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Studies on dew formation under semi-arid conditions
Agricultural Meteorology - Elsevier Publishing Company, Amsterdam - Printed in The Netherlands
STUDIES ON DEW F O R M A T I O N U N D E R SEMI-ARID CONDITIONS W. BALER
Department of Agricultural Technical Services, Agricultural Research Institute of the Highveld Region, eotchefstroom, Republic of South Africa 1 (Received October 26, 1964)
SUMMARY
In this study results of dew recordings by means of the Kessler dew recorder above various soil surfaces under semi-arid climatic conditions in the interior of South Africa are reported. Dew formation on lawn was heaviest in autumn but lowest in late winter and spring. Because dew deposition totalled approximately 12 mm over the year--or only 2 ~ of the average annual rainfall--on 167 days, it was concluded that dew under the local conditions could not have a significant bearing on the water economy of the soil. However, in late summer and autumn dew occurred on 20 days per month and dew formation continued for 12-15 h at night. It was assumed that these conditions may reduce transpiration stress and favour the germination and spread of pathogenic spores on the plant surfaces. Compared with the amount of dew on lawn (100 ~), dewfall in growing wheat approached 200 ~ when the sampling plate was placed slightly below the surface of the crop. The ratio between dew above uncropped soil and on lawn was approximately 55 ~ throughout the year. It is emphasized that continuous dew recordings in crops are useful for field studies concerned with the effect of weather conditions on plant growth and plant diseases. In addition, standard recordings of dew occurrence on lawn are also necessary for reference purposes and for evaluations of the relationship between weather conditions and dew formation.
INTRODUCTION
Dew is a form of condensation of special interest to the agrometeorologist. Particularly under semi-arid climatic conditions, where rainfall is concentrated over the summer months and marked seasonal and daily variations of temperature and hui Now at the Agrometeorological Section, Plant Research Institute, Canadian Department of Agriculture, Ottawa, Ont., Canada.
Agr. Meteorol., 3 (1966) 103-112
104
w. BAIl
midity occur, dew affects plant growth in different ways. On occasions it might suppl ment the water available to plants or at least reduce transpiration, but it might al: create favourable conditions for the outbreak and rapid spread of plant diseases. The physical aspects of dew formation were studied by FRANKE~qBERGER(195: HOFMAN (1955), MONTEITH (1957) and LO~rG (1958). Since many papers have be~ published on this subject, the reader is referred to recent literature reviews by GELm (1955), HOFMA~r(1955), MONTWITH(1957) and GEIGER (1961). Most of these studie however, were undertaken in humid, meso-thermal climates. In a comprehensive study on dew formation, LEHMANNand SCHANDERL(194 found that dew deposition sometimes was increased by wind protection, soil cull vation and irrigation and decreased by covering the soil surface. They estimat~ that plants receive five to ten times more dew than the amount registered by a de recorder. Some plants were even capable of taking in dew through their leaves. ( 21 plants investigated, a plant of the Cape flora (Mesambrianthemum cristallinu~ was found to be the most efficient in absorbing and utilizing dew. The authors coJ cluded that valuable information in respect of periods of reduced transpiratic intensity by plants can be obtained from instruments which produce a continuol record of dew formation. From dew records in southern England, MONTEITH (1957) distinguishe between "distillation" of water vapour from soil to grass (1-2 mg/cm 2 h) on veJ calm nights and "dewfall", the turbulent transfer of water vapour from the atmo phere, negligible when the wind at 2 m height fell below 0.5 m/sec but reaching 3mg/cm 2 h with stronger winds. He concluded that in crops taller and denser than sho grass, distillation rates could approach 7-8 mg/cm 2 h in calm English summer an autumn nights. In tropical and subtropical climates, dewfall could be two to thr~ times greater than in Britain. M~,DE (1956) contributed valuable information in regard to the measuremel of dew and the occurrence of dew in different crops in central Germany. The de records on rye are of special interest here, as the results are in line with those obtaine in the present study for wheat at Potchefstroom (South Africa). During 10 days i June the total amount of dew deposition as registered by a Kessler-Fuess dew record~ in a 173 cm high rye crop amounted to 0.15, 4.00 and 3.65 g/100 cm 2 at 16, 104 an 173 cm, respectively. The corresponding amount of dew at 16 cm height above sho~ grass was 3.15 g/100 cm 2 Thus the amount of dew in the upper part of rye was highe than that above short grass, but negligible near the ground in the rye crop. Th pattern remained unchanged until harvest when the heaviest dew deposition of 2.00 100 cm 2 on the stubble field was observed at 16 cm height above ground level; but th was still less than the amount of dew at the same height above short gra, (3.45 g/100 cm2). Over the Highveld Region--part of the interior plateau of South Africa--evapc ration is high, summer rainfall irregular and erratic, and droughts in summer an autumn are the prevalent hazard for crop production. On the other hand, favourab] weather conditions for dew formation, viz. clear skies, intensive outgoing radiatio
Agr. Meteorol., 3 (1966) 103-11
STUDIES ON DEW FORMATION
105
and light winds, frequently prevail. In their thorough study on dew formation and utilization by plants, LEHMANNand SCHANDERL(1942) found that moderately moist soil exhaled sufficient water vapour for a moderate dew formation, if atmospheric conditions were favourable. Thus, the question arose, whether dew under the local conditions could have any significant effect on the water economy of soils and plants. As far as the writer is aware, no continuous recordings of dew formation above various soil surfaces have been published for this region. Only few measurements under arid and semi-arid conditions have been presented, although the occurrence of dew in such areas is believed to be of considerable importance (DuvDEVANI, 1947, 1951; ANGUS, 1958). Considerable work on dew, its measurement and effect on plants, has been done in Israel, where the conditions for dew formation are unusually favourable during the almost rainless summer months. In a network of dew recording stations organized by the Israel meteorological service, for about 10 years dew was reported in terms of "dew-scale numbers" obtained from a Duvdevani dew gauge placed at 1 m height above the ground (DuvoEVANI, 1964). NA6EL (1962) discussed results from seven devices for measuring the amount of dew and presented measurements of dew on a lawn at Pretoria over 30 days. Nagel found that both the Hiltner and Kessler instruments recorded well in case of heavy dew, but showed a deficiency that increased with decreasing amounts of dew. At this point it should be remembered that results of dew recordings from different sources are strictly not comparable because of the different methods used. WEGER (1955) concluded from comparative studies on dew devices that the KesslerFuess dew recorder appeared to be the most suitable instrument of this type, although the amount of dew recorded was only 75-98 ~ of that collected with so-called LEICK plates. In the subsequent discussion on dew in different climatic regions, only those data were included which apparently resulted from Leick plates, Kessler-Fuess dew recorder, water-vapour flux and heat balance studies. It was assumed that results from these methods allow some estimation of occurrence and amount of dew under different climatic conditions. In the present study continuous records of dew formation above short grass, in growing wheat and in a maize field by means of a Kessler dew recorder, are presented in respect of amount of dew and duration of dew occurrence at Potchefstroom.
