ON THE ISOTOPIC COMPOSITION OF PRECIPITATION IN TROPICAL STATIONS ( * ) .

Roberto Gonfiantini (**)

Some oft the tnendò and characterÍÁtÃ.cò of-, the iòotopic composition ο{, precipita -Uon in tropical stations are discussed. Stations in small Vacile islands show a tmiation with latitude., with lower &-values between 15°N and 1S°S and hiahe.fi valuer at

higher. Inland òtationò ant depleted in heavy isotopes with respect to coastal stattons ut sometimes this continental e^{ect is rather complex, as ^or instance in kfaricaMean nthly 5-valueA show a remarkable correlation with the. amount 0¾' precipitation,but the opt variations do not show a clean, dependence on the mean lona term &-value,as should expected theoretically. In Southern American stations the seasonal variations oi the

an monthly 6-values are correlated and they are greater in inland stations dueto con-n<L)itality. The possible ejects o/\ recycLLnq o& water vapour by evapotranspiration ι alio dis,cussed.

INTRODUCTION

Recently the IAEA has published the statistical treatment of the isotopic data of

precipitation of the stations of the IAEA/WM0 global network (IAEA, I 9 8 1 ) . On the basis

of these results, and of other results available in the IAEA files, I shall try in this

paper to briefly review some characteristics of the stable isotope compositon of preci*£

Í tat ion in tropical stations. The stations, the data of which will be discussed here,are

shown in fig. 1. They are also listed in Table 1, together with their geographical

co-ordinates, altitude, mean temperature and mean annual precipitation.

2. LONG TERM MEAN ISOTOPIC COMPOSITION OF PRECIPITATION

The long term mean compositon os stable isotopes in precipitation in tropical

stations is included in Table 1, The length of the observation period in the last

(*) Trabalho apresentado no "International Workshop on Precipitation and Water Recycling

in Tropical Forests", 10-13 de novembro de 1981 - CENA.

(**) International Atomic Energy Agency , P.O. Box 100, A-1400 Vienna, Austria.

column refers to the oxygen-18 analyses, which are in general more frequentlyavai 1 able,

and it has been computed by dividing the total amount of precipitation for which the 18

δ 0 values are available by the mean yearly precipitation.

For some stations, the observation period is rather short, well below 5 years,

which is probably the minimum period to obtain fully reliable mean values. The values

reported from these stations might change therefore in future, when more results are

available, but I believe that these changes will not affect this discussion very much, 18 18 The mean δ 0 values are also shown in fig. 2. The 6D and the δ 0 values are

also plotted in fig. 3 : they line up very well along the so-called world meteoric water

line (Craig 1961; Dansgaard 1964).

2.1 Island stations

Most of the stations located on small islands are in the Pacific Ocean. For these

stations one can assume that the precipitation derives directly and exclusively from

condensation of atmospheric vapour of oceanic origin and that the influence of the land

on the isotopic composition of vapour and of precipitation is negligible, at least at

low altitude.

The isotopic composition of precipitation of 11 Pacific island stations dependson

the latitude: at stations in the latitude belt between 15°N and 15°S the 6-values are

significantly lower than those at stations at higher latitudes, between 15° and 3 0 ° , as

is shown in fig. k. (At even higher latitudes the heavy isotope content of precipita­

tion decreases again, because the effect of cooling becomes predominant).

The reason of the above trend of δ-values with latitude seems to be mainly thai 18

"amount" effect: the correlation coefficient between amount of precipitation and δ 0 is]

- 0.571, which is significant at the 90¾ level. Th i s means that the fraction of atmospheric

vapour condensed to produce rains is greater in sub-equatorial islands than in subtrop­

ical islands. There are however important exceptions to this behavior: for instance,at 18

Hilo (Hawaii), where the precipitation is high ( 3 ^ 7 0 mm/year) and the δ 0 is in the less

negative range (-2.29¾). The amount of precipitation at Hilo is probably determined by

the high mountains of the island, the altitude of which exceeds the 4000 m at the two

volcanoes Mauna Kea and Mauna Loa.

