factors affecting effective rainfall - Scheduling
Crop water requirement - Crop
coefficients for various crops.
Estimation of Crop water
requirement - field water balance
Irrigation requirement is the total
amount of water applied to the land
surface in supplement to the water supply
through rainfall and soil profile, to meet
the water needs of crops for optimum
The water can be directly measured
with the help of water measuring devices
like flumes, notches, orifices and water
NET IRRIGATION REQUIREMENT
The net irrigation requirement is the amount
of irrigation water just required to bring the
soil moisture content in the root zone depth
of the crops to field capacity.
( Mfci Mbi ). Ai.Di
= net amount of water to be applied during an irrigation, cm
= field capacity moisture content in the ith layer of the soil
= moisture content before irrigation in the ith layer of the soil
= bulk density of the soil in the ith layer
= depth of the ith soil layer, cm, within the root zone and
= number of soil layers in the root zone D.
THE GROSS IRRIGATION REQUIREMENT (IR) AT THE HEAD,
i 1 Ea
IR = seasonal gross irrigation requirement at
the field head
d = net amount of water to be applied at
Ea = water application efficiency
n = number of irrigation in a season
ESTIMATING WATER REQUIREMENT OF CROPS
Water requirement, includes the losses due to
ET (or Cu) plus the losses during the
application of irrigation water (unavoidable
losses) and the quantity of water required for
special operations such as land preparation,
transplanting, leaching, etc.
WR = ET or Cu + application losses + special needs
Water requirement is a ‘demand’ and the
‘supply’ which would consist of contributions
from any of the sources of water, the major
sources being the irrigation water (IR),
effective rainfall (ER) and soil profile
WR - (ER + S)
PET rarely occurs in most of the irrigated
field crops, with an exception of low land
rice and probably for two to three days
immediately after irrigation or rain.
For converting PET values into ET suitable
crop coefficients (kc) should be evolved for
different crops, soils and climatic conditions
and also for different stages of growth for
the same crop.
ET (crop) = kc .PET
Effective rainfall - factors
affecting effective rainfall Scheduling – Irrigation
EFFECTIVE RAINFALL (Re)
Precipitation falling during the growing
period of a crop that is available to
meet the evapotranspiration needs of
the crop is called effective rainfall.
From the point of view of the
water requirement of crops
FAO (Dastane, 1974) has
defined the annual or seasonal
effective rainfall as the part of
the total annual or seasonal
rainfall, which is useful directly
and / or indirectly for the crop
production at the site where it
falls, but without pumping.
Ineffective rainfall is that
portion which is lost by
surface runoff, unnecessary
deep percolation losses,
the moisture remaining in
the soil after the harvest of
the crop and which is not
useful for next season crop.
FACTORS AFFECTING EFFECTIVE RAINFALL
Characteristics of the soil
Ground water characteristics
Carry-over soil moisture
Ground water distribution
Surface and sub-surface in-and out-flows
The water content at field capacity
(the upper limit of the regime) was
considered as 100 percent available
for crop growth and that at the
permanent wilting point as 0
percent available. The safe limit of
allowable soil water depletion (the
lower limit of regime) for a crop was
determined by experimentation and
it was taken as a criterion for a
Climatic parameters play a predominant role in
governing the water needs of crops and the criterion of
soil water availability for scheduling of irrigations cannot
be considered in isolation from that of climatic factors.
The concepts of evapotranspiration and evaporation is
used as the criteria for timing of irrigations.
According to this concept the plant was considered
simply as an aqueduct pumping water retained in the
soil and losing it to the atmosphere under the influence
of atmospheric sink.
Some physiological stages of growth were found to be
more critical in their demand for water than others. In
this way, the soil, the plant and the climatic factors
influence the water needs of crops.
Crop response to water at different
The water requirement of crops vary with
the stage of its growth.
When water supply is limited, it is necessary
to take into account the critical stages of
crop growth with respect to moisture.
The term critical stage is commonly used to
define the growth when plants are most
sensitive to shortage of water causing
drastic reduction in yield.
Growth stages of cereals in relation to irrigation
(Salter and goode, 1967);
The appearance of the radicle
The formation of tillers, i.e, branches produced
from the base of the stem
The stage when 2 nods can be seen, i.e., the
beginning Of shooting
The stage of elongation of internodes
The end of the shooting stage and just prior to the
Emergence of ears
The emergence of the ear from the formed by
the leaf Sheaths
The opening of the flowers
Grain formation The period of grain development from fertilization
Grain formation subdivided
: Grain contents have a
Dough stage: Grain contents have a
: Grain contents have a
: Grain contents are hard
: Grain ripe for harvesting.