THE STUDY AREA
The study was undertaken at the weather station of the Agricultural Research Institute of the Highveld Region, Potchefstroom, located in the vast interior plateau of South Africa at 26044 ' S, 27°05 ' E, 1352 m above sea level. According to K6ppen's classification, the climate is of the middle-latitude steppe type (BSkw) with 84 ~ of the annual rainfall of 621 mm concentrated over the summer months from October to March. Like all stations situated in the belt of high atmospheric pressure and at a similar elevation, the area received a mean daily sunshine of 8-9 h throughout the
Agr. Meteorol., 3 (1966) 103-112
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w. BA
year. Due to the high elevation and the frequent clear skies, wide daily fluctuations temperature of approximately 19°C in winter and 14°C in summer occur. A distij climatic feature is the low relative humidity, the monthly averages ranging from 2f in September and October to 48 ~ in February. An agricultural climatology of 1 area has recently been presented by BALER(1964).
METHOD OF STUDY
Since 1957 dew has been continuously recorded at the Potchefstroom weather stati, by means of a Kessler-Fuess dew recorder 1. The instrument essentially consists of t dew receiver, fastened to the end of an arm, a torsion wire to which the arm is clampe an oil-damping device, a scriber, a clock, a drum and a case. The dew receiver, blackened conically shaped thin (0.1 mm) aluminum sheet, is exposed 10 cm abo the soil surface. The recording mechanism is fitted with an oil damper which elirr nates the effect of wind gusts; it is, however, sufficiently sensitive to respond to de amounts o f approximately 0. I g or 0.01 ram; the accuracy increases with the amouJ o f dew. The Kessler dew recorder was described by GELBKE (1955), M.~.DE (195' and NAGEL (1962). One instrument o f this type was permanently installed on short, dense law (Kikuyu grass) which was usually not irrigated. Other instruments were temporariJ placed on uncropped soil, in a maize field and in growing wheat. The amount of de recorded at these sites was expressed as a percentage o f the dew recorded on gras The amount of dew was registered in g/110 cm 2 effective surface o f the sampling plat~ To facilitate comparison with the rainfall measurements, the dew records were cor verted to depth o f water in mm.
EXPERIMENTAL RESULTS
The mean monthly totals o f rainfall and dew deposition on lawn for 1957-195 are given in Table I. The monthly amounts of dew varied from approximately 0. mm in late winter and spring to above 2 mm in autumn. The number of nights pe month with dew fluctuated between less than 10 days in spring and 20 days in autumn For the year as a whole, there were 167 days with dew and altogether 12.6 mm of dew which is approximately 2 ~ of the annual rainfall. The relationship between rainfall and dew formation is illustrated in Fig.1 From this figure it may be observed that, besides the seasonal trend of dew formatioi with a maximum in autumn and a minimum in spring, the amount o f dew is to som~ extent also related to rainfall. In Table II quarterly rainfall and dew totals are given for the four seasons o: 1 Catalogue No. Fuess 64 a.
Agr. Meteorol., 3 (1966) 103-112
107
STUDIES ON DEW FORMATION
TABLE I MEAN MONTHLY RAINFALL AND DEW AMOUNTS
Period
(1957-1958)
Rainfall 1957-1958
Dew amount average
1957-1958
(mm)
average (1906-1955) (mm)
(%)
(ram)
January February March April May June July August September October November December
143.6 53.7 112.2 56.3 9.1 38.3 32.3 14.8 70.5 40.6 57.2 59.3
107.6 99.6 84.2 35.8 19.8 7.4 8.8 9.6 16.5 49.6 78.8 103.1
133 54 133 157 46 518 367 154 427 82 73 58
1.0 1.1 2.3 2.1 1.1 1.5 0.9 0.4 0.6 0.5 0.4 0.7
13 19 20 20 18 19 17 5 10 9 7 10
Year
687.9
620.8
111
12.6
167
days with dew (n)
140
130 120
:::::::::::::::::::
iiii!!!!!iiiii!iill RAINFALL 1906-'55 2.4
110
2.2
100
2.0
90
1,8
E
E 80
1.6
._1
"J 70 I,l_
z
60
.2 N
50
.0
40
0,8
30
0.6
20
0.4
10
0,2 0.0 JAN. FEB. MAR. APR. MAY JUN. JUL. AUG. SER OCT. NOV. DEC.
Fig.1. Monthly totals of rainfall and dew formation at Potchefstroom.
Agr. Meteorol., 3 (1966) 103-112
W. BAIE]
108 TABLE II IvlEAN QUARTERLY RAINFALL AND DEW AMOUNTS
Period
RainfaU Dew total amount (mm) (mm)
Autumn (March-May) 177.6 Winter (June-Aug.) 85.4 Spring (Sept.-Nov.) 168.3 Summer (Dec.-Feb.) 256.6 Year
687.9
(1957-1958)
Dew amount -
-
Days with dew (n)
100
X
rainfall (%)
Dew amount per night (ram)
5.5 2.8 1.5 2.8
3.1 3.3 0.9 1.1
58 41 26 42
0.095 0.068 0.058 0.067
12.6
1.8
167
0.075
the year. Dew deposition was highest in autumn (5.5 mm) and lowest in spring (1.: mm) and amounted to 2.8 mm in each winter and summer. The number of days witl dew per season showed a similar pattern with a maximum o f 58 days in autumn anc a minimum of 26 days in spring, whereas in winter and summer respectively 41 anc 42 days with dew occurred. Expressed as a percentage of the rainfall, dew formatiol was approximately 3 ~o o f the quarterly rainfall in autumn and winter and 1 ~ il spring and summer. Interesting results were obtained from dew records at 10 cm height in a fielc of growing wheat from May to October 1957. The relationship between the heigh
f-..\
90
80 E
i--
200
/DEw',
100
70
,,/
/
60
150
I,LI "1" ii O
~U.J
:=
50 40
. . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . .
