Agriculture Reference
In-Depth Information
tAble 19.1
world Population and Irrigated land
Irrigated Area
(million ha)
Per capita Irrigated
Area (ha person
−1
)
year
Population (billions)
1900
1.5
40
0.027
1950
2.5
94
0.038
1970
3.9
169
0.043
1980
4.5
211
0.047
1990
5.2
239
0.046
2003
6.3
277
0.044
agreement that water scarcity is the biggest single threat to global food and fiber
production. In her topic, Postel (1999) made the case that water productivity (output
from each unit of water abstracted from rivers, lakes, and underground aquifers)
will have to double to have any hope of fulfilling the water requirements of 8 billion
people while protecting the natural ecosystems on which economies and life itself
depend. Indeed, this is a challenge that will not only require significant changes in
how much of the world's water is managed and used, but in new scientific findings
and technologies that will increase the efficient use of water.
defInItIon of teRms
Although it is universally accepted that agriculture is the biggest user of water both
from precipitation and that abstracted from rivers, lakes, and underground aquifers,
the efficiency of water use by agriculture is not always expressed in the same manner
and can be misleading. Therefore, it is very important that writers clearly define vari-
ous terms, and perhaps even more important, readers must make sure they under-
stand specifically what is meant by terms they see in an article. For example, water
use efficiency is a term commonly used in the popular press and scientific literature,
but it is a term that is often used in different ways. The following is a list of terms and
their definitions as used in this chapter:
Transpiration
: Transpiration is the evaporation of water from parts of plants,
especially leaves but also stems, flowers, and fruits. Transpiration is a result
of the need of the plant to open its stomata to obtain carbon dioxide from
the air for photosynthesis. Transpiration also cools plants and enables mass
flow of mineral nutrients from roots to shoots.
Transpiration ratio
: The ratio of the units of aboveground biomass produced
divided by the units of water transpired. For example, a plant with a transpi-
ration ratio of 0.002 requires 500 g of water to be transpired to produce 1 g
of aboveground biomass. Transpiration ratios are seldom determined under
field conditions because it is difficult to separate water used by transpiration
from water evaporated from the soil surface.
