by
C. E . H O U N A M ,
Commonwealth Bureau of Meteorology, Melbourne
I N T R O D U C T I O N
Although evaporation from large water storages depends o n local meteorological conditions and expo-sure, it is not as sensitive to variations as is the equipment set u p to measure evaporation losses.
T h e measured loss b y an evaporimeter depends, in addition to the above factors, o n the type a n d d i m e n -sions of the p a n or tank, the material from which it is constructed, a n d its colour, a n d s o m e of these factors m a y be varied to bring about a change as high as 50 per cent in the measured loss. It is essential to standardize one particular tank, a n d in Australia the 3-foot tank with a 6-inch annulus has been selected, but even so there is the danger that incorrect maintenance m a y lead to appreciable errors. For a complete discussion of p a n types a n d performances see the report of the United States C o m m i t t e e [19]1 on "Standard E q u i p m e n t for Evaporation Studies".
In s o m e instances, m o r e particularly in arid areas, the tank water has been covered with a wire screen to eliminate interference b y birds a n d animals. This m a y result in such a distortion of evaporation losses that their value becomes doubtful.
P A N COEFFICIENTS
Extensive investigations have been conducted in the United States into the magnitude a n d variation of the p a n coefficient (ratio of lake evaporation to p a n evaporation) relative to pans in use in that country.
M e a n annual values of coefficients approximate to 0.70 for the United States Class A p a n , 0.83 for the Colorado p a n , and 0.91 for the Bureau of Plant Industry p a n . Coefficients for each type of p a n exhibit seasonal variations; the times of occurrence of m a x i m a and m i -n i m a depe-nd o -n the amplitudes of the t w o evaporatio-n curves, modified b y a n y phase lag in lake evaporation.
If they are in phase, coefficients appear to exhibit a m i n i m u m in winter, but phase lag in lake evaporation m a y delay this m i n i m u m until a u t u m n .
Evaporation losses from a large water surface have not been measured in Australia. In recent years, three experiments have been set u p to measure concurrently the evaporation from large tanks '(10-foot to 12-foot) and the standard 3-foot tank. T h e chief interest in these is that the evaporation from such a large tank has been s h o w n to b e very close to that from a large water surface [2]. M o n t h l y values of the ratios of these t w o evaporation measurements are s h o w n in Table 1.
Generally speaking, values of ratios in Table 1 are least in the winter a n d greatest in the s u m m e r .
It has been demonstrated at L a k e Elsinore, Cali-fornia, that, although the evaporation from a 12-foot tank is approximately equivalent to that from a lake on an annual basis, considerable variation occurs in the monthly values. T h e L a k e Elsinore ratios of evaporation from a lake to that from a 12-foot p a n ranged between 0.75 in winter a n d 1.15 in late spring. H e n c e true p a n coefficients, i.e. the product of these t w o ratios, would be lower in the cold season and higher in the w a r m 1. The figures in brackets refer to the bibliography at the end of this chapter.
T A B L E 1. Value of ratio of evaporation from 10-foot or 12-foot pan to that from 3-foot pan
Station Jan. Feb. Mar. April M a y June July Aug. Sept. Oct. Nov. Dec.
Dry Creek [4]: 10-foot tank 0.75 0.81 0.82 0.78 0.82 0.72 Kirkleigh [18]: 12-foot tank 1.00 1.07 1.11 1.03 0.94 0.87 Griffith: 12- foot tank * — — 0.99 0.84 0.87 0.68 1. Results of 1955 observations received from Officer-in-Charge, CSIRO Research Station, Griffith.
0.88 0.88
0.92 0.81
0.96 0.80
1.01 0.78
1.02 0.95
1.00 0.92
Evaporation pan coefficients in Australia season t h a n the ratios noted in Table 1, provided the
L a k e Elsinore results c a n b e applied in principle to the Australian data.
It has b e e n the practice for a n u m b e r of years for s o m e hydrologie authorities in Australia to use a coeffi-cient of 0.90 w h e n estimating annual loss from reservoirs from Australian t a n k data. This coefficient, h o w e v e r , appears to b e a n "intelligent guess" p u t forward b y R . Follansbee [10] o n the basis of the nearest c o m p a r -able United States records.