,
/\
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1oo IJ.l
30 50
20 HEIGHT OF DEWPLATE
10 0 PERIOD:
I JUNE
i JULY
i AUGUST
I
0
SEPTEMBER
Fig.2. Relationship between height of wheat and amount of dew recorded within a wheat crop a Potchefstroom, 1957. (Dew in wheat expressed as a percentage of amount of dew registered at 10 cn height above lawn.)
Agr. Meteorol., 3 (1966) 103-11"
STUDIES
ON
DEW
109
FORMATION
o f wheat and the amount o f dew, expressed as a percentage o f the dew amount above lawn, is illustrated in Fig.2. During the first two months after planting (15th May), when the wheat crop was still less than 10 cm in height, the dew recorded at the height o f the sampling plate was 78 % of the amount o f dew recorded on lawn. When the wheat crop had reached a height of approximately 20 cm, and the sampling plate of the dew recorder at 10 cm above the ground between the 20 cm wide rows was just below the top of the plants, the amount of dew in wheat, as expressed of that on lawn, increased to 197 %. This was the period o f maximum dew deposition registered in wheat. The ratio of dew in wheat to that above lawn dropped to 65 and 25 % when the wheat crop was respectively 60 cm (August 6-September 9) and 90 cm (September 10-October 14)in height. Similar results were obtained in maize. The dew recorded near the ground in a 180 cm high maize crop was negligible, although heavy dew was recorded on the lawn nearby. On the other hand, the ratio of dew above uncropped soil to that on lawn was fairly constant throughout the year, viz. approximately 55 %. Dew records for 1957-1958 were also used for an analysis o f the periods during which dew is formed. The mean and extreme hours of beginning, end and duration o f dewfall are graphically presented in Fig.3. The times of sunset and sunrise are also indicated. It may be seen that dew formation commenced on the average 2-3 h after sunset, but in extreme cases, particularly in winter, it commenced even before sunset. Dewfall usually continued until sunrise or even one hour after sunrise. The differences SUNSET
SUNRISE MEAN
JAN.
_ma__x~__'__
.
.
.
.
12.oo
.
12.15
NAY
b--
.
.
.
.
.
.
.
.
i
.
% % !
15.OO 10.24 --15.00
I
JUN.
- ~
9.54 .
'
4
"" ! "--~i. • !
9.54 . . . .
.
"
9.00 , -- 11.00 . . . . . .
I I-----"
!
%
!
i
11.36
i
15.30
.
I
I
-1
I0,36
I
4
15.30 • • I"
I I
•
I
10.30 " 13.OO 9.00 - - 14.00
"I'-
OCT
I
°. °'j
10.18
I
13.00 l
NO~
8.30
I-
9.15 :
i,- 6 . 1 8
DEC. 1600
.
i
APR.
SER
.
I
MAR.
AUG.
.
i
FEB.
JUL.
10.12
I I
I
1800
2000
I
I
I
MEAN .I M A X .
8.15 I
I
2200 2400 0200 0400 SOUTH AFRICAN STANDARD TIME
~"
I
I
0600
0800
Fig.3. Mean and extreme hours of beginning, end and duration of dew deposition above lawn at Potchefstroom, 1957-1958.
Agr. Meteorol., 3 (1966) 103-112
110
w. BA
between the mean and extreme times of the beginning of dew deposition in the even were greater than for the end of dew deposition in the morning. The mean duration of dew occurrence was 8-11 h or 33--46~o of a full 2, day, but the maximum increased to 15 h or 62 ~ of a day in winter.
DISCUSSION
The experimental results of the present study showed some relationships betwe season, rainfall, soil cover, height of plants and the amount of dew recorded. general, low rainfall was correlated with low dewfall. High rainfall caused high d, formation particularly in winter because of the additional moisture contributed i the rains. On the other hand, above normal rainfall in summer, usually accompani, with excessive cloudiness, inhibited dew formation. Consequently, preceding rai seemed to be a condition for heavy dew formation. Unfortunately, a detailed examination of temperature, vapour pressure at wind gradients above the various soil surfaces was not possible at the time of the de recordings. A distinction between "distillation" and "dewfall", as suggested t MONTEITH (1957), is therefore not possible, but for practical reasons this appea to be of less importance under the local conditions. The partial relationship between rainfall and dew formation indicates that rail provide the moisture for dew distillation. As the climate of Potchefstroom is characte ized by summer rains (October-March) and a sharp drop in temperature from Apt to May (BAIER, 1964), it is obvious that dew formation is most frequent and intensix in autumn, when the soil is still adequately supplied with moisture and wide dail temperature fluctuations occur. For the central parts ot the Highveld Region of which the climate of PotcheJ stroom is typical, a mean annual amount of dew of 12 mm or 2 ~ of the annual rain tall is estimated from the experimental results. This comparatively small amount i in agreement with the values for the dry climate in central Europe of 8-11 mm pe year or 1.4-2.6 ~ of the annual rainfall as published by M~DE (1954). These figure are 2-3 times higher than the "dewfall" (moisture condensate resulting from tN downflux from the atmosphere) estimated by MONTEITH (1957) for the British Isles On the other hand, there are many reports of an annual dew deposition as high a~ 30-45 mm in dry climatic regions (WALTER, 1936; DUVDEVANI, 1947; HOFMANN 1955; NAGEL, 1962). Similarly, the annual number of days with dew of 167 for Potchefstroom is lower than the values for Israel (210 days) reported by DUVDEVANI (1947), central Europe (185 days) by ANTO~m (1951), the Namib Desert (200 days) by WALTER (1936) and Pretoria (276 days) by NAGEL(1962). From Table I and Fig.3 it is possible to calculate the mean and maximum amount of dew per night or per hour. Averaged over the year the mean amount of dew per dew night was approximately 0.075 ram, varying between 0.095 mm in autumn Agr. Meteorol., 3 (1966) 103-112
STUDIES ON DEW FORMATION
111
and 0.058 mm in spring (see Table II). In a single night with conditions favourable for dew formation, a maximum of 0.3-0.4 mm may be reached. As the average duration of dew formation is approximately 10 h per night, the mean hourly rate of dew formation was approximately 0.7 mg/cm ~. These values for Potchefstroom are close to the maximum amount of dewfall of 3-4 mg/cm 2 h for southern England as reported by MONTEITH(1957) and of 2.4 mg/cm 2 h for northern Germany as given by FRANKENBERGER (1955). For central Europe where the average annual rainfall is approximately 490 mm, M~,DB (1954) found an average amount of dew per night of 0.03-0.08 mm depending on the type of weather distribution. Results of the present study indicated that the amount of dew recorded at 10 cm height in growing wheat can considerably exceed the amount of dew recorded at the same height above lawn. This is in agreement with findings by M~DE (1956) in a rye field earlier discussed in this paper, and may be explained by the typical distribution of temperature in and above field crops. CORDUKES and ROBERTSON (1963) showed that the minimum temperature within a 52 cm high oat field occurred between 2 a.m. and 6 a.m. at a height approximately 30 cm above the ground, well within the crop canopy. GEIGER(1961), in reviewing studies on the distribution of temperature within field crops, including his own measurements in a growing rye crop, showed that the lowest temperature may be expected in the upper part of the plant cover, where outgoing radiation is most intensive. M,g.DE (1956) concluded from the results of his dew measurements in a rye field that during a calm, clear night the conditions for outgoing radiation, and therefore dew formation, are fairly uniform over a comparatively large zone within the upper part of the vegetation. Because this study was primarily concerned with the measurement and occurrence of dew under semi-arid climatic conditions, no attempt was made to establish the effect of dew on plants and plant diseases. DUVDEVANI (1964) reported that in regions with heavy dew formation, such as Israel, dew had a beneficial effect on the growth and development of a number of crops, but also tended to increase considerably the infection of certain plants, for example, cucumber by mildew. Duvdevani concluded that in Israel dew can contribute a very considerable amount of moisture in those periods in which there is no precipitation. From the literature cited and the results presented in this study, it may be said that the dew under the local semi-arid climate has no hydrological significance and is dependent on preceding rains.
