THE RAINFALL MEASUREMENT PROBLEM

J O H N C. R O D D A Hydrological Research Unit,

Wallingford, Berkshire, United Kingdom.

RÉSUMÉ

Les hydrologues emploient des mesures de pluie afin d'évaluer la hauteur moyenne des chutes de pluie dans un bassin versant et pour bien d'autres raisons. Ces mesures sont prises au moyen d'un pluviomètre étalon dont les bords se trouvent à une distance invariable du sol. Un tel pluviomètre est censé mesurer la pluie qui atteint le sol, mais à vrai dire la quantité de pluie qui atteint ie sol est toujours plus grande que celle qui est enregistrée par un pluviomètre étalon. De là vient qu'il y a une erreur systématique dans une mesure de pluie prise de la façon normale, erreur qui peut influencer sensible­ment tout calcul qui s'y rattache.

Pendant une période de 5 années à Wallingford la hauteur pluviométrique totale marquée sur un instrument installé au ras du sol a été de 6,6% plus grande que celle enregistrée par un pluviomètre haut d'un pied (30,5 cm) (hauteur normale) qui se trouvait à une distance d'environ 16 pieds (5 m). La différence entre les lectures a atteint son minimum en été et son maximum en hiver et au début du printemps ; mais d'un jour à l'autre il y a eu une variation considérable entre les écarts. Pour examiner les raisons de ces différences, on a mesuré la vélocité du vent et l'angle des chutes de pluie, mais sans réussir tout à fait. Aucune régularité n'a pu être constatée dans la distribution de pluie dans le bassin de réception. On a estimé que le pluviomètre étalon était probablement plus accessible à l'éclaboussement.

Sans réussir à trouver une explication complète des différences entre les prises d'eau on a conclu que le pluviomètre installé au ras du sol fournirait des mesures qui corres­pondraient le plus exactement à la pluie effective. Pour les besoins de recherches, et surtout dans les bassins expérimentaux et représentatifs, on a recommandé l'installa­tion de pluviomètres au ras du sol à côté de pluviomètres étalons.

SUMMARY

Hydrologists employ measurements of rainfall to estimate the mean rainfall over a river basin and for many other purposes. These measurements are made in standard gauges installed with their rims at a fixed distance above the ground surface, but they are assumed to measure the rain actually reaching the ground. However the quantity of rain reaching the ground is invariably greater than the amount recorded by a standard gauge. Hence there is a systematic error in a measurement of rainfall made in the conventional way; an error which may affect appreciably any estimates employ­ing these measurements.

At Wallingford, over a period of 5 years, the total rainfall measured by a gauge level with the ground surface was 6.6% greater than that recorded by a gauge at the standard height of 1 ft (30.4 cm), some 16 ft (5 m) away. The difference between the gauges had a summer minimum and a maximum in the winter and early spring, but there was considerable variation in the difference from day to day. Measurements of wind speed and angle of rainfall were used to investigate the reasons for these differ­ences, but without complete success. No patterns were found in the distribution of rainfall across the site. Splash was considered more likely to affect the standard gauge.

Although the differences in catch could not be entirely explained, it was concluded that the ground level gauge would produce measurements nearest the real rainfall. It was recommended that ground level rain gauges should be installed alongside standard gauges for research purposes, especially in experimental and representative basins.

For most purposes it is assumed that a standard rain gauge measures the quantity of water reaching the earth's surface. However this assumption is not valid, because a number of investigators have shown that the amount of rain falling on the ground is not what is measured in a conventional rain gauge. It is surprising that this fact is not generally recognised and applied in hydrological studies for tests of performance of

2 1 5

rain gauges commenced rather more than 100 years ago. Since then there have been numerous experiments in design and comparisons of various types at a range of sites. New techniques have been devised such as the use of radar, and the separate but allied problem of snowfall measurement has also been studied. There is still no method of measuring the quantity of rain falling at a particular point on the earth's surface to a known degree of accuracy.

