Agriculture Reference
In-Depth Information
precipitation might also decrease yields of wheat
and maize in these bread-basket regions. A com-
bination of increased temperatures (+2 °C) and
reduced precipitation could lower average yields
by over a fi fth.
The large-scale public surface irrigation sys-
tems built during the green revolution dominated
the landscape until the early 1980s and had a pro-
found impact on the fl ow of many rivers. Over
the last 30 years, private investments, stimulated
by the availability of cheap pumps and well-
drilling capacity, have been directed to tapping
groundwater. Consequently, aquifers are being
depleted in countries with key agricultural pro-
duction systems, including China, India, and
the USA.
The role climate change will play with regard
to water in agriculture must be considered in con-
text of rapid increases in water withdrawals, the
degradation of water quality, and the competition
for water at all levels.
Changes in the distribution of precipitation,
with longer periods between rainfall events and
more intense precipitation, are expected every-
where. This may lead to increased occurrence of
extreme weather events, including fl oods and
droughts. Dry spells, the short periods of rainfall
defi cit during the cropping season, are expected
to increase in duration and frequency. This will
directly affect soil moisture and the productivity
of rainfed crops. Such changes will be felt mostly
in areas already subject to climate variability,
such as in the semiarid and subhumid areas of
sub-Saharan Africa and South Asia, where, in the
absence of alternative sources of water, the risk
of increased frequency of crop failures is high.
Reductions of rainfall in arid and semiarid
areas will translate into a much larger reduction
in river runoff (in relative terms). In Cyprus, for
example, analyses have shown that a 13 % reduc-
tion in rainfall translates into a 34 % reduction in
runoff (Faurès et al. 2010 ). In rivers receiving
their water from glacier or snow melt (about
40 % of the world's irrigation is supported by
fl ows originating in the Himalayas), the timing of
fl ows will change, with high fl ows occurring ear-
lier in the year. However, the mean annual runoff
may be less affected.
The impact of climate change on groundwater
recharge will be reduced in arid and semiarid
areas, where runoff will decline (Table 4.10 )
(Turral et al. 2011 ). In arid and semiarid areas,
climate change will place additional burdens on
already stretched water resources. However, agri-
culture will fi rst need to respond to the challenges
posed by increasing human pressures on these
resources.
Finally, the expected rise in sea levels will
affect agriculture in coastal areas, particularly
river deltas. Higher sea levels combined with
upstream changes (variations in runoff distribu-
tion, more frequent fl oods) will result in an
increased incidence of fl oods and saltwater intru-
sion in estuaries and aquifers.
4.4.1
Changes in Hydrological
Regimes and Shifts
in Precipitation Patterns
The hydrological regimes in which crops grow
will surely change with global warming. While
all GCMs predict increases in mean global pre-
cipitation (because a warmer atmosphere can
hold more water vapor), which are not uniformly
distributed, decreases are forecast in some
regions. The crop water regime may further be
affected by changes in seasonal precipitation,
within-season pattern of precipitation, and inter-
annual variation of precipitation. Increased con-
vective rainfall is predicted to occur, particularly
in the tropics, caused by stronger convection cells
and more moisture in the air.
Too much precipitation can cause disease
infestation in crops, while too little can be det-
rimental to crop yields, especially if dry periods
occur during critical development stages. For
example, moisture stress during the fl owering,
pollination, and grain-fi lling stages is espe-
cially harmful to maize, soybean, wheat, and
sorghum.
The amount and availability of water stored
in the soil, a crucial input to crop growth, will
be affected by changes in both the precipitation
and seasonal and annual evapotranspiration
regimes. Some GCMs predict mid-continental
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