Part 2
Fundamentals of Irrigation
1
Fundamentals of irrigation
Topics
• Why irrigate?
• Water needs:
– Plants and water
– Soil and water
– The Irrigation cycle
2
Why irrigation is good
• May be the only means to permit
agriculture (mainly in arid regions)
• Increase in annual yields (double
cropping)
• May enable higher value crops to be
grown
• Crops can be harvested at chosen
times – not when the rain falls
• Yields can be controlled
3
Why irrigation is bad
• Badly applied water can permanently
damage soil
– Erosion
– Salination
– Leaching
• Standing water can spread disease
• Social control is needed
– State control – dependence, loss of control
– Private control – marginalisation, loss of
control
4
Why irrigation is bad (cont’d)
• Farmers may become vulnerable to
outside forces beyond their control
– Fuel for pumping
– Pump competition
– Competition for water sources
• Reservoirs (dams)
• Potential for failure – ruin – unrest
5
Why irrigation is bad (cont’d)
Considerations
• Biologically optimum water may not
match commercial optimum water
– Water efficiency of crop
• Irrigation must exceed water deficit
– Non-uniform application
– Unintended losses
– Irrigation water is impure (salination)
6
Why irrigation is bad (cont’d)
Considerations
• Good management essential
– Too little water – dead or stunted crop
– Too much water – dead or stunted crop
– Water needs met in an untimely way –
dead or stunted crop
– If water supply does not meet demand
there will be conflict
7
Considerations: Planning
8
Considerations: Planning
9
Considerations: Some criteria
• Energy requirement
• Capital intensity
• Labour intensity
– In building
– In running/maintaining
• Efficiency
– Losses
– Excess runoff
– Excess wash through (percolation)
10
Considerations: Water sources
Source Energy
needs
Rivers either coming from a wetter zone or maintained by aquifers low
during dry season
Reservoirs or lakes filled during rains and drawn down during irrigation 0
season
Naturally sustained aquifers (water stored in the ground) accessed via med-high
wells
Fossil (unsustained aquifers) until they deplete high
Artificially sustained aquifers replenished by controlled percolation or med-high
injection
Waste water from a household or a city med
11
Considerations: Methods of application
Application method Labour Capital ‘Energy’ Efficiency
Recession irrigation low 0 0 n.a.
Gravity-fed surface methods – basin, low- low 0 0.3 – 0.6
border, furrow med
Sub-surface pipes low high low n.k.
Pitcher/drip (continuous slow release) med high med 0.7 – 0.9
Spraying med- high high 0.6 – 0.8
high
Bucket (very small scale agriculture) v high v low 0 ~0.7
12
Fundamentals of irrigation
Is it worth it?
• Value of crop
– will it repay the investment? Is it worth
employing sophisticated methods?
• Climate
– Is the land marginal? Will some temporal
readjustment be beneficial (more or better
crops)?
• Topography
– how will the system be laid out? Will
pumping be needed?
• Water
– How much water do you need? Is it
available?
13
Fundamentals of irrigation
Crops and water: Transpiration
long wave radiation
Solar radiation
convection
evaporation
Reflection
Negligible thermal mass
Measured in mm/day
14
Crops and water: Crop coefficient
ETcr = Crop evapotranspiration
(mm/day)
Kc = Crop coefficient
ETcr Kc ETo
ETo = Reference crop
evapotranspiration
(mm/day)
15
Crops and water: Crop coefficient
16
Crops and water: Crop coefficient
Relative humidity >70% (humid) <20% (dry) Growing
period
Mid Final Mid Final (days)
season growth season growth
Barley 1.1 0.25 1.2 0.2 120-165
Green beans 0.95 0.85 1.0 0.9 75-90
Maize 1.1 0.55 1.2 0.6 80-110
Millet 1.05 0.3 1.15 0.25 105-140
Sorghum 1.05 0.5 1.15 0.55 120-130
Cotton 1.1 0.65 1.2 0.65 180-195
Tomatoes 1.1 0.6 1.2 0.65 135-180