It is also worthwhile to notice that other low latitude Pacific stations located

on large islands, that is Manila, Djajapura and Madang, as well as Darwin in Northern

Australia, also follow rather well the isotopic trend observed in small islands. 18

The same trend of δ 0 with latitude is not found In the Atlantic Ocean, where

unfortunately the number of island stations in insufficient. However, the stations on:]

the eastern coast of South America, for which the influence of the continent is probably

minimal due to the prevailing wind direction, also do not show any lati tude i sotopic trend-

The occurrence of the isotopic variation of precipitation with latitude over the

Pacific is most probably linked with the position of the intertropical convergence zone;

the seasonal displacement of which is rather limited. On the contrary,over the Atlantic

the position of intertropical convergence zone is more distorted by the continents and 122 Gonf iantim

the occurrence of cold marine currents, and its seasonal displacement is rather large. 18

The 5 0- SO correlation computed for the 18 island tropical stations of Table

is: 50 = (7.40+ 0.38) 5 1 8 0 + ( 7 . 6 + 1.1)

with a orrelation coefficient of 0 . 9 7 6 . The deuterium excess is 9.3+0.8. The slope of

above correlation is definitely greater than that reported for island stations by

Yurtserver & Gat (I 9 8 1 ) : 1 o

50 = (6.17+0.51) δ 0 + (4.0+1.5)

and than that reported by Dansgaard (1964). For the latter author, the slope is about

3.5 while the intercept ranges from +6,5 to - 9 . 0 1 .

2.2 Coastal s t a t i o n s 18

The δ 0 values of precipitation in coastal tropical stations range from -1.5

(Bombay) to -7.8¾ (Madang) . Rather negative values, similar to that of Madang, are

unexpectedly frequent, as for instance those observed at Singapore, San Salvador, Hong

Kong, Bangkok, Manila. A combination in various proportions of several effects.such as

the amount and the continental effects (the atmospheric moisture derives in part from

inland), is probably responsible for the low 5 values. Also the contribution of recy -18

cled moisture by evapotranspirat ion might be important. The correlation of 5 0 with the amount of precipitation is not significant (r = - 0 . 0 2 0 ) .

18

The 5 0-5 0 relationship for coastal stations is practically identical to that

for island stations:

50 = (7 .60+0.25) <5180 + (7.4+1.2)

with r = O . 9 8 7 and d (deuterium excess) = 9.2+0.6%0.

2.3 Inland stations

Tropical inland stations frequently - but not always - show heavy i sotope contents

of precipitation which are significantly lower than those of coastal stations,as should

be expected as a consequence of the so-called continental effect.

In the Amazonian Basin, the most negative δ values are observed at Porto Velho,

which is the most inland station. Intermediate δ values occur at other inland stations

(Uaupés, Manaus, Cuiabá, Brasília) which are less distant from the coast.

There is no correlation, in the Amazonian Basin, between isotopic composition and

amount of precipitation. This however is not surprising in inland stations,where such a

relationship is quite complex: in fact, low δ values can also be associated with low

precipitation, if the rain derives for instance from an atmospheric vapour already

depleted in heavy isotopes due to previous condensation processes. In addition, any

correlation between isotopic composition and amount of precipitation can be completely

masked by intense evapotranspiration processes, as are likely to occur in Amazonia. If

plant transpiration is the prevailing process, then the isotopic composition if water

vapour returned to the atmosphere is equal to that of precipitation. The isotopic com­

position of rain deriving to a significant exten from recycled moisture, is no longer

related to the amount in a simple way.

Also rather complex is the continental effect on the isotopic composition of

precipitation in Africa. The amount of precipitation decreases steadily from West to

East, i.e. from Bamako, to Kano, N'Djamena, Geneina and Khartoum, in the latitude belt

between 12 and 16°N. Here, however, the strongest precipitation gradient is in the

South-North direction, i.e. from the coast to the Sahel and to the Sahara desert, and

this is mainly responsible for the aridity of the most northern of the above stations,

Khartoum. 18

The 0 content of precipitation is practically the same at Bamako, Kano and

N'Djamena (but in this last station the D content is 5-8|o higher),and then it increases

by about 2%o at Geneina and Khartoum. This trend is also confirmed by the isotopic

composition of groundwater samples of recent age (high tritium and/or carbon-1 A content),

which behave exactly as precipitation (fig. 5, from Dray et a1., 1981). This is a conse

quence of the fact that in the Northern tropical belt of Africa the atmospheric mo is tuie

derives from the Atlantic Ocean in the West, the Gulf of Guiana in the South West and

the Indian Ocean in the South East: the influence of the last source is responsible for

the less negative ô-values observed in precipitation in Eastern stations, inc 1 uding Addis

Ababa, in spite of its elevation (2360 m a.s.l.).

