Methods of Irrigation Scheduling and

Determination of Irrigation threshold triggers

Introduction

Principle of irrigation Scheduling

Methods of irrigation scheduling

Determination of Irrigation Triggers

Worked example – Water Balance Method

Determination of Field Capacity

1 meter deep

A kilometer wide

www.maps-world.net/africa-asia.htm

7 Million Km Long 180 times around the world

Water Required

3000 Calories of food

Every Day for a year

6.7 Billion Inhabitants

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

Principle Irrigation Scheduling

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

Threshold

Upper Threshold -FC

Lower Threshold -%FC

Irrigation Scheduling Methods

Soil Water Monitoring

Plant Water Monitoring

Soil Water Balance Modelling Approach

Irrigation Scheduling Methods

Irrigation Scheduling Soil Water Monitoring

Soil Water Monitoring

Feel and Appearance

Gravimetric

Direct

Volumetric

Neutron Probe

Dielectric Sensor

Indirect

Tensiometers

Tensiometric

Volumetric Electrical Resistive Sensor

Others Others

Why Irrigation Scheduling by Standalone sensors?

Soil Moisture Sensors simplifies these complexities into one measurement

Trigger can be a function of FC or AWC

Crop Factor Soil Water Factor Climatic Factor

Complex and Variable Processes Trigger

20-27th August 2008

15cm sensor

45cm sensor Average trend at 25cm Commencement of irrigation

Days

Irrigation Scheduling Methods

Irrigation Scheduling Methods

Plant Water Monitoring

Stem/Leaf Water Potential

Canopy Temperature

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

Plant water monitoring Leaf Water Potential

Water - xylem vessels Water - under tension (negative pressure)

Plant water monitoring Leaf Water Potential

Xylem vessel extends to the leaves

Leaf Water Potential

Preparation Board

Specimen Holder

Pressure Gauge

Hoses

Hose Connection

Metering Valve

The Pressure bomb

3-way ball valve

Plant water monitoring Leaf Water Potential

Integrate Soil factors Environmental Plant factors

indirect

-12.0

-10.0

-8.0

-6.0

-4.0

-2.0

0.06 8 10 12 14 16 18 20

Mid

day

stem

WP

Time of Day (hrs)

Plot of Midday Stem Water Potential (MSWP) Vs. Time for walnut

Fully Watered Mild stress Moderate Stress

Mid day sampling time Period 1-3 pm

Threshold range

-3 to -5 bars

-5.5 to -7.5 bars

-8.0 to -10.0 bars

Plant water monitoring Canopy Temperature

Transpiration cools the leaves below ambient temperature. If Tcanopy > Tambium, this imples reduced evapotransipration and increased stress

-4.0

-2.0

0.0

2.0

4.0

6.0

8.0

10.0

12.0

(Tc-

Ta) °

C

Date 1983

Plot of Difference Between Canopy and Air Temperature

Stressed

Reduced Stressed

Irrigation Intervals

Difference in Canopy temperature used in conjunction with Soil water potential (0.4 MPa at 76mm depth) to irrigation Kentucky Blue grass turf

Irrigation Scheduling Methods Irrigation Scheduling

Methods

Soil Water Balance Modelling Approach

Combination of Plant, soil and

Climate

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

Water Balance Model

Inputs

Evaporation

Runoff

Rainfall

Irrigation Transpiration

Capillary Rise Deep

Percolation

Root Zone

= Outputs

S

± ∆S

Water Balance Model Outputs Inputs

ETat

Ground Level SMDt-1

Dt

Rnt

SMDt

Threshold

SMDt = SMDt-1- (ETat+Dt +Rnt) + (Rt + It+C)

Summary

Soil Water Monitoring

Plant Water Monitoring

Soil Water Balance Modelling Approach

Irrigation Scheduling Methods

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

Determination of Irrigation Trigger Points

Three important concepts are necessary Field Capacity Permanent Wilting Point Available water capacity Readily Available Moisture Maximum Allowable Deficit

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 -0.1 (10kPa) to -0.33 bars (33Kpa).

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

Plant Available Water

Water within the soil profile between FC and PWP

PWP

AWC = θfc - θwp

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

Soil Texture Field capacity (by% volume)

Permanent Wilting Pt

(by %Volume)

Available water (by %volume)

Sandy 15

(10-20) 7

(3-10) 8

(6-10)

Sandy Loam 21

(15-27) 9

(6-12) 12

(9-15)

Loam 31

(25-36) 14

(11-17) 17

(14-20)

Clay Loam 36

(31-42) 18

(15-20) 18

(16-22)

Silty clay 40

(35-46) 20

(17-22) 20

(18-23)

Clay 44

(39-49) 21

(19-24) 23

(20-25)

Available water

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

Sample Problem1

Field Capacity Soil Available Water

Sample Problem A soil sample taken after gravitational

drainage has a total volume of 50 cm3, of which 12 cm3 is water. Find the field Capacity?

a. Field capacity = ? 12cm3/50cm3=0.24 or 24%

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?

60cm

-0.24

-0.11 Available Water Capacity

Available water capacity (AWC) = (field capacity - wilting point) × rooting depth

(0.24-0.11)*60cm=7.8cm or 78mm Answer 78 mm or 7.8 cm

Sample Problem C. When this soil has a

water content of 0.18 what is the Available water?

Available water = ?

Available water (AW) = (Moisture Content- wilting point) × rooting depth

(0.18-0.11)*60cm=4.2cm or 42mm Answer 42 mm or 4.2 cm

60cm

-0.24 FC

-0.11 PWP Available Water -0.18

Readily Available Water

Readily available soil moisture

AWC = θfc - θwp

PWP

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.

RAW = ½* AWC

θfc

θPWP

θAWC θRAW

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

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

Relationship between FC and AWC

Sandy loam FC= 24% PWP=11% AWC=24-11=13% 50% AWC = 0.5*13 = 6.5% In terms of Moisture Level = 6.5 +11 =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%

-0.24

-0.11

Available Water Capacity 50% AWC

In terms of AWC 50%FC = 1/(24-11)=7% AWC

73% FC

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

-0.24

-0.11 7% AWC 50% FC

Available water content

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

DateRain (mm)

Eto (mm) Kc

Etcrop (mm)

Total available water (mm)

New Soil Moisture Level

(mm)

Irrigation Amount

(mm)

16.56/1 0.8 4.5 0.4 1.8 15.56/2 44.4 4.0 0.4 1.6 30.06/3 0 3.7 0.4 1.5 28.56/4 0 6.0 0.4 2.4 26.16/5 0.4 7.8 0.4 3.1 23.46/6 1.6 6.4 0.4 2.6 22.46/7 0 4.4 0.4 1.8 20.76/8 1.6 2.7 0.4 1.1 21.26/9 0 7.0 0.4 2.8 18.46/10 0 3.0 0.4 1.2 17.26/11 8.4 2.5 0.4 1.0 24.66/12 0 7.8 0.4 3.1 21.56/13 0 4.8 0.7 3.4 18.16/14 0 7.6 0.7 5.4 12.7 30.0 17.36/15 0 7.4 0.7 5.2 24.86/16 0 6.5 0.7 4.6 20.2 60.0 39.86/17 0 5.0 0.7 3.5 56.56/18 0 4.4 0.7 3.1 53.46/19 9.4 3.4 0.7 2.4 60.56/20 0 5.8 0.7 4.0 56.4

http://www.ipm.iastate.edu/ipm/icm/node/2614

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

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