Irrigation Water Requirements
Introduction
ü Irrigated
agriculture is facing new challenges that require refined management and
innovative design.
ü Formerly,
emphasis centered on project design; however,current issues involve limited
water supplies with several competing users, the threat of water quality
degradation through excess irrigation, and narrow economic margins.
ü Meeting
these challenges requires improved prediction of irrigation water requirements.
ü Irrigation
water requirements can be defined as the quantity, or depth, of irrigation
water in addition to precipitation required to produce the desired crop yield
and quality and to maintain an acceptable salt balance in the root zone.
ü This
quantity of water must be determined for such uses as irrigation scheduling for
a specific field and seasonal water needs for planning, management, and
development of irrigation projects.
ü The
amount and timing of precipitation strongly influence irrigation water
requirements. In arid areas, annual precipitation is generally less than 10
inches and irrigation is necessary to successfully grow farm crops.
ü In
semiarid areas (those typically receiving between 15 to 20 inches of annual
precipitation), crops can be grown without irrigation, but are subject to
droughts that reduce crop yields and can result in crop failure in extreme
drought conditions.
ü Subhumid
areas, which receive from 20 to 30 inches of annual precipitation, are
typically characterized by short, dry periods.
ü Depending
on the available water storage capacity of soils and the crop rooting depth,
irrigation may be needed for short periods during the growing season in these
areas.
ü In
humid areas, those receiving more than 30 inches of annual precipitation, the
amount of precipitation normally exceeds evapotranspiration throughout most of
the year.
ü However,
drought periods sometimes occur, which reduce yield and impair quality,
especiallyfor crops grown on shallow, sandy soils or that have a shallow root
system.
ü Irrigation
is not needed to produce a crop in most years, but may be needed to protect
against an occasional crop failure and to maintain product quality.
Irrigation requirements
ü The
primary objective of irrigation is to provide plants with sufficient water to
obtain optimum yields and a high quality harvested product.
ü The
required timing and amount of applied water is determined by the prevailing
climatic conditions, the crop and its stage of growth, soil properties (such as
water holding capacity), and the extent of root development.
Water
within the crop root zone is the source of water for crop evapotranspiration.
ü Thus,
it is important to consider the field water balance to determine the irrigation
water requirements.
ü Plant
roots require moisture and oxygen to live.
ü Where
either is out of balance, root functions are slowed and crop growth reduced.
ü All
crops have critical growth periods when even small moisture stress can
significantly impact crop yields and quality.
ü Critical
water needs periods vary crop by crop.
ü Soil
moisture during the critical water periods should be maintained at sufficient
levels to ensure the plant does not stress from lack of water.
The
calculation of irrigation water requirements
ü Delineation
of major irrigation cropping pattern zones.
ü These
zones are considered homogeneous in terms of types of irrigated crops grown,
crop calendar, cropping intensity and gross irrigation efficiency.
ü Represented
on the map of Africa, they should be viewed as regions where some homogeneity
can be found in terms of irrigated crops.
ü The
cropping pattern proposed for the zone should be viewed as representative of an
'average' rather than a 'typical' irrigation scheme.
ü Definition
of the area of influence of the climate stations (in GIS) and quality check on
the climate data.
ü Combination
of the irrigation cropping pattern zones with the climate stations' zones (in
GIS) to obtain basic mapping units.
ü Calculation
of net and gross irrigation water requirements for different scenarios.
ü Comparison
with existing data and final adjustment.
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