CONCLU~ONS
The quantity of dew recorded at Potchefstroom would probably not have a significant bearing on the soil-moisture relationships of plants. However, the frequent occurrence of dew in summer and autumn, viz. 13-20 days, and the long duration of dew occurrence as long as 15 h/night, do affect plants under the local semi-arid climatic conditions. Firstly, dew may benefit the internal water balance of the plants because transpiration is reduced during the morning hours when dew is still present on the leaves;
Agr. Meteorok, 3 (1966) 103-112
112
w. BAIEI
secondly, dew forms a water film on the plant surface, which may facilitate the sprea( and germination ofpathogene spores. Thus, dew can have beneficial and harmful effects on plants. The chief advantag~ of a dew recorder lies in providing information on the duration of dew occurrenc~ which is even more important than the amount of dew recorded by the instrument Continuous dew recordings in crops are therefore recommended for all studies wher~ the effect of weather conditions on plant growth and on the occurrence of plan disease is involved. The maximum dew formation in a growing crop was registered when th, sampling plate remained just below the top of the plants. To obtain maximum den deposition in a particular field, the dew recorder should be placed within the uppeJ part of the crop canopy. In addition to these recordings and in order to evaluate th, effect of weather conditions on dew formation, standard recordings of dew on lawr at a constant height, say 10 cm above the ground, without any obstacles in the nero vicinity, should always be available for reference purposes.
REFERENCES ANGUS, D. E., 1958. Measurements of dew. Climatology and microclimatology. U.N.E.S.C.O., Aric Zone Res.,Proc. Canberra Symp., 11 : 301-303. ANTONIK, B., 1951. Die H~iufigkeit des Vorkommens von Tau und Reif nach den Potsdamer Beobach tungen. Z. Meteorol., 5 : 52-55. BAmR, W., 1964. Studies on Macroclimates and Microclimates and their Influence on Crops. M. Sc Thesis, Univ. of Pretoria, 191 pp. CORDUKES, W. E. and ROBERTSON, G. W., 1963. Note on the temperature distribution within an ow crop. Can. J. Plant Sci., 43 : 235-239. DUVDEVANI, S., 1947. A n optical method of dew estimation. Quart. J. Roy. Meteorol. Soc., 73 282-296. DUVDEVANI, S., 1951. Dew observations and their significance. New methods in dew estimations Proc. U.N. Sci. Conf. Lake Success, New York, 1949, 4 : 45-47. DUVDEVANI, S., 1964. Dew in Israel and its effect on plants. Soil Sci., 98 : 14-21. FRANKENBERGER, E., 1955. Llber vertikale Temperatur-, Feuchte- und Windgradienten in den unter. sten 7 Dekametern der Atmosphfire. Ber. Deut. Wetterdienstes, 3 (20) : 17 pp. GEIGER, R., 1961. Das Klima der bodennahen Luftschicht--Sammlunff "Die Wissenschaft". Vieweg Braunschweig, 78 : 646 pp. GELBKE, W., 1955. Untersuchungen zur Methodik der Taumessung an Hand dreijiihriger Taumessrei. hen in Greifswald. Abhandl. Meteorol. HydroL Dienst. D.D.R., 5 (38) : 67 pp. HOFMANN, G., 1955. Die Thermodynamik der Taubildung. Ber. Deut. Wetterdienstes, 3 (18) : 45 pp. LEHMANN, P. und SCHANDERL, H., 1942. Tau und Reif-- Wissenschaftliche Abhandlunffen. Springer Berlin, 9 (4) : 20 pp. LONG, I. F., 1958. Some observations on dew. Meteorol. Maff., 87 (1,032) : 161-168. M.~DE, A., 1954. Ober die HOhe des niichtlichen Taufalles im mitteldeutschen Trockengebiet. Wiss Z. Univ. Halle, Math.-Naturw., 4 (1) : 185-190. M3~DE, A., 1956. Zur Methodik der Taumessung. Wiss. Z. Univ. Halle, Math.-Naturw., 5 (3) : 483-512 MONTEITH, J'. L., 1957. Dew. Quart. d. Roy. Meteorol. Soc., 83 : 322-341. NAGEL, J. F., 1962. On the measurement of dew. Arch. Meteorol. Geophys. Bioklimatol., Ser. B, 11 : 403-423. WALTER, H., 1936. Die 5kologischen Verhfiltnisse in der Namib-Nebelwiiste (Siidwest Afrika). Juhrb. Wiss. Botanik, 84 : 58-222. WEGER, N., 1955. Ergebnisse vergleichender Taumessungen. Mitt. Deut. Wetterdienstes, 2 (14): 185-187.
