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7.5CLIMATICOBSERVATION AS

AN IDEX TOCONSUMPTIVE

E


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E How temperature, humidity, wind velocity,vapor pressure, and solar radiation

infuence consumptive use has beenstudied by several researches.

Penman, in England, has made the most

complete analysis using several climaticvariables, whereas temperature has beenused as the principal variable to obtain anindex to consumptive use.

Jensen and Haise used temperature andsolar radiation to estimate the consumptiveuse o a reerence crop (alala).Harrea!esor grass as a reerence crop.


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A!A!A

Alala is a long"lived perennialin the Fabaceae(or issometimes put in

the Papilionaceae) amily withleguminous fowers which varyin color rom purple to yellow,trioliate clover"li$e leaves, anda deep penetrating taproot (some sources say that

taproots have been ound

reaching down %& eet into thesoil'). Alala is native tosouthwest Asia with wildspecies occurring in theaucasus, and in mountainous

regions o Aghanistan and ran,

and is very widely cultivatedthroughout the world.


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Me$%"d

*enman as made the most completetheoretical approach, showing thatconsumptive use is inseparablyconnected to incoming solar energy.

His ormula representing the potentialevapo"transpiration (consumptive use)ia as ollows in the modi+ed orm


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-here

Etp " reerence crop potential evapo"

transpiration, well water alala in cals/.cmper day.

0 " slope o saturation vapor pressure"temperature curve (ded1) in mbardegree elsius.

2 " psychometric constant3n " net radiation in cals/.cm per day4 " soil heat fux in cals/.cm per day56 " wind movement in $mday at 7 m

es" saturation vapor pressure, mean o valuesobtained at daily maximum and daily minimumtemperatures in mbar

e8 " mean actual vapor pressure in mbar.

w9, w6 " wind term coe:cients


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1he ollowing additional e/uations arere/uired or solution o the +rstE/uation

-here


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An empirical e/uation or

estimating the soil heatfux is

-here


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n some locations wind movement may beavailable rom an instrument at a height

di;erent than 7 meters. 1he ollowingrelationship provides a reasonable estimate othe wind movement at 7 meters when theanemometer is at height


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*enman ound that coe:cients werenecessary to reduce the potential rate oconsumptive, used to actual water use opasture growing in England. >ery goodcorrelation has been obtained betweencomputed and measured consumptive"use

values. 1he modi+ed *enman e/uation hasbeen used to estimated consumptive use ortime intervals o ? hour or less or researchstudies with comprehensive instrumentation.

1his method has been used extensively bygovernment agencies and private companieswho are providing computeri


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Jensen&Haise Me$%"d@ensen and Haise used observations oconsumptive use rom the western 5nitedtates to develop a linear relationship orestimating Etp as

-here

t" a temperature coe:cient1 " temperature in degree celsius1x " the intercept on the temperature axis

3s " incident solar radiation in langleysday


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1he values o t and 1x were determinedas B.B7C and "D, respectively. 1hesecoe:cients were urther de+ned or otherareas as

and,

and,


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Estimate o crop consumptive use are

calculated rom Etp, determined romeither *enman or @ensen"Haise method,by use o a crop coe:cient which relates

growth stage bac$ to the reerence crop.n e/uaton orm

-here Et" estimated crop consumptive use.

co " crop coe:cient


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Harrea!es E'(a$i"nHargreaves based his wor$ on data rom grass

lysimeters. 1he method is similar in concept to@ensen"Haise and is subFect to similar limitations.t has been used extensively in atin America toestimate crop water re/uirements. n e/uation

orm

-here

Etg " reerence crop potential consumptive use,well"watered grass in langleysday.

1 " average daily temperature in degree celsius3s " incident solar radiation in langleysday


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Hargreaves indicates that a reerencecrop o alala would have a consumptive

use about 7B percent greater than grass,thus Etp can be estimated rom Etg by,

Hargreaves has also suggested thatwhere measured 3s values are notavailable, a air estimate is,

-here G percent o possible sunshine


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C"n!ersi"n )a*$"r +"r E$,+r"m Lan-es ,er Da $"

Mi--ime$ers ,er Da1he preceding three methods or

estimating reerence crop consumptiveuse all present the Etp and Etg values inunits o langleys per day. 1he units o alangley (calories per s/uare centimeter)when multiplied by ?B and divided by the

latent heat o water are converted tomillimeters o water per day, which is ausual and more desirable unit orconsumptive use calculation.


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B-ane&Cridd-e Me$%"d

laney and riddle developed asimpli+ed ormula using temperatureand day"time hours or the arid westernportion o the 5nited tates. 1heirormula has been used extensively bythe oil onservation ervice o the5nited tates Iepartment o Agriculture

( ..) wherein considerable data hasbeen collected to determine the valueso the coe:cients to be used or various

crops.


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y multiplying the mean monthlytemperature (t) by the monthly

percentage o day"time hours o the year(p), there is obtained a consumptive useactor !. Expressed mathematically, it is

where the ;. /uantities must bedetermined or the same period


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!or monthly calculations, lower caseletters are re/uently used or clarity as

ollows

-hereu " monthly consumptive use, in mm$ " monthly crop coe:cient " monthly consumptive"use actor

(t x p)?BB


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1able J.K illustrates use o the method tocompute monthly consumptive use by

alala in alinas >alley, aliornia.


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5sually, irrigation systems must bedesigned to meet the maximum

monthly water needs rather than theseasonal average.

Lodi+ed laney"riddle E/uation


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E!a,"ra$i"n

Evaporation rom a 5nited tates-eather ureau pan has been ound tobe a good index to pea$ consumptive

use. ome observations in low"humidity areas indicate that pea$ usemay be as much as ?B to 7B percent

higher than the evaporation rom a-eather ureau pan or a short periodo time.Exposure o the pan and the

crop vary the rate considerably.


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Pi*%e E!a,"r"me$ers5sed extensively in manycountries, also give good

indices to maximum rates oconsumptive use. 1he picheunit is essentially a test tube+lled water, inverted with a

blotter over the end, andinstalled in a conventionalshelter with other weather

instruments.


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A$m"me$ers-hich are porous ceramicbulbs, have also been usedsuccessully as indices to

consumptive use.Ii;erences betweenreadings obtained rom blac$and white bulb atmometersare indicative o maximumconsumptive use rates thatcan be expected at the time.


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*rincipal limitations o evaporation pansare that they are expensive and re/uire

considerable water. *iche evaporometersand atmometers have one drawbac$ incommon. 1he evaporating surace o

each is subFect to contamination by dust,oil and other oreign material. lotters or*iche units are inexpensive and can bechange readity. However, since

atmometers are relatively moreexpensive than porous ceramic bulbs,they are re/uently not replaced when

dirty

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