METHODIC RESULTS OF EXPERIMENTAL RESEARCH OF EVAPORIMETER EFFICIENCY

Artùr S E R M E R Hydraulic Research Institute, Bratislava, Czechoslovakia

SUMMARY

Different types of evaporation pans with different surfaces, depths and different kinds of location in the field have been used in the practice of evaporation measure­ments in our country, as well as abroad. Values measured by the different types of evaporimeters have been varying to a great extent. Converting the evaporation from evaporimeters into the real evaporation from reservoir water surfaces the conversion coefficients determined in the process of research, or conversion equations derived from functional relationships of evaporation from liydrometeorological elements, have been used in practice. The points dispersion of the values is fluctuating between more or less extended limits. This fact is caused to a great extent by inaccuracies in the measured evaporation values. The results of the research show, even a small evaporimeter is highly efficient, if suitably exposed to the effects of liydrometeoro­logical elements, its measuring device being accurate. The paper presented contains methodical results of experimental research of evaporimeter efficiency introduced in our country as well as abroad. On the basis of analysis performed a standard type of evaporimeter for the station network in Czechoslovakia is proposed. The proposed devices for evaporation measurement can be, as far as their physical efficiency is concerned, compared with a basin for evaporation measurement, with an area of 20 sq.m. At the same time a method converting evaporation from different types of evaporimeters into the actual evaporation from reservoir water surfaces is proposed. Research results and relationships computed therefrom are elaborated on the basis of data obtained successively on six experimental stations, situated in different ele­vations above sea level and in different climatic conditions since 1957 till the end of 1962.

1. RESEARCH LOCATIONS AND MEASUREMENTS PERFORMED

Experimental research of the efficiency of different types of evaporation pans was performed on :

a) prototype floating evaporation stations (Fig. 1), under conditions of micro­climate of small and big water reservoirs in different elevations above sea level — Orava, Senec, 2ihârec 2,

b) evaporation stations under conditions of microclimate of dry land weather conditions, (Fig. 2), Ziharec, Hurbanovo, Tisice, Hlasivo 2 and 3, on which evapori­meters presented in Table 1 were verified.

For the verification of the efficiency of evaporation pans the values measured on above mentioned experimental stations from 1957 to 1962 were used.

Pan evaporation was measured on these stations every day at 7.00 o'clock a.m. Besides the pan evaporation on dry land and on water reservoir the temperature of water table was measured in all evaporation pans at 7.00, 14.00 and 21.00 o'clock. Anemometric and psychrometric measurements were performed in the height of 200, 100.50 and 5 cm above the evaporation basin II with an area of 20 sq.m. The water temperature was measured by means of thermometer closely at the water surface in evaporation pans and water reservoirs. The vapour tension (<?o) was computed from water surface temperatures for each evaporation pan respectively. The vapour tension in the height of 200 cm (e2oo) was calculated from values observed in meteorological shelters.

253

Fig. 1 — Evaporation floating station on Orava water reservoir.

Fig. 2 — Evaporation station on dry land at Ziharec.

2. R E S E A R C H OF RELIABILITY A N D SUITABILITY OF EVAPORATION PANS INVESTIGATED

The relations between evaporation values from evaporation pans investigated and values of such selected étalons, which are physically nearest to the reality were

254

determined by means of mathematic and statistic computations. As such were con­sidered :

a) values of evaporation from floating evaporation pan with an area of 3 sq.m. b) values of evaporation from evaporation basin II with an area of 20 sq.m. c) values obtained from the dependence of evaporation upon the hydrometeorol-ogical elements. From these relationships the equations of evaporation pans tested were computed

and the degree of their accuracy and reliability was determined according to the computed correlation coefficients and intervals for a 90-per cent accuracy. The depen­dencies were worked out graphically. For the evaporation pans tested we have devel­oped comparative graphs of intervals, which made possible the determination of their sequence of reliability and the suitability of their introduction into network of stations.

