Notes on irrigation scheduling in water resources engineering

scheduling

1. •SoilTexture:Soiltextureisdeterminedbythesizeandtypeofsolidparticles
thatmakeupthesoil.Soilparticlesmaybeeithermineralororganic.Inmost
soils,thelargestproportionofparticlesismineralandisreferredtoas"mineral
soils."Formineralsoils,thetextureisbasedontherelativeproportionofthe
particlesunder2millimeters(mm)or5/64thofaninchinsize.Asshownin
Figure1,thelargestparticlesaresand,thesmallestareclay,andsiltisin
between.
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2. 2/12/2023 MISOF 4
Determination:Soiltexturecanbe
determinedbyseparatingandweighing
thesand,siltandclay.Forexample,ifa
1000gramssampleofsoilwassifted
throughscreensandfoundtocontain
450gofsand,350gofsiltand200gof
clay,thenthesoilwouldbecomposed
of45%sand,35%siltand20%clay.
AsshownbythedottedlinesinFigure
2,thissoilhasaloamtexture.

3. Soil characteristics of particular importance to irrigated agriculture include
(1)Thecapacityofthesoiltoholdwaterandstillbewelldrained;
(2)Theflowcharacteristicsofwaterinthesoils;
(3)thephysicalpropertiesofthesoilmatrix,includingtheorganicmattercontent,soildepth,
soiltexture,andsoilstructure;
(4)Soilchemicalproperties,includingthetranslocationandconcentrationofsolublesaltsand
nutrientsduetothemovement,use,andevaporationofthesoilwater.
Knowledgeofalltheserelationshipsandhowtheyinfluenceeachotheriscriticaltoall
whodesiretoimproveirrigationpracticesandobtainthebest,mostefficientuseofwater.
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4. 2/12/2023 MISOF 6
Soilorganicmatter(SOM)isanotherfactor
thatcanhelpincreasewaterholdingcapacity.
Soilorganicmatterhasanaturalmagnetismto
water.Ifthefarmincreasesthepercentageof
soilorganicmatter,thesoilwaterholding
capacitywillincrease.SOMisdecayed
materialthatoriginatedfromalivingorganism.
SOMcanbeincreasedbyaddingplantor
animalmaterial

5. 2/12/2023 MISOF 7
ExampleNo.1:Asoilsampleweighed230ginamoisturebox.Themassofthemoisturebox
was78g.Afterdryingat105degreesCtoaconstantmass,thesoilandboxweighed204g.
Thesoilsamplefilleda1000cccontainerasitwastakenfromthefield.Findthemoisture
percentageinthesoilbymassandbyvolume.
Water(%)bymass:
Example No. 2:The following data represent a
soil sample:
Soil mass at field capacity = 85 g
Soil mass at wilting point = 71 g
Air-dry mass = 64 g
Oven-dry mass = 58g

6. Gravimetric Water content (GWC) and Volumetric Water content (VWC)
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GWCisrelatedtothemassofwaterandsoil,whereasVWCisrelatedtothevolumeofwaterand
soil.
GWCisthemassofwaterpermassofdrysoilinagivensample.TodetermineGWC,collect
soilsamplefromthefield,weighthatsoilthendryitintheovenat105°Cforatleast3days(or,
moreprecisely,weighthesoilperiodicallyuntilthereisnomorelossinweightwhichindicates
thatallofthewaterhasbeendriedfromthesample).TheequationforGWC(θ
g)is:
Where:WetistheweightofthesoilsamplefromthefieldandDryistheweightofthedrysoilsample.

7. 2/12/2023 MISOF 9
VWCis thevolumeof water pervolumeof soil. To determine VWC, you will need to measure Wetand Dryas described
above for GWC. You will also need to know the bulk densityof the soil sample(BD).
TheequationforVWC(θ
v)[usedtocalculatevolumetricsoilwatercontent]isthengivenby:
θ
v=θ
g*BD (2)
Soilscientistsalsoincorporatethedensity
ofwater(WD)intoθ
vas:
θ
v=θ
g*(BD/WD)(3)
However,thevalueofWDiscloseto1andistypicallyignoredwhencalculatingθ
v

8. 2/12/2023 MISOF 10
▪Soilbulkdensity(ρ
bulk
orrepresentedasBD)isusedforρ
soil
andistheratioofsoildrymassto
samplevolume.Thedensityofwateriscloseto1andoftenignored.
▪Anotherusefulproperty,soilporosity(ε),isrelatedtosoilbulkdensityasshownbythefollowing
expression.
Worked 1

9. 2/12/2023 MISOF 11
Example 2

10. 2/12/2023 MISOF 12
Thepointbelowfieldcapacitywhereplantsbecomestressediscalledthemaximumallowabledepletion(MAD).
TheMADvalueisexpressedasapercentoftheavailablewatercapacity.Table1.2showstypicalMADvaluesfora
fewselectedcrops.
Crop Type Maximum Allowable Depletion (MAD) Effective Root Depth
(inches)
Grass 50% 7
Table beet 50% 18
Sweet corn 50% 24
Strawberry 50% 12
Winter Squash 60% 36
Peppermint 35% 24
Potatoes 35% 35
Orchard apples 75% 36
Leafy Green 40% 18
Cucumber 50% 24
Green beans 50% 18
Cauliflower 40% 18
Carrot 50% 18
Blueberries 50% 18

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12. 2/12/2023 MISOF 14
SoilMoistureTension
Threesoilmoisturestates,saturation,fieldcapacityandpermanentwiltingpointareusedto
describewatercontentacrossdifferentwaterpotentialsinsoilandarerelatedtotheenergy
requiredtomovewater(orextractwaterfromsoil).