Of course there are other problems in the use of rainfall measurements for practical and research purposes, such as the conversion of point values to areal estimates. In attempting to solve these and other difficulties the errors of measurement have often been forgotten. However the problems of collecting a representative sample of rainfall by siting a gauge so that it is free from faults of exposure, cannot be ignored as they are linked with the efficiency of the gauge itself. For the actual catch of a gauge is a function

\ RAIN I [Drop a,amttrr

•

GAUGE Gtomttry, bright,

or.hct I-/*, nalurt of ititround.

CATCH

\ESTIMA \ RAIt

\ A1 \ P0

TE or I

JFAU 1 A

NT 1

\ WIND j

ENCE \

H IN

/ GAU

\ ipiaih out,

GF \ n, Iraki, j loporohon, I ondmsotion j

j ERROR

\MEA$Ufi

S Of 1

1EMENT

TOPOGRAPHY OF THE AREA

AROUND THE GAUGE 1 Scale km.)

SITE OP THE GAUGE

{Scale m.)

Fig. 1 — Conceptual model of the processes involved in determining rainfall with a conventional rain gauge.

216

of the true rainfall, gauge and site characteristics and the meteorological condition pertaining, (fig. 1).

Although there is evidence of earlier measurements of rainfall, i1) (2) the invention of the rain gauge is often ascribed to Castelli in 1639. Accounts of rainfall observations are to be found in many of the scientific journals of the Eighteenth Century. By the second half of the Nineteenth Century some countries were standardising their techni­ques of observation and design of instruments. About the same time it was recognised that the increase in wind speed with height lessened the catch of more elevated gauges (3) (4). Although subsequent improvements in design have eliminated or reduced some of the other sources of error, the effects of wind have not been overcome.

Wind acts in several ways to reduce the catch of a standard rain gauge. It causes drops to fall obliquely and it interacts with the gauge and with features of its environ­ment to produce turbulence and eddies. These further affect the falling drops, particu­larly over the gauge itself where local acceleration alters the drop trajectories, diverting the smaller ones from the funnel (5) (6). Various attempts have been made to overcome these effects by employing fences and walls to surround the gauge. Some gauges have

Fig. 2 — The site at Wallingford.

217

been installed in pits and others have been fitted with shields ; while even rotating gauges with apertures that are kept perpendicular to the falling rain have been built. The aim of most of these devices has been to create a more uniform and parallel air flow over the gauge. However their success has very rarely been judged in those terms, but rather by

ï ï

Fig. 3 — The ground level gauge, standard gauge and anemometer.

Fig. 4 — Ground level and standard catches, a comparison September 1961-August 1966 (inc.). Excluding January and February 1963.

218

the increase in catch above that of an adjacent gauge not similarly treated. The greater the increase, the more successful the device, is a rule which has usually been followed. Whether the increase is due to the efficacy of the device, or to some spurious effect such as splash or a similar factor, is rarely investigated.

A gauge installed with its rim at ground level avoids most of the disadvantages of an elevated instrument, particularly those that result from wind. On the other hand, the likelihood of splash into the gauge is far greater, so that gauges employed in this way have to be surrounded by a surface that eliminates splash. Brush bristles were used with one of the earliest ground level gauges (7). In more recent investigations devices such as wire mesh screens, door mats( s) and honeycomb iron grids (9) have been employed. Short grass and piassava(10) have been used as gauge surrounds and following the suggestion of the use of angled louvers to splash drops away from the gauge(n) , surfaces of that type were tested at several sites in England.

Because of the importance of rainfall as the primary element in the hydrological cycle, and because of the difficulties of measuring it, the Hydrological Research Unit has been testing the performance of rain gauges as part of its preparatory work for the studies of representative and experimental basins. Most of the experiments have been carried out on a level open site at Wallingford using a number of different methods of installing the standard 5 in. (12.7 cm) diameter British Meteorological Office Mk II rain gauge (12) (fig. 2). Replicates of some installations have been made and a number of other instruments are operated at the same site. These include several cup counter anemometers at different heights, with the lowest producing a print out of run of wind each hour. Non recording instruments are read daily at the standard time of 0900 hours G.M.T. Since October 1964, weight of rain has been recorded rather than volume to make the observations finer and more accurate.