Agr. Meteorol., 3 (1966) 103-112
STUDIES ON DEW F O R M A T I O N U N D E R SEMI-ARID CONDITIONS W. BALER
Department of Agricultural Technical Services, Agricultural Research Institute of the Highveld Region, eotchefstroom, Republic of South Africa 1 (Received October 26, 1964)
SUMMARY
In this study results of dew recordings by means of the Kessler dew recorder above various soil surfaces under semi-arid climatic conditions in the interior of South Africa are reported. Dew formation on lawn was heaviest in autumn but lowest in late winter and spring. Because dew deposition totalled approximately 12 mm over the year--or only 2 ~ of the average annual rainfall--on 167 days, it was concluded that dew under the local conditions could not have a significant bearing on the water economy of the soil. However, in late summer and autumn dew occurred on 20 days per month and dew formation continued for 12-15 h at night. It was assumed that these conditions may reduce transpiration stress and favour the germination and spread of pathogenic spores on the plant surfaces. Compared with the amount of dew on lawn (100 ~), dewfall in growing wheat approached 200 ~ when the sampling plate was placed slightly below the surface of the crop. The ratio between dew above uncropped soil and on lawn was approximately 55 ~ throughout the year. It is emphasized that continuous dew recordings in crops are useful for field studies concerned with the effect of weather conditions on plant growth and plant diseases. In addition, standard recordings of dew occurrence on lawn are also necessary for reference purposes and for evaluations of the relationship between weather conditions and dew formation.
INTRODUCTION
Dew is a form of condensation of special interest to the agrometeorologist. Particularly under semi-arid climatic conditions, where rainfall is concentrated over the summer months and marked seasonal and daily variations of temperature and hui Now at the Agrometeorological Section, Plant Research Institute, Canadian Department of Agriculture, Ottawa, Ont., Canada.
Agr. Meteorol., 3 (1966) 103-112
104
w. BAIl
midity occur, dew affects plant growth in different ways. On occasions it might suppl ment the water available to plants or at least reduce transpiration, but it might al: create favourable conditions for the outbreak and rapid spread of plant diseases. The physical aspects of dew formation were studied by FRANKE~qBERGER(195: HOFMAN (1955), MONTEITH (1957) and LO~rG (1958). Since many papers have be~ published on this subject, the reader is referred to recent literature reviews by GELm (1955), HOFMA~r(1955), MONTWITH(1957) and GEIGER (1961). Most of these studie however, were undertaken in humid, meso-thermal climates. In a comprehensive study on dew formation, LEHMANNand SCHANDERL(194 found that dew deposition sometimes was increased by wind protection, soil cull vation and irrigation and decreased by covering the soil surface. They estimat~ that plants receive five to ten times more dew than the amount registered by a de recorder. Some plants were even capable of taking in dew through their leaves. ( 21 plants investigated, a plant of the Cape flora (Mesambrianthemum cristallinu~ was found to be the most efficient in absorbing and utilizing dew. The authors coJ cluded that valuable information in respect of periods of reduced transpiratic intensity by plants can be obtained from instruments which produce a continuol record of dew formation. From dew records in southern England, MONTEITH (1957) distinguishe between "distillation" of water vapour from soil to grass (1-2 mg/cm 2 h) on veJ calm nights and "dewfall", the turbulent transfer of water vapour from the atmo phere, negligible when the wind at 2 m height fell below 0.5 m/sec but reaching 3mg/cm 2 h with stronger winds. He concluded that in crops taller and denser than sho grass, distillation rates could approach 7-8 mg/cm 2 h in calm English summer an autumn nights. In tropical and subtropical climates, dewfall could be two to thr~ times greater than in Britain. M~,DE (1956) contributed valuable information in regard to the measuremel of dew and the occurrence of dew in different crops in central Germany. The de records on rye are of special interest here, as the results are in line with those obtaine in the present study for wheat at Potchefstroom (South Africa). During 10 days i June the total amount of dew deposition as registered by a Kessler-Fuess dew record~ in a 173 cm high rye crop amounted to 0.15, 4.00 and 3.65 g/100 cm 2 at 16, 104 an 173 cm, respectively. The corresponding amount of dew at 16 cm height above sho~ grass was 3.15 g/100 cm 2 Thus the amount of dew in the upper part of rye was highe than that above short grass, but negligible near the ground in the rye crop. Th pattern remained unchanged until harvest when the heaviest dew deposition of 2.00 100 cm 2 on the stubble field was observed at 16 cm height above ground level; but th was still less than the amount of dew at the same height above short gra, (3.45 g/100 cm2). Over the Highveld Region--part of the interior plateau of South Africa--evapc ration is high, summer rainfall irregular and erratic, and droughts in summer an autumn are the prevalent hazard for crop production. On the other hand, favourab] weather conditions for dew formation, viz. clear skies, intensive outgoing radiatio
Agr. Meteorol., 3 (1966) 103-11
STUDIES ON DEW FORMATION
105
and light winds, frequently prevail. In their thorough study on dew formation and utilization by plants, LEHMANNand SCHANDERL(1942) found that moderately moist soil exhaled sufficient water vapour for a moderate dew formation, if atmospheric conditions were favourable. Thus, the question arose, whether dew under the local conditions could have any significant effect on the water economy of soils and plants. As far as the writer is aware, no continuous recordings of dew formation above various soil surfaces have been published for this region. Only few measurements under arid and semi-arid conditions have been presented, although the occurrence of dew in such areas is believed to be of considerable importance (DuvDEVANI, 1947, 1951; ANGUS, 1958). Considerable work on dew, its measurement and effect on plants, has been done in Israel, where the conditions for dew formation are unusually favourable during the almost rainless summer months. In a network of dew recording stations organized by the Israel meteorological service, for about 10 years dew was reported in terms of "dew-scale numbers" obtained from a Duvdevani dew gauge placed at 1 m height above the ground (DuvoEVANI, 1964). NA6EL (1962) discussed results from seven devices for measuring the amount of dew and presented measurements of dew on a lawn at Pretoria over 30 days. Nagel found that both the Hiltner and Kessler instruments recorded well in case of heavy dew, but showed a deficiency that increased with decreasing amounts of dew. At this point it should be remembered that results of dew recordings from different sources are strictly not comparable because of the different methods used. WEGER (1955) concluded from comparative studies on dew devices that the KesslerFuess dew recorder appeared to be the most suitable instrument of this type, although the amount of dew recorded was only 75-98 ~ of that collected with so-called LEICK plates. In the subsequent discussion on dew in different climatic regions, only those data were included which apparently resulted from Leick plates, Kessler-Fuess dew recorder, water-vapour flux and heat balance studies. It was assumed that results from these methods allow some estimation of occurrence and amount of dew under different climatic conditions. In the present study continuous records of dew formation above short grass, in growing wheat and in a maize field by means of a Kessler dew recorder, are presented in respect of amount of dew and duration of dew occurrence at Potchefstroom.
