Methods of Irrigation Scheduling and Determination of Irrigation threshold triggers

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1 Methods of Irrigation Scheduling and Determination of Irrigation threshold triggers

2 Introduction Principle of irrigation Scheduling Methods of irrigation scheduling Determination of Irrigation Triggers Worked example Water Balance Method Determination of Field Capacity

3 1 meter deep

4 A kilometer wide

5 7 Million Km Long 180 times around the world

6 Water Required 3000 Calories of food 6.7 Billion Inhabitants Every Day for a year

7 Irrigation Scheduling Combined management and technical tool which dictates: When to Irrigate How much water your crop requires How fast to apply water to your crop How often to irrigate Irrigation scheduling is the key to improve irrigation Efficiency

8 Principle Irrigation Scheduling

9 FC-Soil water content in the soil after a saturated soil has drained by gravity

10 Upper Threshold -FC Lower Threshold -%FC Threshold

11 Irrigation Scheduling Methods Irrigation Scheduling Methods Soil Water Monitoring Plant Water Monitoring Soil Water Balance Modelling Approach

12 Irrigation Scheduling Soil Water Monitoring Soil Water Monitoring Direct Indirect Feel and Appearance Volumetric Tensiometric Gravimetric Dielectric Sensor Tensiometers Volumetric Neutron Probe Electrical Resistive Sensor Others Others

13 Why Irrigation Scheduling by Standalone sensors? Crop Factor Soil Water Factor Climatic Factor Trigger Complex and Variable Processes Soil Moisture Sensors simplifies these complexities into one measurement Trigger can be a function of FC or AWC

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16 20-27 th August 2008 Days 15cm sensor Average trend at 25cm 45cm sensor Commencement of irrigation

17 Irrigation Scheduling Methods Irrigation Scheduling Methods Plant Water Monitoring Stem/Leaf Water Potential Canopy Temperature

18 Plant Water Monitoring Based on plant response Does not answer the question HOW MUCH water is required but WHEN TO IRRIGATE.

19 Plant water monitoring Leaf Water Potential Water - xylem vessels Water - under tension (negative pressure)

20 Plant water monitoring Leaf Water Potential Xylem vessel extends to the leaves

21 Leaf Water Potential

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23 The Pressure bomb Hoses Pressure Gauge Hose Connection Specimen Holder Preparation Board Metering Valve 3-way ball valve

24 Plant water monitoring Leaf Water Potential Integrate Soil factors Environmental Plant factors indirect

25 Plot of Midday Stem Water Potential (MSWP) Vs. Time for walnut Time of Day (hrs) Fully Watered Mild stress Moderate Stress Midday stem WP Threshold range -3 to -5 bars -5.5 to -7.5 bars -8.0 to bars Mid day sampling time Period 1-3 pm

26 Plant water monitoring Canopy Temperature Transpiration cools the leaves below ambient temperature. If T canopy > T ambium, this imples reduced evapotransipration and increased stress

27 Difference in Canopy temperature used in conjunction with Soil water potential (0.4 MPa at 76mm depth) to irrigation Kentucky Blue grass turf (Tc-Ta) C Plot of Difference Between Canopy and Air Temperature Irrigation Intervals Reduced Stressed Stressed Date 1983

28 Irrigation Scheduling Methods Irrigation Scheduling Methods Soil Water Balance Modelling Approach Combination of Plant, soil and Climate

29 Water Balance Model A soil water accounting system Daily withdrawals Daily inputs Change in storage Accounting is done up to some predetermined threshold. Soil is irrigated back to field capacity

30 Water Balance Model Inputs = Outputs ± S Evaporation Root Transpiration S Irrigation Rainfall Zone Runoff Capillary Rise Deep Percolation

31 Water Balance Model Outputs Inputs SMD t = SMD t-1 - (ETa t +D t +R nt) + (R t + I t +C) SMD t-1 SMD t ETa t Ground Level D t R nt Threshold

32 Summary Irrigation Scheduling Methods Soil Water Monitoring Plant Water Monitoring Soil Water Balance Modelling Approach Soil moisture approach is simple if the triggers can be accurately calculated. Plant approach measures the stress level of the plant, destructive and answers the question when but not how much. Generally used in conjunction with other methods Water Balance approach is a more holistic methods

33 Determination of Irrigation Trigger Points Three important concepts are necessary Field Capacity Permanent Wilting Point Available water capacity Readily Available Moisture Maximum Allowable Deficit

34 Field Capacity Water contained in a soil after a saturated soil has been drained for at least two day usually occurs typically at pressure heads of (10kPa) to bars (33Kpa).