° 3 loo

M J J

MONTHS

Fig. 5 — The mean monthly difference between standard and ground level catches for the 5 years of the experiment.

Of the various comparisons being made, that between gauges at ground level and at the standard height of 1 ft. (30.5 cm) has been underway the longest. It is also likely to be of greatest consequence. The surround for the ground level gauge was built of 1/8 in. (3.2 mm) galvanised iron strip 4 in. (10.1 cm) wide in the form of a square 6ft by 6 ft (1.8 m by 1.8 m) made up of open ended cubes each 5 in. by 5 in. by 3 in. (12.7 cm by 12.7 cm by 7.6 cm). The grid covers a shallow pit and was designed to

219

fi = Catch of ground level gauge

Rç- - Catch at standard gauge

1000 2000 3000 4000

TOTAL MONTHLY RUN Or WIND (MILES! AT TWO METRES

Fig. 6 — Monthly differences between catches in standard and ground level rain gauges related to wind speed.

KT

* fib.

o o oo

• o o <s

^ *cc So

TOTAL RAINFALL

• UNDER 01 h (0-25mm)

. o 01 to 09 in (025 to 2-28mm)

o OVER -.09 h (228mm]

RG = Catch of ground (evet gauge

Rs = Catch of standard gauge

" 3 4 5 6 7 8 9 10 11 AVERAGE WINDSPEEDIM.RH.)

(MEASURED DURING RAIN AT A HEIGHT OF ONE FOOT)

Fig. 7 — Daily standard and ground level catch differences.

220

minimise any disturbance of the usual air flow profile over the gauge, as well as pre­venting in-splash (fig. 3). During the 5 years of the experiment, the ground level gauge caught 6.6% more rain than the standard gauge some 16 ft (5 m) away (fig. 4). This difference was found to vary with the season (fig. 5), the summer minimum being ascribed to the lower wind speeds and larger drop sizes occuring at that time. However an analysis of the monthly percentage difference in catch and average wind speed for the month (fig. 6), showed no relation between these factors. When average wind speed during rain was used, there was no improvement and even on a daily basis, the percent­age difference in catch between the gauges gave a low correlation with this factor (fig. 7).

To discover whether a relationship of any kind exists between difference in catch and wind speed during rain, the data were grouped for further analysis. The larger falls were included in one category, while in another, rain falls were arranged according to intensity (table 1). Only falls where the intensity was more or less uniform for the dura-

TABLE 1

Analyses of the relation between difference in catch between ground level (RG) and standard gauges {Rsi

and wind speed

Coefficient of No. Analysis determination

r2 or R2.

1 For 35 storms each with a total fall greater than 0.09 in. 0.33 (2.3 mm)

2 For 23 storms each with a total fall greater than 0.06 in. 0.34 (1.5 mm) and intensity more than 0.06 in/h

3 For 18 storms each with a totalfall greater than 0.06 in. 0.19 and intensity 0.03 to 0.06 in/hr (0.8 to 1.5 mm)

4 For 19 storms each with a total fall greater than 0.06 in. 0.35 and intensity less than 0.03 in/h

5 For 17 storms each with a total fall greater than 0.01 in. 0.17 (0.2 mm) with uniform intensity throughout the storm

6 Using data from No. 1 with intensity in a multiple regression 0.42

7 Using data from No. 1 with total catch of ground level 0.47 gauge in a multiple regression

8 For 32 storms each with a total fall greater than 0.09 in. 0.50 and wind speed more than 1.1 m.p.h. (0.4 metres/sec) in which difference in catch is related to

(wind speed)2

log intensity

221

tion of the rain were included in another analysis, but this gave a worse result. However this might be due to the inclusion of some very small falls with their appreciable wet­ting errors. Multiple linear regressions were carried out with wind speed and intensity included in one, and wind speed with total fall in another as independent variables and some improvement resulted. The best result was achieved by using a term combining factors representing drag and drop size, obtained from wind speed and intensity measurements; but even then, only 50% of the difference between ground level and standard catch was accounted for.

Fig. 8 — The special gauge for measuring angle of rainfall.