THE STUDY AREA
The study was undertaken at the weather station of the Agricultural Research Institute of the Highveld Region, Potchefstroom, located in the vast interior plateau of South Africa at 26044 ' S, 27°05 ' E, 1352 m above sea level. According to K6ppen's classification, the climate is of the middle-latitude steppe type (BSkw) with 84 ~ of the annual rainfall of 621 mm concentrated over the summer months from October to March. Like all stations situated in the belt of high atmospheric pressure and at a similar elevation, the area received a mean daily sunshine of 8-9 h throughout the
Agr. Meteorol., 3 (1966) 103-112
106
w. BA
year. Due to the high elevation and the frequent clear skies, wide daily fluctuations temperature of approximately 19°C in winter and 14°C in summer occur. A distij climatic feature is the low relative humidity, the monthly averages ranging from 2f in September and October to 48 ~ in February. An agricultural climatology of 1 area has recently been presented by BALER(1964).
METHOD OF STUDY
Since 1957 dew has been continuously recorded at the Potchefstroom weather stati, by means of a Kessler-Fuess dew recorder 1. The instrument essentially consists of t dew receiver, fastened to the end of an arm, a torsion wire to which the arm is clampe an oil-damping device, a scriber, a clock, a drum and a case. The dew receiver, blackened conically shaped thin (0.1 mm) aluminum sheet, is exposed 10 cm abo the soil surface. The recording mechanism is fitted with an oil damper which elirr nates the effect of wind gusts; it is, however, sufficiently sensitive to respond to de amounts o f approximately 0. I g or 0.01 ram; the accuracy increases with the amouJ o f dew. The Kessler dew recorder was described by GELBKE (1955), M.~.DE (195' and NAGEL (1962). One instrument o f this type was permanently installed on short, dense law (Kikuyu grass) which was usually not irrigated. Other instruments were temporariJ placed on uncropped soil, in a maize field and in growing wheat. The amount of de recorded at these sites was expressed as a percentage o f the dew recorded on gras The amount of dew was registered in g/110 cm 2 effective surface o f the sampling plat~ To facilitate comparison with the rainfall measurements, the dew records were cor verted to depth o f water in mm.
EXPERIMENTAL RESULTS
The mean monthly totals o f rainfall and dew deposition on lawn for 1957-195 are given in Table I. The monthly amounts of dew varied from approximately 0. mm in late winter and spring to above 2 mm in autumn. The number of nights pe month with dew fluctuated between less than 10 days in spring and 20 days in autumn For the year as a whole, there were 167 days with dew and altogether 12.6 mm of dew which is approximately 2 ~ of the annual rainfall. The relationship between rainfall and dew formation is illustrated in Fig.1 From this figure it may be observed that, besides the seasonal trend of dew formatioi with a maximum in autumn and a minimum in spring, the amount o f dew is to som~ extent also related to rainfall. In Table II quarterly rainfall and dew totals are given for the four seasons o: 1 Catalogue No. Fuess 64 a.
Agr. Meteorol., 3 (1966) 103-112
107
STUDIES ON DEW FORMATION
TABLE I MEAN MONTHLY RAINFALL AND DEW AMOUNTS
Period
(1957-1958)
Rainfall 1957-1958
Dew amount average
1957-1958
(mm)
average (1906-1955) (mm)
(%)
(ram)
January February March April May June July August September October November December
143.6 53.7 112.2 56.3 9.1 38.3 32.3 14.8 70.5 40.6 57.2 59.3
107.6 99.6 84.2 35.8 19.8 7.4 8.8 9.6 16.5 49.6 78.8 103.1
133 54 133 157 46 518 367 154 427 82 73 58
1.0 1.1 2.3 2.1 1.1 1.5 0.9 0.4 0.6 0.5 0.4 0.7
13 19 20 20 18 19 17 5 10 9 7 10
Year
687.9
620.8
111
12.6
167
days with dew (n)
140
130 120
:::::::::::::::::::
iiii!!!!!iiiii!iill RAINFALL 1906-'55 2.4
110
2.2
100
2.0
90
1,8
E
E 80
1.6
._1
"J 70 I,l_
z
60
.2 N
50
.0
40
0,8
30
0.6
20
0.4
10
0,2 0.0 JAN. FEB. MAR. APR. MAY JUN. JUL. AUG. SER OCT. NOV. DEC.
Fig.1. Monthly totals of rainfall and dew formation at Potchefstroom.
Agr. Meteorol., 3 (1966) 103-112
W. BAIE]
108 TABLE II IvlEAN QUARTERLY RAINFALL AND DEW AMOUNTS
Period
RainfaU Dew total amount (mm) (mm)
Autumn (March-May) 177.6 Winter (June-Aug.) 85.4 Spring (Sept.-Nov.) 168.3 Summer (Dec.-Feb.) 256.6 Year
687.9
(1957-1958)
Dew amount -
-
Days with dew (n)
100
X
rainfall (%)
Dew amount per night (ram)
5.5 2.8 1.5 2.8
3.1 3.3 0.9 1.1
58 41 26 42
0.095 0.068 0.058 0.067
12.6
1.8
167
0.075
the year. Dew deposition was highest in autumn (5.5 mm) and lowest in spring (1.: mm) and amounted to 2.8 mm in each winter and summer. The number of days witl dew per season showed a similar pattern with a maximum o f 58 days in autumn anc a minimum of 26 days in spring, whereas in winter and summer respectively 41 anc 42 days with dew occurred. Expressed as a percentage of the rainfall, dew formatiol was approximately 3 ~o o f the quarterly rainfall in autumn and winter and 1 ~ il spring and summer. Interesting results were obtained from dew records at 10 cm height in a fielc of growing wheat from May to October 1957. The relationship between the heigh
f-..\
90
80 E
i--
200
/DEw',
100
70
,,/
/
60
150
I,LI "1" ii O
~U.J
:=
50 40
. . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . .
