1

Part B3: Irrigation

B3.1 Fundamentals of Irrigation

2

B3.1 Fundamentals of irrigationTopics

• Why irrigate?

• Water needs– Plants and water– Soil and water– The Irrigation cycle

• Available water– Mass curves, flow-duration curves

3

B3.1.1 Fundamentals of irrigation 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

4

B3.1.1 Fundamentals of irrigation 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

5

B3.1.1 Fundamentals of irrigation 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

6

B3.1.1 Fundamentals of irrigation 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)

7

B3.1.1 Fundamentals of irrigation 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

8

B3.1.1 Fundamentals of irrigation Considerations: Planning

9

B3.1.1 Fundamentals of irrigation Considerations: Planning

10

B3.1.1 Fundamentals of irrigation Considerations: Some criteria

• Energy requirement

• Capital intensity

• Labour intensity– In building– In running/maintaining

• Efficiency– Losses– Excess runoff– Excess wash through (percolation)

11

B3.1.1 Fundamentals of irrigation Considerations: Water sources

Source Energy needs

Rivers either coming from a wetter zone or maintained by aquifers during dry season

low

Reservoirs or lakes filled during rains and drawn down during irrigation season

0

Naturally sustained aquifers (water stored in the ground) accessed via wells

med-high

Fossil (unsustained aquifers) until they deplete high

Artificially sustained aquifers replenished by controlled percolation or injection

med-high

Waste water from a household or a city med

12

B3.1.1 Fundamentals of irrigation Considerations: Methods of application

Application method Labour Capital ‘Energy’ Efficiency

Recession irrigation low 0 0 n.a.

Gravity-fed surface methods – basin, border, furrow

low-med

low 0 0.3 – 0.6

Sub-surface pipes low high low n.k.

Pitcher/drip (continuous slow release) med high med 0.7 – 0.9

Spraying med-high

high high 0.6 – 0.8

Bucket (very small scale agriculture) v high v low 0 ~0.7

13

B3.1.1 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?

14

B3.1.2 Fundamentals of irrigation Crops and water: Transpiration

convection

evaporation

Solar radiation

Reflection

long wave radiation

Measured in mm/day

Negligible thermal mass

15

ETcr = Crop evapotranspiration (mm/day)

Kc = Crop coefficient

ETo = Reference crop evapotranspiration (mm/day)

B3.1.2 Fundamentals of irrigation Crops and water: Crop coefficient

cr c oET K ET

16

B3.1.2 Fundamentals of irrigation Crops and water: Crop coefficient

17

B3.1.2 Fundamentals of irrigation Crops and water: Crop coefficient

Relative humidity >70% (humid) <20% (dry) Growing period (days)Mid

seasonFinal

growthMid

seasonFinal

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

18

B3.1.2 Fundamentals of irrigation Crops and water: Pan coefficient

19

ETcr = Reference crop evapotranspiration

Kp = Pan coefficient

Eoan =Pan evaporation

B3.1.2 Fundamentals of irrigation Crops and water: Pan coefficient

o p panET K E

20

B3.1.2 Fundamentals of irrigation 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

21

B3.1.3 Fundamentals of irrigation Soil and water: The soil reservoir

22

B3.1.3 Fundamentals of irrigation 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

23

B3.1.3 Fundamentals of irrigation Soil and water: Water content of the soil

24

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

25

B3.1.3 Fundamentals of irrigation Soil and water: The root zone

Used 80% of total

60%

40%

20%

Average 50%

26

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

27

B3.1.3 Fundamentals of irrigation 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)

28

Wp = Water used by plant (mm)

f = factor (~0.5)

Wa = Available water (mm/m)

Wpwp = Permanent wilting point (mm/m)

dr = Root depth (m)

B3.1.3 Fundamentals of irrigation Soil and water: Useful water

p a pwp rW f W W d

29

B3.1.3 Fundamentals of irrigation Soil and water: Soil water balance

Precipitation (P)

Drainage (D) & deep percolation

Surface inflow and Irrigation (F) Runoff (R)

Evapotranspiration (E)

30

S = Water stored in soil (mm)

F = Surface inflow and irrigation (mm)

P = Precipitation (mm)

E = Evapotranspiration (mm)

D = Drainage (mm)

R = Runoff (mm)

B3.1.3 Fundamentals of irrigation Soil and water: Soil water balance

S F P E D R

31

B3.1.4 Fundamentals of irrigation 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

32

Wa = Water applied (mm)

f = factor (~0.5)

Wfc = Field capacity (mm/m)

Wpwp = Permanent wilting point (mm/m)

dr = Root depth (m)

B3.1.4 Fundamentals of irrigation The irrigation cycle: How much?

• If application is at permanent wilting point:

a fc pwp rW f W W d

33

T = Time between irrigations (days)

Wa = Water applied (mm)

ETcr = Crop evapotranspiration (mm/day)

P = Precipitation (mm/day)

B3.1.4 Fundamentals of irrigation The irrigation cycle: When?

a

cr

WT

ET P

34

Ta = Application time (hr)

Wa = Water applied (mm)

I = Infiltration rate (mm/hr)

B3.1.4 Fundamentals of irrigation The irrigation cycle: For how long?

aa

WT

I

35

B3.1.4 Fundamentals of irrigation The irrigation cycle: For how long?: Infiltration

36

B3.1.4 Fundamentals of irrigation The irrigation cycle: For how long?: Infiltration

37

B3.1.4 Fundamentals of irrigation The irrigation cycle: For how long?: infiltration

38

B3.1.4 Fundamentals of irrigation 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

39

B3.1.4 Fundamentals of irrigation 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

40

B3.1.5 Fundamentals of irrigation Water supply: Mass curve

0

500

1,000

1,500

2,000

2,500

3,000

1996 1997

Cu

mm

ula

tiv

e r

ain

fall

(mm

)

41

Next: Irrigation techniques