Transcript No Slide Title

Surface Irrigation
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Basic types
• flood retreat
• surface
• furrow
• corrugation
• border strip
• basin
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Advantages and disadvantages
Advantages
 the large expenditure on storage, headworks,
and distribution canals is usually paid for by the
Government, and the capital cost on the farm is
low - this is an advantage (for the farmer) in the
case of peasant agriculture:
 surface irrigation may be more likely to be
traditionally understood,
 surface irrigation is more suitable for some
crops, such as rice and forage crops;
 leaching is easier and cheaper if it is needed
 surface methods can use large flows available
for short periods, and so allows canalised water
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supply to be easily shared by several farmers
Disadvantages
 difficult to get even distribution of water on light
permeable soils which have high infiltration rates;
 surface irrigation is not suitable for crops which
need frequent light watering (shallow rooted
and/or drought susceptible crops);
 efficient surface irrigation usually requires smooth
land and if the land is not naturally smooth, it may
be expensive to level it and the topsoil and fertility
can be disturbed;
 field layout for surface irrigation may restrict
mechanisation unless special measures are adopted
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Land levelling and its effects
Methods




grid surveys;
deep ripping;
land planes;
use of lasers
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Some problems associated with land levelling
An example of the effect of exposing sub-soil on the
surface after land levelling on a field of cotton in
Iran.
Cut areas are where soil has been excavated.
Fill areas are where soil has been deposited during
land levelling
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Flood retreat
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Not really a method of irrigation but many
people depend on flooding for crop
production.
Problem often caused by dam construction
such as in Kenya (Turkwell dam) - see New
Sci. 18/6/87; p35
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Furrow irrigation
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Construction
 Constructed with reversed mouldboard ploughs or
specially designed ridgers
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Shape and size of furrows
• must be sufficiently high to take discharge
without overtopping
• depth typically 15 to 20 cm
• top width typically 25 to 30 cm for most
situations but wide, flat ridges needed for
some (clay) soils or crops
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Following figures shows water distribution from
smaller closely spaced and larger, widely spaced
furrows.
Closely spaced furrows
entire root zone is wetted before the wetting
front reaches the moist soil below, and deep
percolation losses are minimised.
Distance between furrows is more than twice the
distance to the moist soil,
water penetrates to moist soil below and is lost by
deep percolation while the crop roots continue to
be dry.
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• spacing function of the crop and tillage
machinery used - for single row crops = 75
to 105 cm
• plant on side near top to avoid salinity (if
planted at top of ridge) and waterlogging
(if planted at bottom of furrow)
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Furrow slopes
 discharge is function of S1/2
 upper limit is about 2% slope but if there is high
intensity rainfall and erodible soils, upper limit may
be as little as 0.3%
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Allowable slopes for furrows :-
Soil
Maximum
1
slope
sand
0.25%
sandy loam
0.4%
silt loam
0.5%
clay
2.0 – 2.5%
loam
5 to 6.25%
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Need not follow slope of land with furrow irrigation some side-slope is permissible.
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Distribution problem and control of inflow
 Advance & recession wave
 need water to infiltrate to bottom of root zone at
end of field (this changes as the crop is
established)
 Apply maximum possible inflow to get wetting front
to the bottom of the field then cut back to reduce
wastage - ideally water should continue to reach
bottom of field but only a small amount should
drain off
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 Inflow regulated by adjusting spile orifice or
removing one or more of a bank of siphons
(perhaps 5 to start reduced to 2)
 Using more small siphons -> better control but
more expensive
 Infiltration rate onto a steep field will be less
than onto a lower slopes
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Another consideration ….
If the water is turned before wetting fron reaches wet soil
below, there will be negligible loss through deep percolation
because the excess water that drains out of the large pores
of the soil behind the wetting front is utilised in wetting the
dry soil beneath.
