Maclean et al. - 2002 - Rice almanac source book for the most important e

egions where modern rice technologies have yet
to make an impressive impact; area expansion
has been an important source of growth in rice
production. Strategic research is needed for
these regions, not only for maintaining the
natural resource base but also for genetic
improvements of germplasm, so that constraints
to increased rice production imposed by abiotic
and biotic stresses—droughts, floods,
waterlogging, salinity, weeds, pests, and
diseases—can be minimized.
Irrigated rice ecosystem
Physical description
Irrigated rice is grown in bunded fields with
assured irrigation for one or more crops a year.
Rainfall variability is the basis for subdividing
the irrigated ecosystem into irrigated wet season
and irrigated dry season.
Irrigated wet season areas are those where
irrigation water may be added to the rice fields
during the wet season as a supplement to rainfall.
Relatively small volumes of water early in the
season or during a midseason dry period can pay
large dividends in assuring the success of a crop
threatened by erratic precipitation.
Irrigated dry season areas are those where
no rice crop can be grown without supplemental
water during the rice season. Rainfall is usually
very low, cloud cover is minimal, and the levels
of incoming solar radiation are markedly above
those of the rainy season. In the dry season,
evapotranspiration is high and water needs are
considerably greater than during the wet season.
In most areas, the combination of assured water
throughout the season, high solar radiation, low
pest incidence, and high input levels results in
high yields.
Worldwide, about 79 million ha of rice are
grown under irrigated conditions. Average yields
vary from 3 to 9 t/ha. Because the risk of crop
failure is lower than in the other ecosystems,
farmers of irrigated land use more purchased
inputs than in nonirrigated production. Even
though irrigated rice represents only about onehalf
of the world’s rice land, more than 75% of
the world’s rice supply is produced from
irrigated rice. Improved rice cultivars that have
been developed for irrigated rice are short in
height and responsive to N fertilization. Equally
important is their short duration, allowing two
and sometimes three crops per year. Additionally,
most improved cultivars have resistance to
several insects and diseases and some tolerance
for adverse soils.
East Asia, 93% of which is irrigated,
accounts for about 43% of the world’s irrigated
rice area. In the tropics of South and Southeast
Asia, only about 40% of the rice area is irrigated.
In West Africa, only about 10.5% of the rice area
is irrigated.
Productivity
Based on current national or regional yield
averages, the irrigated rice ecosystem can be
divided into high-yielding areas where yields are
>5 t/ha, medium-yielding areas with yields of 4–
5 t/ha, and low-yielding areas that typically
achieve yields of