A COMPARISON OF BLACK POROUS DOUBLE-DISC

AND BELLANI PLATE ATMOMETERS *

by

G. D. V. WilliamsfAgrometeorology Section

Plant Research Institute, Research Branch

Canada Department of Agriculture, Ottawa, Ontario

INTRODUCTION

Evapotranspiration data are oftenused in estimating irrigation requirements by means of soil moisture budgettechniques. Such techniques have beendescribed by Ayers (1), and others assummarized by Robertson (5). Evapotranspiration data can also be utilized in various other agricultural engineering applications including thedesign of irrigation and drainage systems, water conservation structures andcrop drying installations, and the scheduling of tillage operations. Baier (2),in reviewing the relationships betweenweather factors, soil moisture, andplant growth, has discussed the use oflatent evaporation data in estimatingactual evapotranspiration in soil moisture budget calculations.

Latent evaporation, which can beconsidered as an index of the dryingability of the air, is measured with anatmometer featuring a small horizontalblack evaporating surface (6). Theblack Bellani plate atmometer, whichhas been widely employed in Canada inmeasuring latent evaporation (7), isvery susceptible to frost damage. Thisseriously limits its usefulness since important amounts of latent evaporationmay occur during the spring and fallwhen the risk of frost is great.

In the latter half of the 1950's,Robertson began experimenting withan atmometer of his own design at theCentral Experimental Farm at Ottawato try to overcome frost susceptibilityand some other problems associatedwith the use of the Bellani plate (4).The instrument which he developed,known as the black porous disc atmometer, has continued to evolve andis now produced by a commercial firmin Ottawa. The manufacturer incorporates modifications and improvementsafter they have been tested by theAgrometeorology Section.

In 1960, Carder (3) reported that ablack porous disc atmometer generally

tSeconded from Meteorological Branch, CanadaDepartment of Transport.

66

evaporated more than a "shieldedburette mount" Bellani plate but lessthan a "shielded plastic mount" Bellaniplate (sometimes known as the "Central Experimental Farm plasticmount"). These three atmometers allevaporated more than the "standard"Bellani plate (with Livingston-Thonemercury valve) and several other unshielded Bellani plate ass;mblies. Usingdata obtained at Beaverlodge, Alberta,during the summers of 1956-59, hefound that the daily latent evaporationfrom the standard Bellani plate washighly correlated with that from theblack porous disc (r=0.988), althoughthe average latent evaporation from theblack porous disc appeared to be about6.2% higher than from the Bellaniplate on days without rain and 5.8%higher on all days. In a test using 40days of data, he did not find thedifference between the latent evaporation from the standard Bellani plateand the black porous disc to be significant.

The present study was undertakento find out how the version of the blackporous disc atmometer in use in 1965compared with the Bellani plate.

MATERIALS AND METHODS

At least two black porous disc atmometers are in routine use at the

agrometeorological observing site atthe Central Experimental Farm. In themodel in use throughout the 1965 season, a metal cup held two discs (oneresting on top of the other), in ahorizontal position 4 feet above theground (figure 1). The water whichevaporated from the surface of theupper disc was replaced by distilledwater which fed by gravity via a plasticfeeder tube from a vertical cylindrical250 cc graduated plastic reservoirabove the discs, and air bubbles ascended the feeder tube to replace thewater used from the reservoir.

The lower end of the feeder tube

was set in a "well" formed by holes inthe centres of the discs and was supported so as not to protrude below the

•mfmm

Figure 1. Black porous disc atmometer in operation.

lower surface of the bottom disc. The

cup and cylinder were supported byclamps mounted on a metal stand setin the ground. Each disc was 2.9 inches in diameter, Vs, inch thick, had a9/16 inch centre hole, and was madeof "Alundum", a black crystallinealuminous abrasive. (This instrumentand its operation have been describedin detail in Agroclimatological Obser-

CANADIAN AGRICULTURAL ENGINEERING, JAN. 1967

vations Bulletin #16, available in eitherEnglish or French from the Agrometeorology Section).

This instrument and the black porous disc atmometer used in Carder'sexperiments in 1956-59 differed in thatthe earlier model had only a single discwith no centre hole, so that the end ofthe feeder tube rested on the upper surface of the disc. The use of the twodiscs with centre holes in the latermodel was to minimize occurrences of

water loss by wind action during periods of strong gusty winds. Anotherdifference was that in the later model,the whole apparatus was of very frostresistant material, whereas in the earlier model the feeder tube and reservoir had been of glass and thereforesubject to breakage by freezing of thewater.

