Post on 23-May-2022
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551' 508' 7 : 551' 573
Some experiments with the U. S.A. Standard Evaporimeter
S. P. YEXKITEsm VARAN. P. JAGAX",ATHA", and S. S. RA~rAKRISHNAN
Meteorological Office , Poona
IReceiV/'d 7 August 1956)
t , Inlro<luelloD
The U.S.A. Standard Evaporimeter is inregular usc at a number of selected Agricultural Farms in the Indian States and in theObservatories of the India MeteorologicalDepartment . The evaporimeter consist. of acvlindrical reservoir 48· in diameter and10· high supported on a wooden fmme 4high. The level of the water in the reservoiris maintained at about 2- from the rim so thatthe height of the level of water is nearly 12above ground. The evaporimeter is of 20SWG copper sheet. The evaporation ismeasured by accurately determining with thehook-gauge the change in the level of waterin the reservoir after any given interval oftime. The hook gauge is placed on a stillwell provided with levelling screws.
This instrument is fairly in wide use atmany stations in India for measurement ofevaporation. To ensure the comparability ofobservations recorded at different stations,care is taken as far as possible to keep to thestandard of specifications prescribed for theinstrument and also to maintain similarconditions of exposure at all sta tions. But, itwasnoticed that some condit ionsof exposuresuch as the colour of the paint on the evapo'rimeter, the depth of water level in the tanketc, could not be maintained identical atall the places at all times. It was, therefore,considered desirable to investigate the extentof the effects of such uncontrollable variations in the conditions of exposure. In theseries of experiment s designed for such apurpose some more factors of variation suchas the height of the tank above the ground,size of tank etc have also been included for
testing. The series of experiments were conducted in the Central Agricultural Meteorological Observatory at Poena and the resultsof the observations are summarised below.
It may also be mentioned here tbat in anobservatory tbere is a contribution to theloss of water from an open tank evaporimeterby birds etc. In the series of experimentsdiscussed bere such losses have been avoidedby suitable watcb and maintenance conditions.
2. Colour or paint OD Ill• •nportm.ler
When the evaporimeters are supplied to theobservatories, they are painted wbite outsideand are either painted white or tinnedinside. It may happen in course of time thatthe paint may wear away or become discoloured. To study the effect of colour ofpaint on evaporation, observations weretaken with 6 USA type evaporimeters andthey Were painted as follows-
A-Tinned inside, white paint outsideB-Painted white, both inside and
outsideC-No paint inside, white outsideD-No paint botb inside and outsideE-Painted black inside and outsideF-No paint inside, painted black
outsideTaking the mean evaporation of the Stan.
dard A as unit, the ratios of the other meansare given in Table 1.
F rom Tables 1 and 2 it will be seen that theeffect of the colour of the paint on the evaporat ion is not negligible. While the temperature of the water in the tank in the morning