It is difficult to compare the results obtained by other workers with those for Wallingford, because of differences in gauge height, type and method of measuring the 'true' rainfall as well as by reason of contrast in site. Indeed in one paper to which reference is often made, (13) results from a variety of sites most with different gauges are combined to demonstrate the effect of wind on catch. Results from the investigations that are readily comparable with that carried out at Wallingford show that there is only a loose association between wind speed and the percentage difference in catch between

222

a gauge at ground level (or close to it), and the catch in another more elevated gauge. In the Netherlands, de Zeeuw (14) compared a 40 cm high gauge with an identical one in a turf wall and found that there was only a rough relation between catch ratios and wind speed, even when intensity was accounted for. On the other hand, studies carried out in the Soviet Union (15) (1G) (17) have shown that there is a relationship between wind speed and the difference between the catch of a 2 metre high Tret'yakov gauge, and another installed in a conical pit. However, this result was achieved by grouping a large number of observations from different sites, whereas for short periods at one site, the dependence of under-registration on wind speed could be rather tenuous. This was found to be the case in Project Pluvius (18), when a gauge at ground level and another at 1.5 metre were compared. For individual storms the percentage difference in catch was roughly proportional to wind speed and roughly in inverse proportion to total rainfall. By grouping observations and accounting for intensity the effect of wind beca­me a little clearer.

Because of the apparent lack of dependence of catch reduction on wind speed at Wallingford, other factors were sought that could be important in controlling the difference between the quantities of rain caught at ground level and at the standard height. One major factor was thought to be the angle at which the rain fell. Drops with a large horizontal component to their motion were considered as being more suscep­tible to gauge eddies and less likely to be caught in the funnel. So as special gauge was built to measure the incident angle of rain, by the use of separate vertical and horizontal apertures (fig. 8). It has been shown (19) that for a rain gauge with separate apertures, one in a vertical plane and the other in the usual horizontal plane, the mean tangent

TABLE 2

Analyses of the relation between difference in catch between ground leoel (RQ) and standard gauges (Rs) and

angle of rain

Coefficient of No. Analysis determination

r2 or R2

9

10

11

12

13

14

For the 35 storms in analysis No. 1

For the 23 storms in analysis No. 2

For the 18 storms in analysis No. 3

For the 19 storms in analysis No. 4

Using data from No. 9 with intensity in a multiple regression

Using data from No. 9 with total catch of ground level gauge

0.46

0.45

0.24

0.07

0.52

0.56 in a multiple regression

15 For the 32 storms in analysis No. 9 in which difference in 0.58 catch is related to angle of rainfall/log intensity

223

tan i of all the incident angles is:

Y (r sin i) tan i = ^ -

£ (r cos i) where a £{r sin i) = catch in the vertical aperture and a S (/-cos/) = catch in the horizontal aperture.

Results from this special gauge were employed in further analyses of the difference between standard and ground level gauges, but with little improvement. One reason for this might be that this gauge would be as susceptible to eddies as any other, so that angle of rainfall would not be measured properly (table 2),

Splash was a factor suspected of upsetting the performance of one or both the gauges and several experiments were conducted to try and determine its importance. Splash-out was considered less likely than splash-in, so tests were made with grids of different shapes and sizes, and a special compound gauge, to find out whether some of the catch reached the gauges indirectly. Of course the results of splash-in would act in opposite directions on the directions on the difference between the gauges. It would

Fig. 9 — Types of ground level grid employed at Plynlimon in Mid Wales.

224

reduce the difference if it affected the standard gauge alone, but the reverse would be true if only the ground level gauge was susceptible to splash. Gold (20) has shown that splash occurs most readily from a wet surface and that under these conditions certain drops can rebound to a height greater than 1 ft. (30.5 cm). Yet in another experiment^1), no evidence for splash into a standard gauge was found, even for heavy rain. However, on several of the occasions when the catch in the standard gauge exceeded that at ground level, there was intense rain, and it must be concluded that splash added to the catch of the standard gauge in these cases. It is not known on how many other occasions the difference between the gauges was altered by splashing into the standard gauge, but from tests carried out at two separate sites, with different grids, it seems unlikely that splash affects the ground level gauge.

