IRRIGATION REQUIREMENT
(i)
NET IRRIGATION REQUIREMENT (NIR)
It is the amount of irrigation water required to bring the soil moisture level in the
effective root zone to field capacity.
It is the difference between field capacity and the soil moisture content in root zone
before starting irrigation
NIR can also be defined as the depth of irrigation water exclusive of precipitation
carry over soil moisture or ground water contribution or other gains in soil moisture
that is required consumptively for crop
(ii)
GROSS IRRIGATION REQUIREMENT
It is the total amount of water applied through irrigation e.i. it is the net irrigation
requirement plus the losses in the water application and other losses. It is computed a
follows
Gross irrigation requirement ( in the field)= Wet irrigation requirement/field
efficiency of system(%)
Example.
Given that the net amount of irrigation is 8cm and the field efficiency is 75 % what is
the gross amount of water to be applied to the field
Gross irrigation requirement = 8cm / 0.75= 10.67 cm
(i)
(ii)
(iii)
(iv)
(v)
(vi)
(vii)
(iii)
Unavoidable losses of water in the field are caused through
Seepage
Leakage in the water conveyor system
Non –uniform distribution of water over the field
Percolation below the crop root zone
Wastage due to surface runoff especially in border and furrows
Evaporation from sprays
Water left on the crop foliage
IRRIGATION INTERVAL (FREQUENTCY)
It refers to the number of days between irrigation during period without rainfall. I t
depends on
(a) The consumptive use rate of a crop
(b) Amount of available moisture in te crop root zone
Irrigation frequency is a function of crop, soil and the climate. Irrigation should have been
used from the zone in which most of the crop roots are concentrated.
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When designing irrigation systems the irrigation system, the irrigation frequency to be used is
the time in days between two irrigation in the period of highest consumptive use of the crop
grown
Design Frequency = Field capacity of soil in the - Moisture content of the same zone at the
time
Effective crop root zone of starting irrigation
Peak period moisture use rate of the crop
( iv ) IRRIGATION PERIOD
It is the number of days that can be allowed for applying one irrigation to a given design area
during peak CU period of the crop being irrigated. It is the basis for irrigation system
capacity and equipment design
The irrigation system must be designed such than the irrigation frequency
Irrigation period = Net amount of moisture in the soil between start of irrigation and lower
limit of moisture depletion / Peak period moisture use rate of crop
(v) IRRIGATION EFFICIENCY
This indicate how efficiently available water supply is being used based on different method
of calculation irrigation efficiency depend on
+Degree of land preparation
+Skill of the irrigator
+ Care of the irrigator
Different types of efficiencies’ include
(a) WATER CONVEYANCE EFFICIENCY (EC)
This is the term used to measure the efficiency of water conveyance associated with the
canal networks, water courses and field channel.
EC=WF/WDX 100
WF- Water delivered to the irrigated plot ( at the field supply channel)
WD- Water diverted from the source
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(b) WATER APPLICATION EFFICIENCY (EA)
It’s the measure of how efficiently the water is applied after the water reached the field
supply channel
EA = Ws/ Wf x 100
Ws - Water stored in the root zone of the plants
Wf - Water delivered to the field (at the field supply channel)
EA is below 100% is due to;
(i) Seepage losses
(ii) Deep percolation below the crop root zone
(iii) Run off losses
Water application efficiently decreases as the amount of water applied during each irrigation
increases
( c) Water storage efficiency (Es)
This measure how completely the water needed prior to irrigation has been store in the crop
root zone during irrigation. It is define as
Es = ws / Wn x 100
Ws – Water store in the root zone during irrigation
Wn - Water needed in the root zone prior to irrigation
This efficiency is important when water supplies are limited and when excessive time is
required to get enough water penetration into the soil which require high water storage
efficiency
(d) Water distribution efficiency (Ed)
This indicates the extent to which water is uniformly disturbed along the run
Ed= (1 – y/d) x 100
y - Average numerical deviation from d
D - Average depth of water stored along the run during irrigation
Other types of efficiency include
+ Water use efficiency
+ Project efficiency
+ Operational efficiency
+economic / irrigation efficiency
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Example 1
Given that a 5cm depth of irrigation water is to be applied to basic 100 long and 6cm wide.
Using a pump discharge rate of 10l /sec and assuming the conveyance system to be 70
calculate
( i ) the total amount of water required
( ii ) The time required in hours to give the 5 cm depth of irrigation
Example 2
A pump discharges water at the rate of 1100 l /hr and works at the rate of 1100 l/ hr and work
for 8 hrs each day. Estimate the area covered by the water lift. If the average rate of
irrigation is 8cm and irrigation period is 15 days
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