some experiments with the u.s.a. standard evaporimeter

• 551' 508' 7 : 551 ' 573 Some exp eriments with the U.S.A. Standard Evaporimeter S. P. YEXKITEsm VARAN. P. JAGAX",ATHA", and S. S. Meteo rolog ical O ffice , Poona IRecei V/'d 7 August 1956) t, Inlro<luelloD The U.S.A. Standard Evaporimeter is in regular usc at a number of selected Agricul- tural Farms in the Indian States and in the Observatories of the India Meteorological Department. The evaporimeter consist. of a cvlindrical reservoir 48· in diameter and 10· high supported on a wooden fmme 4- high. The level of the water in the reservoir is maintai ned at about 2- from th e rim so that the height of the level of water is nearly 12- above ground. The evaporimeter is of 20 SWG copper sheet. T he evaporation is measured by accurately determining with t he hook-gauge the change in the level of water in the reservoir after any given interval of time. The hook gauge is placed on a still well provided with levelling screws. This instrument is fairly in wide use at many stations in India for measurement of evaporation. To ensure the comparability of observations recorded at different stations, care is taken as far as possible to keep to the standard of specifications prescribed for the instrument and also to maintain similar conditions of exposure at all stations. But, it was noticed that some conditions of exposure such as the colour of the paint on the evapo' rimeter, the depth of water level in the tank etc, could not be maintained identical at all the places at all times. It was, therefore, considered desirable to investigate the exten t of the effects of such uncontrollable variations in the conditions of exposure. In the series of experiments designed for such a purpose some more factors of variation such as 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 Meteorolo- gical Observatory at Poena and the results of the observations are summarised below. It may also be mentioned here tbat in an observatory tbere is a contribution to the loss of water from an open tan k evaporimeter by birds etc. In the series of experiments discussed bere such losses have been avoided by suitable watcb and maintenance conditions. 2. Colour or paint OD Ill• • nportm.ler When the evaporimeters are supplied to th e observatories, they are painted wbite outside and are either painted white or tinned inside. It may happen in course of time th at the paint may wear away or become discoloured. To study the effec t of colour of paint on evaporation, observations were taken with 6 USA type evaporimeters and they Were painted as follows- A-Tinned inside, white paint outside B-Painted white, both inside and outside C-No paint inside, white outside D-No paint botb inside and outside E-Painted black inside and outside F-No paint inside, painted black outside Taking the mean evaporation of the Stan. dard A as unit, th e ratios of the other means are given in Table 1. F rom Tables 1 and 2 it will be seen that th e effect of the colour of the pai nt on the evaporation is not negligible. While the temperature of the water in the tank in the morning

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Page 1: Some experiments with the U.S.A. Standard Evaporimeter

•

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