankind.
3. Water resource implications