The relationships between daily evaporation values from comparative floating evaporation pan with an area of 3 sq.m and from evaporation pan tested showed :

a) The coefficients of correlation from values of evaporation pans VI, IX, X, XI XIII and XIV are in the limits of 0.917 and 0.860. The lowest correlation coefficient is shown by the evaporation pan XIV, located 40 cm above the earth surface, on a wooden post (the type Ron, Czechoslovakia) ;

b) Intervals for 90 per cent reliability are in the limits of 1.231 and 2.642. The greatest intervals are shown by values originating from evaporation pans with the smallest area, the least accurate being the evaporation pan XIV, figure 3a;

c) Evaporation pans with the area of 3 000 sqcm and greater, located in the soil, show practically the same accuracy.

The relationships between mean daily evaporation values from the comparative floating evaporation pan with an area of 3 sq.m and from evaporation pans tested showed :

a) Correlation coefficients from the values of evaporation pans II, III, IV, V, VI, IX, X, XI, XII, XIII, XIV and XV are between 0.988 and 0.876. Intervals for the 90-per cent reliability are between 0.308 and 1.202. Small accuracy have evaporation pans with small area. From these the least accurate are evaporation pans XIV and XV, figure 3b;

b) The evaporation pahs with an area of 3 000 sq.cm and greater located in the soil, show practically the same accuracy.

The relationships between daily evaporation values from the comparative evapor­ation basin II, with an area of 20 sq.m and from evaporation pans tested showed

a) Correlation coefficients from the values of evaporation pans III, IV, V, VI, VII, VIII, IX, X, XI and XIV are between 0.968 and 0.693. Intervals for the 90-per cent reliability are between 0.558 and 3.06, figure 3c;

b) The lowest accuracy was demonstrated by evaporation pans located above the soil surface. From these the least accurate was the evaporimeter XIV and then the evaporation pan VIII. Evaporation pans IX, X and XI with an area of 3 000 sq.cm, located in the earth, have practically the same accuracy. The lowest point dispersion have the basins and evaporation pans with greater surfaces;

c) Between the daily values of evaporation pans XV (Wild's evaporimeter at Hurbanovo) and comparative basin with an area of 20 sq.m in Ziharec there was found no dependence. In average daily values of individual months a dependence was shown due to the daily values compensation.

The relationships between mean daily evaporation values of the comparative evaporation basin II with an area of 20 sq.m and of evaporation pans tested showed :

a) Correlation coefficients from values of evaporation pans III-XV are in limits of 0.999 and 0.869. Intervals for 90 per cent reliability are between 0.109 and 0.891 figure 3d;

b) Low accuracy is demonstrated by evaporation pans XIII , XIV and XV; from these the lowest accuracy has the evaporation pan XIV. Evaporation pans IX,

255

x xv vu xiv m rv v vi x vrn xi ix vwxiv iv m iv vu vnixt vi xn v x xtnix xiv vu xi m tv vi n vin tx v x xnxmxiv

Fig. 3 — Interval for 90-per cent reliability of tested evaporation pans II-XV.

X,XI, XII with the area 3 000 sq.cm have practically the same accuracy. The highest accuracy is shown by the evaporation pans with greater areas. In this case it is the evaporation basin III, with an area of 10sq.m.

The relationships between mean daily values of evaporation from the compara­tive evaporation basin II with an area of 20 sq.m and from evaporation pans tested on research stations at Tisice and Hlasivo showed :

a) Tisice : correlation coefficients of three evaporation pans tested are as follows : the evaporation pan X, with an area of 3 000 sq.cm, 0.976; evaporimeter XV (Wild's in the shelter), 0.818 and evaporation pan XIV (type Ron), 0.955. According to the sequence of accuracy the best is the evaporation pan X with an interval of 0.352, evaporimeter XV with an interval of 0.478 and the lowest accuracy is shown again by the evaporation pan XIV with an interval of 0.682, figure 3e.

b) Hlasivo : correlation coefficients of the tested four evaporation pans VII, X, XIV, XV are between 0.966 and 0.814. The highest is at the evaporation (station) pan X 0.966, the lowest at evaporimeter XV 0.814. As far as the order of accuracy is concerned the best is the evaporation pan X with the interval of 0.357, the least accurate being the evaporation pan XIV with the interval of 0.694, figure 3f.