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Furrow stream
 methods of getting water onto the field varies:
 breach bank - poor control - large openings can get
larger through erosion
 siphons
 spiles - usually of aluminium
 Withers and Vipond quote 45/s l/min where s is
the % slope (or 0.6/s in litres/sec) but this is only
the median value - try values either side in field
trials - will depend on soil type, slope, & dimensions
of furrow - OR calculate from Mannings equation
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Flow through an orifice is given in British units
(cusecs and ft) as :
Q  0.61A 2gH
However some trials I did indicated that flow in
siphons was proportional to d 2.5 (where d is diameter)
- may be because the flow at the smaller diameters
were more turbulent
Flow must also depend on the length and material of
the siphon - use published graphs rather than
equations
Stream is typically 3 l/sec on relatively flat land
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initially high then reduce to reduce runoff
If stream is too fast, there will be erosion in the
furrows
If stream is too slow, irrigation will take too long
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Following table shows the relation of maximum nonerosive flow rates to critical slopes in furrows based
on the equation Qm = C/S seen in some books
However, the erosive slope will depend on the type of
soil and also the velocity down a slope is inversely
proportional to the square root of the slope not the
reciprocal of the slope.
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Furrow
slope (S)
[%]
0.1
Maximum flow rate
(Qm)
-1
l sec
gpm
6
100
0.3
2
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0.5
1.2
20
2.0
0.3
5
Comments
Flow rate indicated is about double the
capacity of most furrows in normal use
on 0.1% slope. Erosion is negligible
with furrows flowing to capacity with
this slope.
A slope of 0.3% is near the upper limit
where furrows flowing at full capacity
will not cause serious erosion.
Cultivated furrows with 0.5% slope will
cause erosion unless the flow rate is
considerably less than the furrow
capacity.
This indicates the reduction in flow
rate needed to prevent serious erosion
on a 2.0% slope. This is considered to
be the maximum slope allowable for
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cultivated furrows.
Surge Irrigation
Gated pipes attached to pump- water pressure
varied. Advance faster, leaching less.
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Duration of irrigation
Evaluation of required duration of irrigation, T is
required to be calculated unless tensiometers or
neutron probes are used to indicate when sufficient
water has been applied
Phillips equation is not easy to solve for t. For this
situation, the use Kostiakov's equation is suggested:
I = Kta
log I = log K + a log t
Plot log I against log t to obtain K (from the
intercept) and a (from the slope).
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T ( )
Ir
K
1

where Ir is the required depth of irrigation to be
applied. T should be taken as the uptake
opportunity at the end of the field.
If the time to reach the bottom of the field is >
25% of T then field size needs to be changed.
We will come back to this shortly
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The following figure shows an example scheme for
moving siphons in a furrow irrigation scheme.
The amount of water flowing out of the head ditch is
constant but the block of furrows being irrigated is
changing as time progresses.
The numbers refer to the number of siphons in each
block of 100 furrows.
Thus, a value of 200 means there are 2 siphons per
furrow in order to “push” the wetting front down to
the far end of the field.
Thereafter the number is reduced.
The shaded squares shows the full picture at time =
16 hours and for furrow block D throughout its
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irrigation cycle.
Time
(hours)
Block (100 furrows long)
A
B
C
D
E
F
G
H
0
100
100
100
200
0
4
100
100
100
100
200
0
8
100
100
100
100
100
200
0
12
100
100
100
100
100
100
200
0
16
0
100
100
100
100
100
100
200
0
20
0
0
100
100
100
100
100
100
200
0
24
0
0
0
100
100
100
100
100
100
200
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0
0
0
0
100
100
100
100
100
100
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0
0
0
0
0
100
100
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Furrow length
Shorter fields needed for soils with high
percolation losses
Percolation losses at the top of the field occur due
to extra time for advance wave to reach the bottom
of the field. Rule of thumb is that time to reach end
of field < 1/4 of time required to apply adequate
water.
Empirically I found that
L = L1t0.7
where L1 is the distance advanced in the first hour
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From the design criteria:
L  L1

T
4
0 .7
where T is the time for the replenishment of deficit.
Thus:
L 
L 1T 0 . 7
2 .6
For most crops, T should be < 48 hrs to avoid waterlogging.