Two black Bellani plate atmometerswere operated at Ottawa, at the samesite as the double-disc atmometersfrom July 6 to September 26, 1965.These Bellani plates were similarto the "standard" Bellani plate atmometers described by Carder (3)except that no valves were used. Thepurpose of such a valve is to preventrain which falls on the plates fromcausing a backward movement of waterthrough the system, thus resulting inapparently negative evaporation. In thisstudy, it was felt that sufficient comparative data could be obtained byconsidering only the days without measurable rain, and this avoided the"priming" problems associated withthe use of a valve which Carder mentioned.

The current procedure at Ottawawhen two or more identical atmometersare in use at the same site is to arbitrarily designate them No. 1, No. 2, etc.and to use the data from No. 1 unlessthere is reason to believe that No. 1has malfunctioned. For simplicity wewill refer to the black porous discatmometers used in this study as "Disc1" and "Disc 2" and to the Bellaniplate atmometers as "Plate 1" and"Plate 2". The manufacturing processfor Bellani plates is such that a calibration factor is required for them. Eachevaporation value from each of theBellani plates used in this study wasmultiplied by 0.94, which was thecoefficient for both plates. Calibrationcoefficients are not required for blackporous disc atmometers.

Statistical analyses to compare Disc1 with Plate 1 were performed on thelatent evaporation data for 43 of the

days from July 6 to September 26,1965. Data for other days were rejected because of occurrences of measurable rain or, in one case, becauseof instrument malfunction.

RESULTS AND DISCUSSION

The daily latent evaporation datafrom Disc 1 exceeded those from Plate1 by 2.14 cc or 5.2% on the average(table 1). This difference between thetwo means was significant (t = 3.9 for43 pairs of observations). The twosets of data were closely correlated,however, as indicated by the high correlation coefficient (r = 0.98) and thegoodness-of-fit to a straight line of theplotted data (figure 2).

Figureblack

20 30 40 50

PLATE 1

70 cc.

2. Comparison of daily latent evaporation fromporous disc and Bellani plate atmometers.

The high correlation coefficient between Disc 1 and Plate 1 data is similar to that found between the blackporous disc and the Bellani plate inCarder's study (3). The percentage bywhich the evaporation from the blackporous disc exceeded that from theBellani plate is also similar in magnitude to the percentage difference found

by Carder. His comment that the greater evaporation from the porous discmay be due to the fact that "the wateron its surface is under no tension sinceit is virtually flooded at all times, whilethe water at the surface of the Bellaniplate is under slight tension due tocapillary action" seems to be themost logical explanation of the positivebias of the black porous disc in comparison to the Bellani plate data.

Contrary to Carder's results, thedifference between black porous discand Bellani plate data was found to besignificant in the present study. Sincethis contradiction was apparently notdue to any lack of similarity of meandifferences or of sample size, it suggests the daily differences were morevariable in Carder's experiment than inthe present study. Perhaps if Carderhad performed a "t" test using datafrom all the 230 days for which hecompared the black porous disc andth? standard Bellani plate, instead ofusing only a 40 dayperiod, a "t" whichwas significant would have been obtained even if the variability of thedifferences was higher than in thepresent study.

The importance of pairing the observations in such comparisons shouldperhaps be stressed. In the presentstudy, no significant difference betweenDisc 1 and Plate 1 would have beenfound if a "t" test without pairing hadbeen employed. This is because thegreat day-to-day differences in latentevaporation would have obscured thedifferences between instruments.

The Disc 1 and Disc 2 data werehighly correlated and they did not differ significantly; the average differencein their means being less than 1/10 ofa cc (table 1). Their standard deviations were also quite similar. Theseresults indicate that the procedure ofarbitrarily selecting one of the twoblack porous disc atmometers to use

TABLE 1. STATISTICAL ANALYSIS OF DAILY LATENT EVAPORATION

Atmometer

Disc-1

Plate-1

Disc-1Disc-2

Plate-1Plate-2

Mean Standard Number Mean(cc) deviation of Difference

(cc) days (cc)

43.26

41.12

42.83

42.76

41.12

40.37

14.3511.96

14.2514.12

11.96

11.60

43

42

43

2.14

0.07

0.75

** For P = 0.01, t = ±2.7, r = ± 0.39

Paired"t"

3.9s*

0.2

3.5"

Correlationcoefficient

<« »»

0.980**

0.986**

0.994**

CANADIAN AGRICULTURAL ENGINEERING, JAN. 1967 67

for reporting latent evaporation dataand using the other only for checkingpurposes is justified, since the datafrom the two were essentially the same.