TABLE 3

Comparison of ground level gauges at Stocks Reservoir (No. 10 site)

TEST A

Comparison of large grid (6' x 6') made of 5" x 5" x 3" cubes (RG) and medium size grid (4' x 4') made of 3" x 3" x 3" cubes (R„)

Total Catch (in.) in

For 17 week period 19.94

't' Test of difference between

Number of degrees of freedom W

16

Gauge Ra Total Catch (in.) in Gauge Rg

19.56

. pairs of normalised weekly totals.

1 calculated

1.29

' / ' for 0.05 probability

1.75

TEST B

Comparison of large grid ( 6 ' x 6 ' ) made of 5 " x 5 " x 3 " cubes (RG) and small grid (3 ' x 3 ') made of 3" x 3" x 3" cubes (Rgi)

Total Catch (in.) in

For 19 week period 23.07

Gauge RG Total Catch (in.) in Gauge Rg\

23.20

'f ' Test of difference between pairs of normalised weekly totals

Number of degrees of freedom

18

't' calculated

0.64

't' for 0.05 probability

1.74

225

At a hilltop site, alongside Stocks Reservoir, in Northern England, a gauge in a grid identical to those at Wallingford was compared with another ground level gauge in a different type of grid. This second grid was smaller (4 ft by 4 ft; 1.2 m by 1.2 m) and built of members with narrower spacing; but even when its overall size was reduced to 3 ft by 3 ft (0.9 m by 0.9 m), there was no significant difference between the catch of the gauges (table 2). A second experiment was carried out at a site in Mid Wales. The per­formances of five gauges, each in a different grid, were compared with that of a sixth gauge in a grid, built to a pattern of those at Wallingford (fig. 9). Some of the grids were interchanged to compensate for rainfall variations within the site, but all the differences between catches were found to be small for both grid positions. Catches in the gauge within the Wallingford-type grid (R g) were the smallest of all, suggesting that little or no splash is produced by that grid (table 4).

Another experiment on splash was carried out with a special compound gauge built to a design first suggested by Bleasdale (22). This compound gauge (fig 10) was placed at ground level inside the grid used with the ground level gauge, to see if the

TABLE 4

Comparison of ground level gauges at Plynlimon, Mid Wales

Period RG

Ground level gauge catches (mm)

^ 4 T-96

20 July to 4 Sep. 222.8 221.6 224.1 225.3 225.2 224.6

On 8 Sep. Grids. RG and Rg, and grids Rgt and Rg5 interchanged

; Sep. to 5 Oct. 131.4 132.4 133.7 132.7 133.6 134.5

6 0 c t . t o 2 N o v . 114.4 117.3 115.8 116.6 117.3 116.9

3 Nov. to 26 Nov 133.4 133.8 135.2 134.8 135.3 136.1

Total 602.0 605.1 608.: 609.4 611.4 612.1

Rg = grid 6' x (,' made of 5" x 5" x 3" cubes

Rg2 — grid 6 ' x 6 ' made of 5" x 3" lattice Rg3 = grid 6' diameter made of

5" x 3" circles

RgA = grid 1 m x 1 m made of 5" x 5" x 3" cubes

Rg5 — grid l m x l m made of 5" x 3" lattice Rg6 — grid 1 m diameter made of

5" x 3" circles

226

•»•*

£?'

Fig. 10 — A compound rain gauge in a grid at ground level.

catches of the 9 gauges were distributed in a way that could be explained by splash from the grid. It was supposed that if the grid caused spash, the smallest catch would be recorded by the centre gauge and the largest by those at the corners. However no such pattern was apparent (table 5) even when the grid was reduced in size, and wind was taken into account. Of course the apparent lack of splash from the grid may be due to splash between the 9 gauges obscuring it and this possibility is being investigated.

Dew and variations in the distribution of rain over the site were thought to be factors that could account for some of the difference between the standard and ground level gauges. Where the relative efficiency of the gauges for catching dew would vary from season to season so that differences would tend to cancel out, there could be a consistent pattern of rainfall over the site at Wallingford. However, tests of the records from several pairs of similar gauges showed that the differences between them were mostly non-significant on a daily basis, and very small for monthly amounts (table 6).