V Relationships between mean daily values ----- and "W7" from the tested

evaporation pans at 2iharec, where " V" is the evaporation, eo — «200 is the differ­ence of vapour tension near the water surface and in the height 200 cm above the water surface and " W" is the action of wind in m per s in the height 200 cm, showed :

a) Correlation coefficients of tested evaporation pans II-X1V are between 0.960 and 0.818 thus demonstrating that the dependences are close. The least close depen­dence was observed at evaporation pan XIV (type Ron), 0.818;

b) Intervals for the 90 per cent reliability are in the limits of accuracy of 0.057 and 0.210. The greatest interval has the evaporation pan XIV (Ron) and therefore it has the lowest acci racy. Evaporation basins II, III and IV have the smallest intervals and therefore they are most accurate. The intervals of other evaporation pans V-

256

XIII are rather heterogenously distributed in the limits from 0.087 to 0.122 (difference 0.035) that means that their values in the dependence upon hydrometeorological elements used, have practically the same significance, figure 3g.

The relationships between mean daily values of pan evaporation from evapor­ation pans II-XIV and eo — «200 at Ëihàrec showed :

a) Correlation coefficients of tested evaporation pans are in the range of 0.950 and 0.888 thus demonstrating these dependencies are also relatively close;

b) Intervals for 90 per cent reliability are in the ranges from 0.546 to 1.190 mm. The greatest interval (1.190) has the evaporation pan XIV (Ron) and therefore it has the lowest accuracy. Further follow the evaporation pans XIII (interval 0.987)and XII (interval 0.950); intervals of other evaporation pans II-XI are getergenously distributed between limits from 0.546 to 0.847 mm (the difference 0.301), what does mean, that their values in the dependence upon eo — £200 have relatively equal signi­ficance, figure 3h;

V c) Comparing with the relationship between rf~zZ7—\ m& "W" it is evident

that evaporation basins II, III and IV have not the smallest intervals in the case given. This is however dependent upon the more extensive air movement above the basins, which was not considered in this computation.

3. DESIGN OF THE PROTOTYPE OF A STANDARDEVAPORIMETER EQUIPMENT FOR THE NET­

WORK OF STATIONS IN CZECHOSLOVAKIA

Determining the standard type of evaporimeter following criteria were taken into consideration :

a) the efficiency of the evaporimeter b) the reliability and simplicity of the measurement c) the economy Verifying the efficiency according to the analysis mentioned above the evapor­

ation pans of circular shape, with different surface area, depth, diffusion height and location in the evaporation station were considered. It was demonstrated by means of investigations that other than circular shaped evaporimeters were not suitable (6).

The verification of the efficiency showed : a) some measured values of evaporation pans tested are in the limits of reliable

accuracy and some of the evaporation pans are inaccurate. b) In order to obtain higher accuracy in the evaporation measurements the eva­

poration pans with an area smaller than 3 000 sq.cm, deep less than 50 cm, with a diffusion height higher than 5 cm, sunken into the earth, or located loosely above the soil surface and in meteorological shelters can not be recommended as suitable.

c) Circular evaporation pan with an area of 3 000 sq.cm and greater are suitable for the network of evaporation stations.

d) The best accuracy can be obtained with investigated evaporimeters II, III and IV, with an area of 20, 10 and 3 sq.m respectively, with regard to the possibility of better air movement above them.

e) Evaporation pans with areas of 0.3-2 sq.m, sunken into the earth, with upper border 5-6 cm above the ground level, are less accurate with regard to their relatively less possibility of air movement;

/ ) Evaporation pans with a smaller surface area and depth, located loosely above the ground are quickly growing warmer due to the radiation of sun and overnight become cool. The water in them is getting frozen quickly. Thus are caused inaccur­acies in evaporation values.