The differences between the Plate 1and Plate 2 data, while significant,averaged less than 1 cc and the twosets of data were highly correlated. Thedifferences were due, at least partly, tothe fact that Plate 1 remained cleanthroughout the experiment while Plate2 accidentally became encrusted withdirt. This did riot affect the achievement of the main purpose of the experiment in view of the prior decision touse Plate 1 and Disc 1 data for thecomparison of the two types of instrument. While it is strongly recommended that atmometer evaporating surfaces"" be kept clean at all times, thesmall magnitude of the differences andth£ high correlation in this case suggests that data even from a very dirtyatmom&ter may be quite useful inindicating latent evaporation, if nooSier data are available for a particularperiod.

Inspection of Bellani plate and blackporous disc atmometer data gatheredat Ottawa in 1959 and 1960 seems toindicate that the latent evaporationmay vary considerably from one Bellani plate to another, even when bothare kept clean and in similar condition.While the present study suggests thatBellani plates evaporate significantlyless than black porous disc atmometers,the actual quantitative relationship canbe expected to vary from one pair ofthe.two types of instrument to another.

The standard statistical" tests used inthis paper are based on the assumptionthat random independent samples fromnormal populations are involved. Preliminary analyses indicated that thedistributions of the data used here didnot differ significantly from normaldistributions. The observations werenot completely independent, since theywere from groups of consecutive days,and evaporation on a given day wouldtend to be correlated somewhat with

6*

that on the previous day. The lack ofindependence, however, did not appearto be great enough to invalidate any ofthe results of the tests in Table 1. Inmore comprehensive studies it mightbe desirable to employ statistical techniques which do not require independent and normally distributed randomsamples.

CONCLUSIONS

This study indicates that evaporation from the black porous double-discatmometer is about five percent higherthan from the Bellani plate. This is ingeneral agreement with Carder's findings, except that in his experiment hedid not find the difference to be significant.

In research investigations involvinglatent evaporation, where the type ofatmometer has varied from one location or time to another, the possibleeffect of instrumental differences onthe results should be acknowledged.Such differences however can, for mostpractical purposes, probably be ignored.

Results of different correlation

analyses between latent evaporationand some other factor would generallybe comparable even though a blackporous disc may have been used in onecase, and a black Bellani plate inanother, since the data from these twotypes of instruments are highly correlated with each other. If data from bothtypes of atmometer are used togetherin a particular analysis to compare theevaporation with some other factor,this lack of homogeneity in the latentevaporation data will tend to reducethe correlation coefficient, but in mostcases such reduction will probably benegligible.

ACKNOWLEDGEMENTS

The analyses in this study wereperformed with the assistance ofthe Data Processing Service of the

Canada Department of Agriculture,using computer programmes written byMr. D. A. Russelo of the Agrometeorology Section, Plant Research Institute.

REFERENCES

1. Ayers, H. D. Water Deficit andIrrigation Needs in Ontario. Canadian Agricultural Engineering7:37-39, 1965.

2. Baier, W. The Interrelationship ofMeteorological Factors, SoilMoisture and Plant Growth ( areview), International J. of Bio-meteorology 9:5-20, 1965.

3. Carder, A. C. Atmometer Assemblies, a Comparison, Canadian J.of Plant Science 40:700-706,1960.

4. Robertson, Geo. W. in Discussionon Evaporation Pans and Atmometers. Proceedings of HydrologySymposium No. 2, National Research Council, Subcommittee onHydrology 102-105, 1961.

5. Robertson, Geo. W. A Summaryof Literature Pertaining to LatentEvaporation and its Applicationto Soil Moisture Estimation andIrrigation Scheduling. Ag. Met.Tech. Bui. 3, AgrometeorologySection, Canada Department ofAgriculture (Ottawa), 1964.

6. Robertson, Geo. W. and R. M.Holmes. A New Concept of theMeasurement of Evaporation forClimatic Purposes. InternationalAssociation for Scientific Hydrology, I. U. G. G., AssembleeGenerate de Toronto, 1957(Gentbrugge 1958) Tome III,pp.399-406.

7. Wlliams, G. D. V. Report on theMeasurement of Latent Evaporation at Canada Department of Agriculture Establishments. Ag. Met.Tech. Bui. 9, AgrometeorologySection, Canada Department ofAgriculture (Ottawa), 1966.

CANADIAN AGRICULTURAL ENGINEERING, JAN. 1967