Cabbage/Cauliflower 1.0 0.85 1.2 0.95 80-95
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Crops and water: Pan coefficient
18
Crops and water: Pan coefficient
ETcr = Reference crop
evapotranspiration
ETo K p E pan
Kp = Pan coefficient
Eoan =Pan evaporation
19
Crops and water: Pan coefficient
Cropped area Dry Fallow area
Humidity <40% 40-70% >70% <40% 40-70% >70%
Light wind 0.65 0.75 0.85 0.60 0.70 0.80
Moderate wind 0.60 0.70 0.75 0.55 0.65 0.70
Strong wind 0.55 0.60 0.70 0.50 0.55 0.65
Very strong wind 0.5 0.65 0.60 0.40 0.5 0.55
20
Fundamentals of irrigation
Soil and water: The soil reservoir
21
Soil and water: Water content of the soil
• Gravity water: Water that drains
through the soil into the water table –
not usually considered available to
plants
• Capillary water: water held in
interstices in the soil – available to
plants
• Hydroscopic water: water chemically
bonded to the soil - not usually
considered available to plants
22
Soil and water: Water content of the soil
23
B3.1.3 Fundamentals of irrigation
Soil and water: Available water
% mm/m
Fine sand 2-3% 30-50
Sandy loam 3-6% 40-100
Silt loam 6-8% 60-120
Clay loam 8-14% 90-210
Clay 13-20% 190-300
24
Soil and water: The root zone
Used 80% of total
60%
40%
20%
Average 50%
25
B3.1.3 Fundamentals of irrigation
Soil and water: Available water
Root depth (full grown)
Shallow
Beans 0.6-0.7 m
Grass 0.4-0.6 m
Rice 0.5-0.7 m
Medium
Barley 1.0-1.5m
Grains (small) 0.9-1.5 m
Sweet potatoes 1.0-1.5 m
Tomatoes 0.7-1.5 m
Deep
Alfalfa 1.0-2.0 m
Orchards 1.0-2.0 m
Maize 1.0-2.0 m
26
Soil and water: The root zone:
Wilting point
Plant sucks water from interstices in soil
Less water in the soil need greater suction
At some point (the wilting point) the plant is
losing more water than it is gaining
Finally, the plant uses more if its internal water
than it can recover and wilts permanently
(permanent wilting point)
27
Soil and water: Useful water
Wp = Water used by plant (mm)
f = factor (~0.5)
Wp f Wa Wpwp dr Wa = Available water (mm/m)
Wpwp = Permanent wilting point
(mm/m)
dr = Root depth (m)
28
Soil and water: Soil water balance
Precipitation (P) Evapotranspiration
(E)
Surface inflow
Runoff (R)
and Irrigation (F)
Drainage (D) & deep percolation
29
B3.1.3 Fundamentals of irrigation
Soil and water: Soil water balance
S = Water stored in soil (mm)
F = Surface inflow and irrigation
(mm)
S F P E D R P = Precipitation (mm)
E = Evapotranspiration (mm)
D = Drainage (mm)
R = Runoff (mm)
30
The irrigation cycle
• When the wilting point is reached, the
plant needs replenishment
– Application of irrigation is needed
• Water should rise above the field
capacity
– Saturation, ponding, salination
31
The irrigation cycle: How much?
• If application is at permanent wilting point:
Wa = Water applied (mm)
f = factor (~0.5)
Wa f W fc Wpwp dr Wfc = Field capacity (mm/m)
Wpwp = Permanent wilting point
(mm/m)
dr = Root depth (m)
32
The irrigation cycle: When?
T = Time between irrigations
(days)
Wa Wa = Water applied (mm)
T
ETcr P ETcr = Crop evapotranspiration
(mm/day)
P = Precipitation (mm/day)
33
The irrigation cycle: For how long?
Infiltration
Ta = Application time (hr)
Wa
Ta Wa = Water applied (mm)
I
I = Infiltration rate (mm/hr)
34
The irrigation cycle: For how long?
Infiltration
35
The irrigation cycle: For how long?
Infiltration
36
The irrigation cycle: For how long?
infiltration
37
The irrigation cycle: For how long?
infiltration
mm/hr
Sand 30
Sandy loam 20-30
Silt loam 10-20
Clay loam 5-10
Clay 1-5
38
The irrigation cycle: Notes
• So far we have made no account of
irrigation efficiency (~0.3-0.8) so we
will have to increase volume and time
of application
• Water may not be available for the
needed rate of application – action will
need to be taken
39