These experiments serve to show that there is a real difference between the catch of a gauge in the standard position and what is measured by a similar gauge at ground level. However they fail to explain adequately the reason for this difference in terms of conventional measurements of wind speed and other factors. Perhaps this would be the case if the gauges were in an environment where drops were of equal size and all air movement was uniform and free from gusts and turbulence. As such conditions do not exists in a natural environment, far more sophisticated instruments must be required for measurements of the factors that are important. In particular the action of drops in the region above the funnel needs to be studied in more detail for a fuller understanding of the behaviour of a rain gauge in wind.

Over long periods standard rain gauges produce good estimates of the amount of rain reaching the ground, where climate and site conditions are favourable. On the other hand for short periods and at less suitable sites, their measurements may be seriously in error. This is, of course, a systematic error (23) and as such it must have appreciable consequences where standard rainfall measurements are employed for

227

TABLE 5

Results of investigations into splash using the compound rain gauge

Compound gauge in 6' x 6' grid

(a) Total catch for 108 storms expressed as a percentage of centre gauge catch.

N

Mean of 4 Mid-Side Gauges

103.7

100

101.4 Mean of

4 Corner Gauges

W S

(b) For 12 storms with no wind during rain.

103.2

100

100.5

Compound gauge in 1 m x 1' m grid

(a) Total catch for 43 storms expressed as a percentage of centre gauge catch.

102.5

100

97.3

(b) For 20 storms with no wind during rain.

104.2

100

100.8

(c) For 12 storms with wind during rain per­pendicular to side.

Wind

99.6

/ Mean of 3 Windward Gauges

100 99.8

\ Mean of 3 Leeward Gauges

(c) For 12 storms with wind during rain.

Wind

98.1 100 100.1

228

design and research purposes. A gauge installed at ground level within a non-splash surface is considered to give measurements of rainfall closer to the true value than any type of gauge. For research, particularly in experimental and representative basins, such a gauge would seem essential.

TABLE 6

Rainfalls recorded at the Wallingford Site

A. Monthly Totals (in.)

Ground level Gauges Standard Gauges Month

I II 1 2 3 4 5

February 1966 March April May June July August September October November December

January 1967 February March

2.87 0.43 2.82 1.68 2.65 3.06 2.31 1.79 6.04 1.58 2.50

1.59 2.25 1.54

— —

2.82 1.72 2.65 3.10 2.23 1.78 6.02 1.55 2.55

1.59 2.24 1.53

2.66 0.37 2.60 1.56 2.56 2.99 2.21 1.74 5.91 1.48 2.34

1.46 2.11 1.45

— — — — — —

2.27 1.75 5.92 1.50 2.40

1.53 2.16 1.47

— — — — — —

2.27 1.76 , 5.90 1.48 2.42

1.49 2.18 1.48

2.61 0.37 2.61 1.57 2.59 — — — — — — — — —

B. 't' Tests of the Significance of the Difference between Pairs of Normalised Daily Totals.

Number of 't' for Gauges Period of Records degrees of 't' calculated 0.05

freedom probability

I and II 20/6/66 to 5/11/66 65 0.72 1.67 I and 4 6/1/66 to 8/6/66 60 0.77 1.67 I and 2 8/6/66 to 4/11/66 71 2.9 1.67 I and 3 8/6/66 to 4/11/66 72 1.73 1.67

ACKNOWLEDGEMENTS

The author wishes to acknowledge the generous assistance of his colleagues in the preparation of this paper, particularly the help of Dr. D. Plinston, Mr. S.W. Smith, Mr. A. Bucknell, and Miss A. Hitchings. Thanks are also due to Mr. F. Law (Fylde Water Board) and the Forestry Commission.

229

DISCUSSION

Intervention de G. LAMBERT

Question: Avez-vous utilisé des dispositifs analogues à ceux que vous présentez pour mesurer la hauteur des précipitations à bord de bateaux?

Answer: No we have not done this.

