1. 1 Part B3: Irrigation B3.1 Fundamentals of Irrigation

2. 2 B3.1Fundamentals of irrigation Topics Why irrigate? Water needs –Plants and water –Soil and water –The Irrigation cycle Available water –Mass curves, flow-duration curves

3. 3 B3.1.1Fundamentals 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. 4 B3.1.1Fundamentals 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. 5 B3.1.1Fundamentals 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. 6 B3.1.1Fundamentals 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. 7 B3.1.1Fundamentals 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. 8 B3.1.1Fundamentals of irrigation Considerations: Planning

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10. 10 B3.1.1Fundamentals 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. 11 B3.1.1Fundamentals of irrigation Considerations: Water sources SourceEnergy 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 depletehigh Artificially sustained aquifers replenished by controlled percolation or injection med-high Waste water from a household or a citymed

12. 12 B3.1.1Fundamentals of irrigation Considerations: Methods of application Application methodLabourCapital‘Energy’Efficiency Recession irrigationlow00n.a. Gravity-fed surface methods – basin, border, furrow low- med low00.3 – 0.6 Sub-surface pipeslowhighlown.k. Pitcher/drip (continuous slow release)medhighmed0.7 – 0.9 Sprayingmed- high high 0.6 – 0.8 Bucket (very small scale agriculture)v highv low0~0.7

13. 13 B3.1.1Fundamentals 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. 14 B3.1.2Fundamentals of irrigation Crops and water: Transpiration convection evaporation Solar radiation Reflection long wave radiation Measured in mm/day Negligible thermal mass

15. 15 ET cr = Crop evapotranspiration (mm/day) K c = Crop coefficient ET o = Reference crop evapotranspiration (mm/day) B3.1.2Fundamentals of irrigation Crops and water: Crop coefficient

16. 16 B3.1.2Fundamentals of irrigation Crops and water: Crop coefficient

17. 17 B3.1.2Fundamentals of irrigation Crops and water: Crop coefficient Relative humidity>70% (humid)<20% (dry)Growing period (days) Mid season Final growth Mid season Final growth Barley1.10.251.20.2120-165 Green beans0.950.851.00.975-90 Maize1.10.551.20.680-110 Millet1.050.31.150.25105-140 Sorghum1.050.51.150.55120-130 Cotton1.10.651.20.65180-195 Tomatoes1.10.61.20.65135-180 Cabbage/Cauliflower1.00.851.20.9580-95

18. 18 B3.1.2Fundamentals of irrigation Crops and water: Pan coefficient

19. 19 ET cr = Reference crop evapotranspiration K p = Pan coefficient E oan =Pan evaporation B3.1.2Fundamentals of irrigation Crops and water: Pan coefficient

20. 20 B3.1.2Fundamentals of irrigation Crops and water: Pan coefficient Cropped areaDry Fallow area Humidity<40%40-70%>70%<40%40-70%>70% Light wind0.650.750.850.600.700.80 Moderate wind0.600.700.750.550.650.70 Strong wind0.550.600.700.500.550.65 Very strong wind0.50.650.600.400.50.55

21. 21 B3.1.3Fundamentals of irrigation Soil and water: The soil reservoir

22. 22 B3.1.3Fundamentals 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. 23 B3.1.3Fundamentals of irrigation Soil and water: Water content of the soil

24. 24 B3.1.3Fundamentals of irrigation Soil and water: Available water %mm/m Fine sand2-3%30-50 Sandy loam3-6%40-100 Silt loam6-8%60-120 Clay loam8-14%90-210 Clay13-20%190-300

25. 25 B3.1.3Fundamentals of irrigation Soil and water: The root zone Used 80% of total 60% 40% 20% Average 50%

26. 26 B3.1.3Fundamentals of irrigation Soil and water: Available water Root depth (full grown) Shallow Beans0.6-0.7 m Grass0.4-0.6 m Rice0.5-0.7 m Medium Barley1.0-1.5m Grains (small)0.9-1.5 m Sweet potatoes 1.0-1.5 m Tomatoes0.7-1.5 m Deep Alfalfa1.0-2.0 m Orchards1.0-2.0 m Maize1.0-2.0 m

27. 27 B3.1.3Fundamentals 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. 28 W p = Water used by plant (mm) f = factor (~0.5) W a = Available water (mm/m) W pwp = Permanent wilting point (mm/m) d r = Root depth (m) B3.1.3Fundamentals of irrigation Soil and water: Useful water

29. 29 B3.1.3Fundamentals of irrigation Soil and water: Soil water balance Precipitation (P) Drainage (D) & deep percolation Surface inflow and Irrigation (F) Runoff (R) Evapotranspiration (E)

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

31. 31 B3.1.4Fundamentals 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. 32 W a = Water applied (mm) f = factor (~0.5) W fc = Field capacity (mm/m) W pwp = Permanent wilting point (mm/m) d r = Root depth (m) B3.1.4Fundamentals of irrigation The irrigation cycle: How much? If application is at permanent wilting point:

33. 33 T = Time between irrigations (days) W a = Water applied (mm) ET cr = Crop evapotranspiration (mm/day) P = Precipitation (mm/day) B3.1.4Fundamentals of irrigation The irrigation cycle: When?

34. 34 T a = Application time (hr) W a = Water applied (mm) I = Infiltration rate (mm/hr) B3.1.4Fundamentals of irrigation The irrigation cycle: For how long?

35. 35 B3.1.4Fundamentals of irrigation The irrigation cycle: For how long?: Infiltration

36. 36 B3.1.4Fundamentals of irrigation The irrigation cycle: For how long?: Infiltration

37. 37 B3.1.4Fundamentals of irrigation The irrigation cycle: For how long?: infiltration

38. 38 B3.1.4Fundamentals of irrigation The irrigation cycle: For how long?: infiltration mm/hr Sand30 Sandy loam20-30 Silt loam10-20 Clay loam5-10 Clay1-5

39. 39 B3.1.4Fundamentals 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. 40 B3.1.5Fundamentals of irrigation Water supply: Mass curve

41. 41 Next: Irrigation techniques