E S T I M A T I O N O P E V A P O R A T I O N F R O M A L A R G E W A T E R S U R F A C E
Availability of Data
Some of the terms in the equation, such as T0 and e¿, are direct observations, available for practically every climatological station. Others must be estimated in some or all cases. The most important term, observed at only a few stations in Australia, is Q and a good deal of the subsequent work in this paper involves the estimation of this term. The terms n/N [7] and/(u) (Deacon [8]) are also based on estimated data in most cases. The reflec-tion coefficient r has been kept constant in spite of probable variation as the surface ranges between sand, gibber, rocks, grass, or tree cover.
Equation Adopted
The estimation of evaporation from meteorological elements has been given full theoretical treatment by Ferguson [9] and P e n m a n [15], working independently.
Both developed equations, which involve the same general terms and both make similar assumptions.
The form of the equation used here is that of Pen-m a n [15].
F r o m Ferguson's studies on the differential analyser and Penman's discussion on evaporation over the British Isles [16], it appears that both workers are confident that the equation closely approximates to the evaporation from a large water surface.
The equation in the form quoted by P e n m a n is:
the reflection coefficient for the surface;
the measured short-wave radiation/cm. 2/day;
the theoretical black-body radiation at T0° K ; the temperature of the air;
saturation vapour pressure at d e w point in m m . H g ; ratio of actual to possible hours of sunshine;
deJdTa — the slope of the e : T curve at T = T „ ; the standard constant in the psychometric equation
= 0.27 in °F. and nun. H g ;
E0 = the value of E obtained b y putting e, = e<j in the sink strength formula E = (e4 — ea) f (u), where e, = saturation vapour pressure at the water surface
temperature T„ (generally u n k n o w n ) ;
ea = saturation vapour pressure at the air temperature Ta;
T h e equation w a s applied to meteorological data for M e l b o u r n e a n d Alice Springs for the years 1953 a n d 1954, w h e n accurate m o n t h l y values of Q w e r e first available [6].
Table 2 s h o w s estimated values of evaporation from a n o p e n water surface in inches per m o n t h .
In the a b o v e calculations, values of Q w e r e readily available. H o w e v e r , w h e n it is desired to calculate values of evaporation for particular years for stations not equipped with radiation measuring e q u i p m e n t or average values at a n y station including those in the radiation n e t w o r k , it is necessary to b e able to estimate the required radiation fairly accurately.
A t this stage it is therefore necessary to see w h a t data can b e extracted f r o m the n e w l y established Australian radiation n e t w o r k .
E S T I M A T I O N OF R A D I A T I O N Inadequacies of some Expressions
P e n m a n proposes that in cases w h e r e Q has not been w h i c h does not appear to fit the data satisfactorily.
H o w e v e r , as the a b o v e equation does not include a n y T A B L E 2. Estimated values of evaporation from an open water surface (Penman's equation) in inches per m o n t h
Date
Climatology and microclimatology / Climatologie et microclimatologie means of varying turbidity it could not be expected to give reliable values of Q . The precipitable water in the atmosphere, cloudiness, and m e a n length of the air mass path have seasonal variations which produce variations in Q . Such variations have been noted by P e n m a n [15].
Where dust is an additional factor tending to reduce the amount of solar radiation reaching the earth, as over the arid zone of Australia, then greater difficulties are involved in making the required estimates.
Relation between Q / Q0 and n / N
It has been shown by Fritz and McDonald [12] that there is a fairly close relationship on a monthly basis between Q / Q o and n / N , where Q0 is the mean radiation received on a cloudless day at the surface. Use of Q and Qo cancels out the error due to variations in the air mass path, but it does not eliminate variations in water content entirely. For example, there might be a large difference in water content on clear and cloudy days at Townsville, but not at Melbourne; in this case the ratio Q / Q o would be lower at Townsville. This is one reason therefore w h y some scatter of points would still be expected. Another reason might be the influence of dust if there is differential scattering on clear and cloudy days.