Intervention of Mr. VÂSA (Czekoslovakia)

Remarks In CSSR similar results were gained. We have found the connections between the

differences and: 1. wind velocity, 2. height of precipitations. It is possible because of being the dm of the raingage in 1 m.

Questions 1. Evaluation of the influence of wind velocity in 30 cm ? 2. Comparison of another height of the rim? 3. Evaluating of the influence of precipitations divided into intervals according

their height ?

Answers 1. We measured wind speed at the height of the standard gauge (30 cm) using an

anemometer that printed out hourly wind run totals. 2. We have comparisons with gauges at other heights but these are shielded gauges. 3. We took account of the total amount of precipitation in each storm as well as

wind speed and intensity in investigating the difference between the gauges.

Intervention of Henry W. ANDERSON

Question: In relating ground-gage rainfall and standard-gage catch it might be well to have estimated or measured wind velocity at both levels. The standard gage wind might be measured as you have, with the ground-level wind obtained by the gradient between two levels of wind movement, such as by the logarithm of height .(Curvilinearity and thresholds may be involved in the relations.)

Answer: We have measured wind speed at several levels but only the lowest was employed in our investigation i.e. measurements made at the same height as the standard gauge.

Intervention of L. L. HARROLD

Question: At Coshocton, Ohio, U.S.A., weighing lysimeters with ground surface rainfall catch was rather uniformly 0.02 inch (0.5 mm) greater than that in the rain gauge with horizontal surface 1 m above the ground surface.

Answer: No reply needed.

Intervention of J. HARDING (Great Britain)

Mr. Harding, referring first to a question raised by the previous speaker in the discussion, reported that Meteorological Office experiments with raingauges on high poles had produced a design wherein rainfall measurements agreed well with the catch of a standard gauge on the ground. This should now be capable of production for meaningful measurements of rainfall at sea.

230

Mr. Harding reported that Meteorological Office experiments between a standard gauge with rim 30 cm above the ground and a ground level gauge produced results broadly similar to those found by Dr. Rodda at Wallingford.

The complexity of the problem of rainfall measurement has been emphasized by recent field experiments at 10 Meteorological Office stations where now fibre-glass gauges were compared with the standard copper gauge for daily observations. In both cases the gauge rims were at 30 cm and the catch of the new gauges was some 4% higher than that of the standard gauges.

It is premature to amend the mass of rainfall data for Great Britain or to assume that water resources are considerably higher than hitherto assessed. Much more work on raingauge measurements has to be carried out. Moreover, work continues on the more accurate estimations of actual evaporation.

REFERENCES

(*) KAUTILYA, "Arthasastra" trans. R. Shamasastry. Gov. Oriental Library Series, Bibliotheca Sanskrita, no. 37, Pt 2, Bangalore 1915.

(2) HORTON, R.E., The measurement of rainfall and snow, / . New Eng. Water Works Assoc, 33 (1), pp. 14-72, 1919.

(3) JEVONS, W. S., On the deficiency of rain in an elevated rain gauge as caused by wind, London, Edinburgh & Dublin Phil. Mag., 22, pp. 421433, 1861.

(4) STOWE, F.W., Rain gauge experiments at Hawsker near Whitby, Yorkshire, British Rainfall, pp. 16-26, 1871.

(5) SERRA, L., Possibilités d'amélioration des mesures de précipitation, LA.S.H. Toronto 1958, pp. 535-545.

(6) Moss, J. M., Exposure factors affecting the accuracy of precipitation measurements, Seminar on Rain, Sydney 1960.

(7) STEVENSON, T., On the defects of rain gauges with description of an improved form, Edinburgh New. Phil. Journ., 33, pp. 12-21, 1842.

(8) WINTER, E.J. and STANHILL, G., Rainfall measurements at ground level, Weather, 14, pp. 367-368, 1959.

(9) KOSCHMEIDER, H., Methods and results of definite rain measurements 111, Danzig Report (1), Mon. Weath. Rev., 62, pp. 5-7, 1934.

(10) ASLYNG, H. C , Rain, snow, and dew measurements, Actae Agriculture Scandina-vica, 15, pp. 275-283, 1965.

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