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L1 is a function of :
slope,
infiltration rate as a function of time,
inflow rate and
furrow dimensions.
Fields too long  waterlogging, percolation losses,
rise in water-table, salinity, logistical problems with
equipment.
If fields short  very expensive, wastage on roads,
headlands, field channels & surface drains.
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Another equation that will help to determine the
correct inflow rate is:
L1 = C Q0.5
where Q is the inflow rate.
i.e. to double the L1 quadruple the inflow rate and
vice versa.
Note - increasing the inflow rate too much will cause
erosion within the furrow.
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Exponent 
Fraction
of time
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
t
3.5
6.9
10.4
13.8
17.2
20.5
23.8
27.0
30.2
t/2
2.0
4.1
6.1
8.1
10.1
12.1
14.1
16.1
18.0
t/3
1.4
2.9
4.3
5.7
7.2
8.6
10.0
11.5
12.9
t/4
1.1
2.2
3.3
4.5
5.6
6.7
7.8
8.9
10.0
t/5
0.9
1.8
2.7
3.6
4.6
5.5
6.4
7.3
8.2
t/10
0.5
1.0
1.4
1.9
2.4
2.9
3.3
3.8
4.3
Percentage of infiltrated water that is lost below
root zone for various fractions of total irrigation
time (t) that it takes for the water to reach end of
furrow, & for several values of exponent a in
Kostiakov’s equation
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As already pointed out, the velocity of the water will depend
on the inflow rate and the furrow dimensions - the table is
very simplistic
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Furrow irrigation in layered soils
Water entering dry loam soil underlain with coarse
dry sand. Lines indicate positions of wetting
front at different time intervals.
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Water does not enter sand until loam is nearly
saturated.
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Water from overlying loam draining from isolated
place(s) [fingering] leaving the rest of the sand dry.
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In this photograph, water is seen entering loam
underlain by clay.
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This image shows that when the wetting front
reaches the clay, it enters immediately; but the rate
of advance is retarded because of the slower
hydraulic conductivity
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Corrugations
 Constructed with roller
 Cannot run at angle to slope but otherwise field
design similar to furrows
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Border strip irrigation
(also known as “strip checks” in USA)
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Construction
 use scraper
 water runs directly down-slope - therefore
limitation to non- erosive slopes - cannot put in at
angle to main slope as is possible with furrow
irrigation;
 problem with furrow down border, use side bunds
Width
 6 to 30 m between borders depending on soil type
and slope (see table for suggested designs);
 multiple of tractor widths
 reduce for longer fields
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Slopes
heavier soils - <0.5% slope
lighter soils - <0.3% slope
Top
top of field should be levelled
Length
design of field lengths similar to furrow irrigation
See handout for suggested dimensions for
various soil types
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Distribution of water
During trials, if not uniform across strip, try:
- increasing inflow
- reducing strip width
- increasing no. of inlet points
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Basins
 In small scale irrigation, irrigators often use small
square basins to control the flow. Basins are from
5 m to 20 m sides.
 Minimum of land levelling
 Difference is that water is prevented from running
off until irrigator decides by breaking bank or
through spillway arrangement.
 Sometimes used in large-scale projects with
dimensions as shown in following table.
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Optimum basin areas (ha) for different soil type given various
inflow rates
Soil type
Flow rate
l sec-1
m3 hour-1
Sand
Sandy
loam
Clay
loam
Clay
30
108
0.06
0.12
0.20
60
216
0.02
0.04
0.12
0.24
0.40
90
324
0.06
0.18
0.36
0.60
120
432
0.08
0.24
0.48
0.80
150
540
0.10
0.30
0.60
1.00
180
648
0.12
0.36
0.72
1.20
210
756
0.14
0.42
0.84
1.40
240
864
0.16
0.48
0.96
1.60
270
972
0.18
0.54
1.08
1.80
300
1080
0.20
0.60
1.20
2.00
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Use of clay pot to irrigate vegetables
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see tables comparing different
types of surface irrigation
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