257

Fig- 4 — Standard evaporimeter equipment with a device for measurement of evapor­ated water column : 1. Total assembly, 2. Head of the intake pipe, 3. Intake pipe, 4. Removable fixator.

258

The reliability and simplicity of the measurement of evaporated water column in mm was solved according to figure 4 Nos. 2, 3 and 4. Measurement equipment is patented (No. 96303 Patent Office, Prague).

Proposing the evaporation pan for the basic network of stations an account was taken of the economy of its establishing and further operation. Besides the basic network of stations it is necessary to reckon with the purpose network, which is going to be forced by the practice. Evaporation basins and evaporation pans with greater areas, though most accurate, are very expensive, as far as their manufacture and establishing is concerned. In order to avoid deformation they must be located on a concrete slab. When they are in operation, they are pretentious to water in connection with filling and maintaining of levels. During the operation their cleaning, painting, spraying and repairing is very expensive.

On the basis of research results it was proposed to use an evaporation pan with an area of 0.3 sq.m, deep 60 cm and with the diffusion height of 5 cm in the network of evaporation stations. The proposed evaporation pan is settled in the evaporation station according to figure 4, No. 1 in this way :

a) The evaporation station is located in a plain field without growth, which is characteristic for the area investigated;

b) The area of the stations is covered with short cut grass ; c) The evaporation pan is sunken into an earth bank the upper border of which

is 5 cm distant from the upper border of the evaporation pan and the border is on the earth level 25 cm from the upper border of the evaporation pan. The earth bank is created out of slanting grass covered area with a base-line 60 cm on the ground level

Experiences obtained showed, the measured values from the evaporation equip­ment situated and assembled in this way are in the closest physical relation to the hydrometeorological elements. The evaporation pan is relatively most suitably ex­posed to the action of wind and water is warmed in the best ratio to the temperature of the surrounding air, which is the most essential element, contributing to the creation of microclimate of the location observed.

The computations from the values measured during the last two years of evapor­ation equipment control have shown its effectiveness is practically the same as that of the evaporation basin with an area of 20 sq.m. The comparative graph of intervals, (Fig. 5) and integration lines of relative deviations of computed and measured evapor-

p p p p p p p p p p p p

20 m

STANDARD

I I

GGI

- IN

TE

RV

AL

9C

A PAN

x Fig. 5 — Accuracy limits of functional relationships of evaporation pans tested.

ation values, figure 6 show the accuracy of the designed standard type comparing with evaporation pans used in USSR (GG1-3000), USA (Class A Pan) and with the

259

0,8

0,7

2 20 m ® — STANDARD® —

GGI

A PAN

® — © —

V=(0,09SomO,3200)(e0

V=(0,2544W+0,1981)(e0

V=(0,20S3W+0,200U(e0

V=(0,2583W+0,32l7S(e0

-e ) 200

~ezoo -e ) zoo

-w 25 SO 75 100%

Fig. 6 — Integration lines of relative deviations of measured and calculated evapor­ation values according to the equations of evaporation pans.

basin with an area of 20 sq.m. Graphs on figures 5 and 6 were constructed from the V

results of computed relationships between mean daily values and (en—e2oo) for the period March-October 1961 and April-October 1962. Intervals for 90 per cent reliability are with these evaporation pans as follows : evaporation basin with an area of 20 sq.m 0.0558, designed standard with an area of 3 000 sq.cm 0.0671, GGI-3000 0.0880 and Class A Pan 0.1197. Equations of these evaporation pans on figure 6 are valid for the climatic region of the Lower Danube Plain and for similar regions.

