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Tables 1 and 2 below show the average daily irrigationrequirements of citrus trees in litres for different canopysizes in Wokalup and the Upper Swan. Note: gureshave been rounded. Average evaporation rates weretaken from the Bureau of Meteorology website

Imperial mandarinsVarieties with narrow upright growth habits such asImperial mandarins will require slightly more waterthan that calculated according to canopy ground area.Use the above calculations as a base and netune theschedule using soil moisture monitoring devices.

Daily water requirementsThere is good correlation between plant waterrequirements and evaporation. Citrus trees useapproximately 70 per cent of evaporation. If evaporationis 10 mm the trees will use 7 mm of water from the soilroot zone volume.

Daily water use (mm) = Evaporation (mm) x 0.7

A sprinkler system that irrigates the under canopyarea evenly with a known application rate can be usedto schedule irrigations by replacing water used asdescribed above. For drip irrigation systems, where onlya portion of the under canopy area is watered, it is bestto calculate daily water requirements in litres.

Converting water use from mm to litresTo convert millimetres water use to litres, multiply dailywater use in millimetres by the canopy ground area.

Daily water use (L) = Evaporation (mm) x 0.7 xCanopy Ground Area (m 2 )

Water use is proportional to the area of exposed leaf,which relates to the area of land covered by the canopy.Calculate canopy ground area in m 2 by measuring thehorizontal dimensions of the canopy. Example: If thetrees are growing in a hedge and the canopy areas aretouching, multiply the tree spacing by the average widthof the row. If the canopies do not touch, work out thearea as a circle.

Figure 3: Measuring canopy size

Table 1: Average daily water requirements in lit res per tree per day – Wokalup

Average monthly Evaporation (E) →

A v e r a g e

D a

i l y I r r i g a

t i o n

R e q u

i r e m e n

t ( L / t r e e

)

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

9 8.6 6.8 4.3 2.7 2.1 2.1 2.5 3.2 4.4 6 7.8

Small Tree1 m 2 canopy (E x 0.7 x 1 m 2 )

6.5 6 5 3 2 1.5 1.5 2 2.5 3 4.5 5.5

Medium Tree9 m 2 canopy (E x 0.7 x 9 m 2 )

57 54 43 27 17 13 13 16 20 28 38 49

Large Tree16 m 2 canopy (E x 0.7 x 16 m 2 )

101 96.5 76 48 30 23.5 23.5 28 36 49.5 67 87.5

Table 2: Average daily water requirements in litres per tree per day – Upper Swan

Average monthly Evaporation (E) →

A v e r a g e

D a

i l y I r r i g a

t i o n

R e q u

i r e m e n

t ( L / t r e e

)

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

9.9 9.1 7.3 5 3.1 2.5 2.5 2.6 3.3 4.8 6.7 8.5

Small Tree1 m 2 canopy (E x 0.7 x 1 m 2 )

7 6.5 5 3.5 2.5 2 2 2 2.5 3.5 5 6

Medium Tree9 m 2 canopy

(E x 0.7 x 9 m2

)

62.5 57.5 46 31.5 20 16 16 16.5 21 30 42 53.5

Large Tree16 m 2 canopy (E x 0.7 x 16 m 2 )

111 102 82 56 35 28 28 29 37 54 75 95



RainfallRainfall during the irrigation season may reduce the irrigationrequirement of your trees. Rainfall events greater than 5 mmshould be factored into your irrigation schedule. Not all rainthat falls is available to be taken up by your trees; some islost to run-off, drainage below the root zone, or interceptionby leaf litter or mulch. If rain exceeds the amount of water

that can be held in the root zone only consider the amountof water that can be held in your irrigation schedule; theremainder will be lost to drainage. Keep a close eye on soilmoisture during dry winters and in spring as the soil canquickly dry out if there is insuf cient rainfall or irrigation.

Soil moisture monitoringIt is best practice to use a combination of evaporationreplacement and soil moisture monitoring whenscheduling irrigation. Monitoring tools give a pictureof what is happening in and below the crop rootzone, allowing for more informed irrigation schedulingdecisions. There is a wide range of devices available sobe sure to match your chosen tool’s capabilities withyour requirements.

Scheduling irrigationsTo schedule irrigation, compare the amount of wateravailable in the crop root zone with the tree’s daily waterrequirement. If the daily water requirement exceeds theamount of water that can be held in the root zone, youwill need to irrigate more than once a day. If the soil canhold more than the daily water requirement there is an

option of irrigating when the available water is depleted(this may be every second or third day), daily or severaltimes a day.

Scheduling exampleUsing the example pictured above, on an average dayin January in the Upper Swan area these trees wouldrequire 62.5 litres of water per day. The drip-irrigated treewould require two irrigations per day, one in the morningand one part-way through the day. The sprinkler-irrigatedtree would require irrigating every third day.

DRIP IRRIGATED TREE SPRINKLER IRRIGATED TREE

9m 2 CANOPY AREA 9m 2 CANOPY AREA

17L17L 189L

30cm

ROOT

ZONE

DEPTH

WETTED AREA

SPRINKLERDRIPPER

LOAM SOILSOIL

Figure 4: Comparing daily water requirement with water holding capacity



2045-12/07-ID8456 ISSN 0726-934X

© State of Western Australia, 2007

Further readingFarmnote 198 Calculating Readily Available Water

Farmnote 206 Soil Texturing

Farmnote 196 Converting Readily Available Water (mm)to litres for drip systems

Farmnote 35/1990 Evaluating sprinkler and trickleirrigation systems

Farmnote 26/1990 Soil moisture monitoring equipment

Average monthly evaporation gures can be found on theBureau of Meteorology website at www.bom.gov.au

Irrigation pointersWhere drip irrigation is used, you may need to•

irrigate more than once a day to meet peak waterrequirements. If the drip system drains out after eachirrigation, break the irrigation down into the longestpulses possible to reduce losses to drainage.

Redesign the irrigation system if the wetted area is•

too small (limiting) and pulsing is not an option. Toperform pulse irrigation ef ciently, a non-drainingdrip system is required.

Sprinkler irrigation systems cannot be used for pulse•

irrigation. Short pulses of sprinkler irrigation do notpenetrate deep enough into the soil resulting in highevaporative losses.

Check the uniformity of the irrigation system at the•

beginning and throughout the irrigation season.

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