,
/\
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1oo IJ.l
30 50
20 HEIGHT OF DEWPLATE
10 0 PERIOD:
I JUNE
i JULY
i AUGUST
I
0
SEPTEMBER
Fig.2. Relationship between height of wheat and amount of dew recorded within a wheat crop a Potchefstroom, 1957. (Dew in wheat expressed as a percentage of amount of dew registered at 10 cn height above lawn.)
Agr. Meteorol., 3 (1966) 103-11"
STUDIES
ON
DEW
109
FORMATION
o f wheat and the amount o f dew, expressed as a percentage o f the dew amount above lawn, is illustrated in Fig.2. During the first two months after planting (15th May), when the wheat crop was still less than 10 cm in height, the dew recorded at the height o f the sampling plate was 78 % of the amount o f dew recorded on lawn. When the wheat crop had reached a height of approximately 20 cm, and the sampling plate of the dew recorder at 10 cm above the ground between the 20 cm wide rows was just below the top of the plants, the amount of dew in wheat, as expressed of that on lawn, increased to 197 %. This was the period o f maximum dew deposition registered in wheat. The ratio of dew in wheat to that above lawn dropped to 65 and 25 % when the wheat crop was respectively 60 cm (August 6-September 9) and 90 cm (September 10-October 14)in height. Similar results were obtained in maize. The dew recorded near the ground in a 180 cm high maize crop was negligible, although heavy dew was recorded on the lawn nearby. On the other hand, the ratio of dew above uncropped soil to that on lawn was fairly constant throughout the year, viz. approximately 55 %. Dew records for 1957-1958 were also used for an analysis o f the periods during which dew is formed. The mean and extreme hours of beginning, end and duration o f dewfall are graphically presented in Fig.3. The times of sunset and sunrise are also indicated. It may be seen that dew formation commenced on the average 2-3 h after sunset, but in extreme cases, particularly in winter, it commenced even before sunset. Dewfall usually continued until sunrise or even one hour after sunrise. The differences SUNSET
SUNRISE MEAN
JAN.
_ma__x~__'__
.
.
.
.
12.oo
.
12.15
NAY
b--
.
.
.
.
.
.
.
.
i
.
% % !
15.OO 10.24 --15.00
I
JUN.
- ~
9.54 .
'
4
"" ! "--~i. • !
9.54 . . . .
.
"
9.00 , -- 11.00 . . . . . .
I I-----"
!
%
!
i
11.36
i
15.30
.
I
I
-1
I0,36
I
4
15.30 • • I"
I I
•
I
10.30 " 13.OO 9.00 - - 14.00
"I'-
OCT
I
°. °'j
10.18
I
13.00 l
NO~
8.30
I-
9.15 :
i,- 6 . 1 8
DEC. 1600
.
i
APR.
SER
.
I
MAR.
AUG.
.
i
FEB.
JUL.
10.12
I I
I
1800
2000
I
I
I
MEAN .I M A X .
8.15 I
I
2200 2400 0200 0400 SOUTH AFRICAN STANDARD TIME
~"
I
I
0600
0800
Fig.3. Mean and extreme hours of beginning, end and duration of dew deposition above lawn at Potchefstroom, 1957-1958.
Agr. Meteorol., 3 (1966) 103-112
110
w. BA
between the mean and extreme times of the beginning of dew deposition in the even were greater than for the end of dew deposition in the morning. The mean duration of dew occurrence was 8-11 h or 33--46~o of a full 2, day, but the maximum increased to 15 h or 62 ~ of a day in winter.
DISCUSSION
The experimental results of the present study showed some relationships betwe season, rainfall, soil cover, height of plants and the amount of dew recorded. general, low rainfall was correlated with low dewfall. High rainfall caused high d, formation particularly in winter because of the additional moisture contributed i the rains. On the other hand, above normal rainfall in summer, usually accompani, with excessive cloudiness, inhibited dew formation. Consequently, preceding rai seemed to be a condition for heavy dew formation. Unfortunately, a detailed examination of temperature, vapour pressure at wind gradients above the various soil surfaces was not possible at the time of the de recordings. A distinction between "distillation" and "dewfall", as suggested t MONTEITH (1957), is therefore not possible, but for practical reasons this appea to be of less importance under the local conditions. The partial relationship between rainfall and dew formation indicates that rail provide the moisture for dew distillation. As the climate of Potchefstroom is characte ized by summer rains (October-March) and a sharp drop in temperature from Apt to May (BAIER, 1964), it is obvious that dew formation is most frequent and intensix in autumn, when the soil is still adequately supplied with moisture and wide dail temperature fluctuations occur. For the central parts ot the Highveld Region of which the climate of PotcheJ stroom is typical, a mean annual amount of dew of 12 mm or 2 ~ of the annual rain tall is estimated from the experimental results. This comparatively small amount i in agreement with the values for the dry climate in central Europe of 8-11 mm pe year or 1.4-2.6 ~ of the annual rainfall as published by M~DE (1954). These figure are 2-3 times higher than the "dewfall" (moisture condensate resulting from tN downflux from the atmosphere) estimated by MONTEITH (1957) for the British Isles On the other hand, there are many reports of an annual dew deposition as high a~ 30-45 mm in dry climatic regions (WALTER, 1936; DUVDEVANI, 1947; HOFMANN 1955; NAGEL, 1962). Similarly, the annual number of days with dew of 167 for Potchefstroom is lower than the values for Israel (210 days) reported by DUVDEVANI (1947), central Europe (185 days) by ANTO~m (1951), the Namib Desert (200 days) by WALTER (1936) and Pretoria (276 days) by NAGEL(1962). From Table I and Fig.3 it is possible to calculate the mean and maximum amount of dew per night or per hour. Averaged over the year the mean amount of dew per dew night was approximately 0.075 ram, varying between 0.095 mm in autumn Agr. Meteorol., 3 (1966) 103-112
STUDIES ON DEW FORMATION
111
and 0.058 mm in spring (see Table II). In a single night with conditions favourable for dew formation, a maximum of 0.3-0.4 mm may be reached. As the average duration of dew formation is approximately 10 h per night, the mean hourly rate of dew formation was approximately 0.7 mg/cm ~. These values for Potchefstroom are close to the maximum amount of dewfall of 3-4 mg/cm 2 h for southern England as reported by MONTEITH(1957) and of 2.4 mg/cm 2 h for northern Germany as given by FRANKENBERGER (1955). For central Europe where the average annual rainfall is approximately 490 mm, M~,DB (1954) found an average amount of dew per night of 0.03-0.08 mm depending on the type of weather distribution. Results of the present study indicated that the amount of dew recorded at 10 cm height in growing wheat can considerably exceed the amount of dew recorded at the same height above lawn. This is in agreement with findings by M~DE (1956) in a rye field earlier discussed in this paper, and may be explained by the typical distribution of temperature in and above field crops. CORDUKES and ROBERTSON (1963) showed that the minimum temperature within a 52 cm high oat field occurred between 2 a.m. and 6 a.m. at a height approximately 30 cm above the ground, well within the crop canopy. GEIGER(1961), in reviewing studies on the distribution of temperature within field crops, including his own measurements in a growing rye crop, showed that the lowest temperature may be expected in the upper part of the plant cover, where outgoing radiation is most intensive. M,g.DE (1956) concluded from the results of his dew measurements in a rye field that during a calm, clear night the conditions for outgoing radiation, and therefore dew formation, are fairly uniform over a comparatively large zone within the upper part of the vegetation. Because this study was primarily concerned with the measurement and occurrence of dew under semi-arid climatic conditions, no attempt was made to establish the effect of dew on plants and plant diseases. DUVDEVANI (1964) reported that in regions with heavy dew formation, such as Israel, dew had a beneficial effect on the growth and development of a number of crops, but also tended to increase considerably the infection of certain plants, for example, cucumber by mildew. Duvdevani concluded that in Israel dew can contribute a very considerable amount of moisture in those periods in which there is no precipitation. From the literature cited and the results presented in this study, it may be said that the dew under the local semi-arid climate has no hydrological significance and is dependent on preceding rains.