4. THE METHOD FOR CONVERTING THE PAN EVAPORATION VALUES IN CONDITIONS OF

DRY MICROCLIMATE TO REAL EVAPORATION VALUES FROM RESERVOIR WATER SUR­

FACES

This method has been already described and published in works 2 and 5 but in order to obtain a further more comprehensive view of the work with evaporimeters from the point of view of the requirements of the water management practice and because the converting of pan evaporation to the real evaporation from reservoir water surfaces by means of improperly considered converting coefficients is still in use, I shall shortly deal with this method. The real evaporation from reservoir water surfaces was measured in natural conditions of microclimate of water reservoirs, located in different elevations above sea level on Orava, Senec and 2ihârec, by means of prototype floating evaporation stations, (Fig. 1). These stations were anchored far from the waterside as to prevent the influence of the shore microclimate on the course of evaporation. The evaporation was measured with a special prototype device (Fig. 4), No. 2, 3 and 4 from an evaporation basin with an area of 3 sq.m. Micro­climatic conditions were not affected by this floating device and we have obtained practically coincident difference in vapour tension above water surface and thus also a coincident evaporation capacity according to the formula :

V = / (eo — «20o) (D where : V = evaporation; eo = vapour tension near the water surface; £200 = vapour tension in the height of 200 cm above water surface.

260

The recent calculations from measurements on these water reservoirs have shown, for the relation between mean daily value of evaporation and mean difference in vapour tension is valid :

V = 0.6029 (eo - £200) - 0.0799 (2)

where : V = mean daily evaporation value in mm of water column, eo — e200 = mean value of the vapour tension difference in mm/Hg, eo = being the vapour tension, calculated from the water surface temperature in the floating evaporation pan with an area of 3 sq.m and also in the reservoir and £200 is the vapour tension in a height of 200 cm (in a meteorological shelter) on the floating evaporation station.

Relative deviations from measured evaporation values have shown that from the total number of mean points 50 (1 point = 30 daily values arranged according to the magnitude of eo — Ê20O) 40 points are in the limit of average deviation ± 0 . 1 0 ; 6 points are in the limits of average deviation ±0 .15 and 4 points are in the limit of average deviation ±0.18. Similarly also the correlation coefficient 0.98 has shown, the functional relationship is extraordinary close. The interval for 90 per cent relia­bility is 0.38. The regression line of this relationship is presented on figure 7, No. 1. Considering that this functional relationship is in our country valid for different elevations above sea level and different areas of water reservoirs it is to be holded as very favourable. The use of further function " W " in the computation did not bring any improvement of this relationship in conditions of microclimate of water reservoir.

From the relationships " V" and (eo — £200) w e have computed the equations of station evaporimeters. The equations of these evaporimeters were converted to equation (2), figure 7 according to

.<£ <q 1 I I I 1 1 1 | 1 \-^—zz^.. 0 1 2 3 4 5 6 7 0 9 10 11 12

Fig. 7 — Relationship " V " and (eo — £200" from water reservoirs Orava, Senec and Ëihârec.

261

where : Y = real evaporation ; K = regression line of the equation (2) ; Ki = regression line of equations of station evaporimeters on dry land; x = difference in vapour tension eo — e2oo above the station evaporimeter; qi= the section defined on the axis Y by the line of station evaporimeters equations and q = the section defined on the axis " Y" by the line of the equation (2).

For needs of water management practice we substitute into the equation (2) values eo — ^200, obtained from measurements on station evaporimeters and thus we attain an estimation o the real evaporation from reservoir water surface of given region of station. From the equation of evaporimeters we have further constructed for need of water management monographs for direct reading of real evaporation from reser­voir water surfaces, (2), (4) and (5).