The Australian radiation network has been func-tioning n o w for over three years, and two years of record have been analysed and published [5, 6]. These published data include the mean monthly radiation Q in cal./cm.2/day, and the highest and lowest daily recordings for each month.
As values of Q0 were not readily available from the original records, the highest monthly values were assumed to have occurred on clear days; after some graphical smoothing approximate mean values of Q0
were obtained for four stations, Guildford, Darwin, Alice Springs, and B o x Hill for which values of n / N are held.
Values of Q / Q o were plotted against n / N and the correlation shown in Fig. 1 was obtained. This is reason-ably good considering the fact that Q„ is based on only two years of record.
The empirical relationship is found to be Q / Q0
= 0.34 + 0.66 n / N , which agrees with the fact that when n = N then Q = Q0.
This expression agrees closely with that of Fritz and McDonald [12] Q / Q0 = 0.35 -f 0.61 n / N , based on long-term data for the United States, and that of Mateer [14], Q/Q0 = 0.355 + 0.68 n / N , based on Canadian data for the s u m m e r months.
In Mateer's work the s u m of the constants (0.355 + 0.68) was deliberately permitted to exceed 1.00 to compensate for the under-recording of the Campbell-Stokes recorder even on clear days.
Determination of Q0
Before any practical value can be obtained from the relationship shown in Fig. 1 it is necessary to deter-mine Q0.
In the case of the established radiation stations an approximate value of Q0 m a y be found as above. In the case of non-recording stations the estimation of Q0 m a y be quite difficult.
Both Fritz [11] and Albrecht [1] have indicated the dependence of Q0 on water content. Fritz's method using the precipitable water of the atmosphere is very sound but cannot be confidently applied to Australian data owing to the sparseness of the radiosonde network.
Albrecht's method has been developed to give the instantaneous radiation and is not readily adaptable to yield m e a n monthly data. However, an interesting point is that he is satisfied to use surface vapour pressure.
The relationship between values of Q0 and the mean value of the air mass was then tested. The m e a n air mass is a function of latitude and date (or declination of the sun) and was calculated from Kennedy's graph [13]. In view of approximations already m a d e the air mass values were not corrected for use in the southern hemisphere.
The full lines shown on Fig. 2 are the mean curves for Darwin, Garbutt, Williamtown, Guildford, and Box Hill. O n labelling these with the average value of the 0.900 inches of mercury vapour pressure, it is found that they fit in order of descending vapour pressure from left to right and that ¡other isobars (0.800, 0.700, etc., inches of mercury) can be readily fitted in their correct position (broken lines).
Although annual vapour pressure should not give such a satisfactory fit it is probable that the figure approximately represents the seasonal trends also, i.e.
at Darwin the mean is higher than at any other station and so are the highest and lowest monthly values. This is very convenient, as it eliminates the necessity for monthly graphs, which would be almost impossible to construct owing to the small amount of data available over two years.
Precipitable water would be expected to improve the above relationship when such data ultimately become available.
N o w when Alice Springs, the only inland station in the network, is plotted on Fig. 2, it is found to be a poor fit. The vapour pressure 0.267 inches should place it on the extreme right of the family of curves if the above explanation is correct, whereas it falls near Williamtown at a point where a vapour pressure of about 0.48 inches would be expected. This means that for a given air mass, i.e. time of the year, Alice Springs receives considerably less radiation than would be expected by comparison with coastal stations.
There are two reasons w h y the plotted data for this station might vary from the expected. The first, a minor
Evaporation pan coefficients in Australia
5-50
40
• A A L I C E S P R I N G S 1 9 5 4
• D D A R W I N 1 9 5 4 + d M 195 3
• G G U I L D F O R D 1 9 5 4
+ S » 1 9 5 3 d a
• M M E L B O U R N E 1 9 5 4
O
D12
M 7 rri5 + •
M
• M 6 ^ m 6 + / í
/ + m e
A
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Gll • • , 6 1 2 d
gl g* +
D" /
g11+ • G 3/ /^
0 3 • . G 1 0 * G 9 + + • / «A
d u A U A6« " D I O D2 M, \m 3++ +83 m 1 2 + / ,+ ; / id" A i o A 4 « m z « i z / g s + • / m i + g *
&Z Ml M 3
m i l / • G5 D 1 • + / • • 6 6
/ 114 d i+ . M 11 + m i o
* G7 " M 9
5.H2 / m 4 M 8 /
/ M 4 5*
MÍO m 7
o e Q / 00 = 0 0 4 + O
-o e /
• + g '2 / De da *A9 D 5 - - / f
D 9 » « _ + d 7
» / G 2 . \ d8
. A7 Al ^d5
>z « A 8 5
6 6 n / N
O
n/V
F I G . 1. Relation between radiation and sunshine. Numerals next to letters refer to month of the year, e.g., 1 = January; 2 = February, etc.