CONCLU~ONS
The quantity of dew recorded at Potchefstroom would probably not have a significant bearing on the soil-moisture relationships of plants. However, the frequent occurrence of dew in summer and autumn, viz. 13-20 days, and the long duration of dew occurrence as long as 15 h/night, do affect plants under the local semi-arid climatic conditions. Firstly, dew may benefit the internal water balance of the plants because transpiration is reduced during the morning hours when dew is still present on the leaves;
Agr. Meteorok, 3 (1966) 103-112
112
w. BAIEI
secondly, dew forms a water film on the plant surface, which may facilitate the sprea( and germination ofpathogene spores. Thus, dew can have beneficial and harmful effects on plants. The chief advantag~ of a dew recorder lies in providing information on the duration of dew occurrenc~ which is even more important than the amount of dew recorded by the instrument Continuous dew recordings in crops are therefore recommended for all studies wher~ the effect of weather conditions on plant growth and on the occurrence of plan disease is involved. The maximum dew formation in a growing crop was registered when th, sampling plate remained just below the top of the plants. To obtain maximum den deposition in a particular field, the dew recorder should be placed within the uppeJ part of the crop canopy. In addition to these recordings and in order to evaluate th, effect of weather conditions on dew formation, standard recordings of dew on lawr at a constant height, say 10 cm above the ground, without any obstacles in the nero vicinity, should always be available for reference purposes.
REFERENCES ANGUS, D. E., 1958. Measurements of dew. Climatology and microclimatology. U.N.E.S.C.O., Aric Zone Res.,Proc. Canberra Symp., 11 : 301-303. ANTONIK, B., 1951. Die H~iufigkeit des Vorkommens von Tau und Reif nach den Potsdamer Beobach tungen. Z. Meteorol., 5 : 52-55. BAmR, W., 1964. Studies on Macroclimates and Microclimates and their Influence on Crops. M. Sc Thesis, Univ. of Pretoria, 191 pp. CORDUKES, W. E. and ROBERTSON, G. W., 1963. Note on the temperature distribution within an ow crop. Can. J. Plant Sci., 43 : 235-239. DUVDEVANI, S., 1947. A n optical method of dew estimation. Quart. J. Roy. Meteorol. Soc., 73 282-296. DUVDEVANI, S., 1951. Dew observations and their significance. New methods in dew estimations Proc. U.N. Sci. Conf. Lake Success, New York, 1949, 4 : 45-47. DUVDEVANI, S., 1964. Dew in Israel and its effect on plants. Soil Sci., 98 : 14-21. FRANKENBERGER, E., 1955. Llber vertikale Temperatur-, Feuchte- und Windgradienten in den unter. sten 7 Dekametern der Atmosphfire. Ber. Deut. Wetterdienstes, 3 (20) : 17 pp. GEIGER, R., 1961. Das Klima der bodennahen Luftschicht--Sammlunff "Die Wissenschaft". Vieweg Braunschweig, 78 : 646 pp. GELBKE, W., 1955. Untersuchungen zur Methodik der Taumessung an Hand dreijiihriger Taumessrei. hen in Greifswald. Abhandl. Meteorol. HydroL Dienst. D.D.R., 5 (38) : 67 pp. HOFMANN, G., 1955. Die Thermodynamik der Taubildung. Ber. Deut. Wetterdienstes, 3 (18) : 45 pp. LEHMANN, P. und SCHANDERL, H., 1942. Tau und Reif-- Wissenschaftliche Abhandlunffen. Springer Berlin, 9 (4) : 20 pp. LONG, I. F., 1958. Some observations on dew. Meteorol. Maff., 87 (1,032) : 161-168. M.~DE, A., 1954. Ober die HOhe des niichtlichen Taufalles im mitteldeutschen Trockengebiet. Wiss Z. Univ. Halle, Math.-Naturw., 4 (1) : 185-190. M3~DE, A., 1956. Zur Methodik der Taumessung. Wiss. Z. Univ. Halle, Math.-Naturw., 5 (3) : 483-512 MONTEITH, J'. L., 1957. Dew. Quart. d. Roy. Meteorol. Soc., 83 : 322-341. NAGEL, J. F., 1962. On the measurement of dew. Arch. Meteorol. Geophys. Bioklimatol., Ser. B, 11 : 403-423. WALTER, H., 1936. Die 5kologischen Verhfiltnisse in der Namib-Nebelwiiste (Siidwest Afrika). Juhrb. Wiss. Botanik, 84 : 58-222. WEGER, N., 1955. Ergebnisse vergleichender Taumessungen. Mitt. Deut. Wetterdienstes, 2 (14): 185-187.
Agr. Meteorol., 3 (1966) 103-112