TABLE 1

Evaporimeters verified by research

No of eva-por. pan

II

III

IV

V

VI

VII

VIII

IX

X

XI

XII

XIII

XIV

Setting in the soil

m

1.32

1.32

1.34

1.34

1.34

0.54

0.20

1.35

0.55

0.55

0.45

0.20

0.40 above j

Surface area sq.m

20

10

3

2

1

1

1

0.3

0.3

0.3

0.3

0.2

0.2 ;round

Depth

m

1.40

1.40

1.40

1.40

1.40

0.60

0.60

1.40

0.60

0.60

0.50

0.25

0.25

Diffusion height

m

0.10

0.10

0.07

0.07

0.06

0.06

0.06

0.05

0.05

0.05

0.05

0.05

0.05

Note

white painted metal-plate in a concrete ring

white painted metal-plate in a concrete ring

white painted zinc pla­ted metal-plate

white painted zinc pla­ted metal-plate

white painted zinc pla­ted metal-plate

white painted zinc pla­ted metal-plate

white painted zinc pla­ted metal-plate

white painted zinc pla­ted metal-plate

white painted zinc pla­ted metal-plate

white painted zinc pla­ted metal-plate with air-insulation

white painted zinc pla­ted metal-plate with air-insulation

white painted zinc pla­ted metal-plate with air-insulation

zinc plated metal-plate on wooden post

XV I Wild in shelter

262

It is true that equations of station evaporimeters according to relationships V/(e0 — Ê200) and " W " are essentially more accururate as relationships between " V"

V i — v In our network of stations the wind velocity is chiefly observed by

means of estimation according to Beaufort's scale. Thus inaccuracies in determining the wind velocity in m/s are caused. Therefore the use of evaporation relationships " V" from the difference in vapour tension <?o — Ê200, obtained without considering the wind " W" was proposed for these cases.

For the proposed standard evaporation pan at the station Zihârec in Lower Danube Plain this relationship was computed :

V = (0.254 W+ 0.1981) (e0 - «200)

CONCLUIONS

In the sphere of the research of evaporation from reservoir water surfaces I consider as essential contribution the fact that we were able :

a) to determine the method for checking the evaporation pan efficiency and on the basis of this to propose for the evaporation network in Czechoslovakia an effi­cient, very accurate and economically unpretentious evaporimeter ;

b) to determine a method for the conversion of pan evaporation in conditions of dry microclimate (stations evaporimeters) to real evaporation from reservoir water surfaces ;

c) to improve and propose the evaporimetric devices and thus to attain higher accuracy of searched values. These problems have not been successfully solved til now. The investigations are in the stage of making research results more accuratl

LITERATURE

(!) DTJB, O., "Hydrolôgia", SVTL, Bratislava, 1957, CSSR. (2) SERMER, A., "Metodika merania a urcovania strât vody vyparom z vodnych

ploch", Vodohospodârsky casopis SAV, roc. VIII, c. 3, 1960, CSSR. (3) VÂSA, J., "Vypar z volné hladiny — zhrnuti dosavadnich pozorovâni". Dilci

_zâvèrecna zprâva 1959, VÛV Praha, CSSR. (4) SERMER, A., "Vyskum metodiky a techniky merania vyparu z vodnych nàdrzi",

jzâvërecnâ zprâva, VÛV Bratislava, 1960, CSSR. (5) Sermer, A., "Method of Measurement and Determination of Evaporation Losses

from Water Surfaces" IAS G.A. of Helsinki, 1960. (6) URYVAJEV, V.A., "Experimentalnyje gidrologiceskije issledovanija na Valdaji",

Gidrometeoizdat, Leningrad 1954. (7) LAPWORTH, C.F., "Evaporation from the Water Surface of a Reservoir", Sym­

posia Darcy, Dijon, September 1956, Tom I. (8) LONGACRE, L.L. and BLANEY, H.F. , "Evaporation at high Elevations in Cali­

fornia", Journal of Irrigation and Drainage Division, Vol. 88, No. IR2. June 1962, A.S.C.E., USA.

263