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36 Permanent Wilting Point Permanent wilting point (PWP) represents to lower limit of water available to plants. At this stage crops tend to wilt and cannot recover if irrigated. Typically occurs at 15 bars (15MPa) PWP

37 Plant Available Water Water within the soil profile between FC and PWP PWP AWC = θ fc - θ wp

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40 Objective of understanding Plant soil water relationship Maximize crop yields managing soil and water and crop

41 Available water Soil Texture Sandy Sandy Loam Loam Clay Loam Silty clay Clay Field capacity (by% volume) 15 (10-20) 21 (15-27) 31 (25-36) 36 (31-42) 40 (35-46) 44 (39-49) Permanent Wilting Pt (by %Volume) 7 (3-10) 9 (6-12) 14 (11-17) 18 (15-20) 20 (17-22) 21 (19-24) Available water (by %volume) 8 (6-10) 12 (9-15) 17 (14-20) 18 (16-22) 20 (18-23) 23 (20-25)

42 Plant Available Water Content and Available Water Available water (AW) = (water content - wilting point) rooting depth Available water capacity (AWC) = (field capacity - wilting point) rooting depth

43 Sample Problem1 Field Capacity Soil Available Water

44 Sample Problem A soil sample taken after gravitational drainage has a total volume of 50 cm 3, of which 12 cm 3 is water. Find the field Capacity? a. Field capacity =? 12cm 3 /50cm 3 =0.24 or 24%

45 Sample Problem b. If the Permanent wilting point is 0.11 or 11% and the plant rooting depth is 60 cm. Find the available water capacity? cm Available Water Capacity Available water capacity (AWC) = (field capacity - wilting point) rooting depth ( )*60cm=7.8cm or 78mm Answer 78 mm or 7.8 cm

46 Sample Problem C. When this soil has a water content of 0.18 what is the Available water? Available water =? FC 60cm Available Water PWP Available water (AW) = (Moisture Content- wilting point) rooting depth ( )*60cm=4.2cm or 42mm Answer 42 mm or 4.2 cm

47 Readily Available Water Readily available soil moisture PWP AWC = θ fc - θ wp

48 Readily Available Water water which can be removed from the soil with minimal energy applied. It is common to consider about 50% of the available water as readily available water. θ fc θ AWC θ RAW θ PWP RAW = ½* AWC

49 Readily Available Water All of the available water can be used by the plant, The closer the soil is to the wilting point, the greater the stress is that the plant experiences when water is being removed from the soil. Plant stress and yield loss are possible after the readily available water has been depleted

50 Maximum Allowable Deficit The maximum level of depletion to which the soil can dry without causing water deficit stress in a crop that has a fully expanded root zone For most vegetable crops its 30-40% of AWC The MAD therefore become the lower trigger and field FC the upper trigger

51 Sandy loam FC= 24% PWP=11% AWC=24-11=13% 50% AWC = 0.5*13 = 6.5% In terms of Moisture Level = =17.5% What does 17.5% AWC represent in terms of a fraction of FC? %FC = 17.5/24=73% What does 50% FC represent in terms of AWC? 50%FC=24%*0.5 = 12% 1% Relationship between FC and AWC 73% FC Available Water Capacity % AWC In terms of AWC 50%FC = 1/(24-11)=7% AWC

52 Relationship between FC and AWC What does 50% FC represent in terms of AWC? 50%FC=24%*0.5 = 12% 1% more than PWP AWC =13% In terms of AWC, 50%FC = 1/(24-11)=7% AWC 50% FC % AWC Available water content

53 Example of water budget approach for scheduling Irrigation scheduling of tomatoes Location: Castries, St. Lucia Soil Type, FC =21%, PWP =11% Loamy Sand Rooting Depth Before flowering (before June 15) 30 cm After flowering (after June 16) 60 cm Maximum total available water Before flowing 30 mm After flowing 60 mm Allowable soil water Depletion Before flowering - 15 mm After flowering - 30 mm

54 Date Rain (mm) Eto (mm) Kc Etcrop (mm) Total available water (mm) New Soil Moisture Level (mm) Irrigation Amount (mm) / / / / / / / / / / / / / / / / / / / /

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60 Soil Type - loam FC = 31% PWP=11% AWC=20% Rooting Depth =60cm Area of Frame = 1m*1m How much water to Saturate the soil AWC over rooting depth = 0.2*60cm=12cm Vol. of Water required Aear * depth 1m2*0.12m=.12m3 =120 l =120/3.78=31 gallons

61 Data Required Gravimetric water content Bulk density Volumetric water content Sensor reading corresponding to FC