one, is that being situated at approximately 2,000 feet able to scatter or reflect a large proportion of the above sea level the air m a s s would b e slightly less incoming radiation. A s dust would b e constantly present (7 per cent) than if it were at sea level. T h e second is
that the a m o u n t of dust in the atmosphere spread
through a considerable depth owing to instability is tion of Q0.
in varying proportions over the interior of the continent, it presents a serious obstacle to the satisfactory
estima-Climatology and microclimatology / Climatologie et microclimatologie
8 0 0
7 0 0
a 6 0 0 13
l./cmí
0
¿ 5 0 0
4 0 0
3 0 0
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.1 , N • >2«S 2 - 6 2 - 7 2 ' 8 2-9 3 - 0 3 1 3 - 2 3-3 3-4 3-5 3-6 3-7 3 - 8 3«9 4 - 0 4-1 4 - 2 4 - 3 4 - 4 4 - 5 4 - 6 4 - 7 4 ' S M E A N AIR M A S S
FIG. 2. Values of Qo at mean sea level at coastal stations plotted against mean air mass. Values on curves are mean annual vapour pressure 9 a.m. (in inches X 103).
A t Alice Springs the reduction in radiation appears to be approximately 20 to 25 per cent during M a y to September, but it is difficult to estimate what it would be in the warmer months. Although the summer months are the wettest, they are still relatively dry.
Interpolation between Alice Springs and the coastline is rather rash, but there m a y be little change in turbidity over a large area of the interior. N o doubt this could be estimated approximately b y reference to average rain-fall and vegetation m a p s , but the only satisfactory method of obtaining data over the interior would be to establish an inner ring of radiation stations at such places as Cloncurry, Halls Creek, Meekatharra, Kal-goorlie, Oodnadatta, Mildura, and Charleville.
Thus for coastal stations, where the atmosphere m a y be expected to be relatively dust free, the value of Q0
can be read from Fig. 2 . For stations situated over the dusty interior Q0 m a y be estimated from vapour pres-sure relative to the curve for Alice Springs, whilst for intermediate stations a less reliable estimate can be m a d e based on vapour pressure and a turbidity factor somewhat less than for Alice Springs and probably with a more seasonal bias depending on climate.
F r o m a knowledge of the value of re/N and a deter-mination of Q0, a value for Q can be obtained from Fig. 1.
If n is not observed at a station a fairly good value for n / N can be determined from cloudiness. This has been determined b y work carried out in the C o m m o n -wealth Bureau of Meteorology [7].
APPLICATION O F E Q U A T I O N USING E S T I M A T E D R A D I A N T E N E R G Y
The station selected for this purpose was Griffith, which is situated in the Murrumbidgee Irrigation Area (lat.
34° 17' S., long. 146° 0 3 ' E . ) at a height of 420 feet above sea level. A s the average monthly rainfall is between one and one-and-a-half inches, the station is obviously not in the arid zone, but the water require-ments of crops are high and cannot be maintained b y rainfall alone. T h e station is therefore beyond the normal agricultural belt.
The reason for the selection of Griffith was that a set of evaporimeters of various diameters has been installed there (in 1955), and a comparison between observed and estimated values of evaporation should be interesting.
The chief difficulty m e t in the estimation was the determination of Q0 for Griffith. This was based on the average annual vapour pressure and an estimate of atmospheric pollution, but it is realized that this latter