In the past, when the climatic conditions were different and the precipitation

more abundant, the West-East continental effect was reversed with respect to the present

one, as shown by the isotopic composition of "old" (low ' β content) groundwater samples

(fig. 5). In relative terms, therefore, the contribution of Indian Ocean moisture was

perhaps less important in the past than nowadays.

The continental effect is also responsible for the local minima of the heavy

isotope content of precipitation observed at Luan Prabang, and at Alice Springs.

In stations at high elevation the altitude effect is summed up with the cont inental

effect producing the negative δ-values observed in many stations in Southern Africa and

in South America (especially Izobamba and Bogota). There are however also stations which

do not show any appreciable influence of elevation, as for instance Addis Ababa.Entebbe

and perhaps Shi 1 long.

3. MEAN MONTHLY ISOTOPIC COMPOSITION OF PRECIPITATION

The discussion of this section is based on the mean weighted monthly values of

the isotopic composition of precipitation. In this way most of the fluctuations of the

individual monthly values are smoothed out. These fluctuations in fact may reflect only

occasional phenomena and in part mask the main processes governing the isotopic com­

position of precipitation.

Only stations having a record of data sufficiently long (5 years) have been

considered. Also, months with less than three measurements or with low mean precipita­

tion (less than 10 mm) are eliminated, because the mean data are considered scarcely

reliable.

3.1 Correlations between mean monthly δ-values and amount of precipitation 18

In Table 2 the correlation between mean monthly 6 0 and precipitation amount at

124 Gonflantlftl

selected tropical stations is shown. Apart from a few exceptions (Salvador, Midway,

Brisbane), this correlation is generally very good, with a significance level above 90¾

in two stations, above 95¾ in four, above 98¾ in five, above 99¾ in ten and above 99.9¾

In fifteen. The correlation coefficient is in all cases much greater than that obtained

hy correlating monthly individual values of isotopic composition and of amount of precip_

itatlon, as reported by IAEA (l98l). The correlation between mean monthly values is

also shown in fig. 6 and in fig. 7 for selected stations in Amazonia and in the Pacific

respect ively.

Assumi ng for the precipitation process a simple model of Ray lei gh d i st i 1 lat ion type

the slope a of the correlation between the mean monthly values of isotopic composition

and the amount of precipitation should be negative (and this occurs, as can be seen

from Table 2 ), a n d should increase in absolute value at stations where the long term δ

values are more negative. This second condition does not seem to be fulfilled, as the - 1 8

correlation coefficient between δ 0 and the slope a is only 0.042 (with each pair of 2 18 values weighted by the determination coefficient r between monthly data of δ 0 and of

amount). - 1 8

Also the intercept b should become more negative together with δ 0: b in fact

represents the mean isotopic composition of the very first liquid fraction which can be

obtained by condensation of the atmospheric vapour reaching the site. In principle,the

isotopic composition of the latter should differ from b by the equi 1 ibrium fractionation -18

factor liquid-vapour. The correlation coefficient between δ 0 and b is 0.462, with a significance level of 35%, which should be considered rather satisfactory. An apparent

-18

correlation between a and δ 0 can be obtained by subdividing the stations in three

groups according to their long term mean isotopic composition. In this way one obtains:

18 i) For the 13 stations with δ 0 > -3¾: ã = -0.0124, σ, = 0.0066, σ = 0 . 0 0 2 6 ; b

I η (intercept) = -0.11, σ, = 0 . 8 2 , σ = 0 . 3 2 . The values of Barbados have not been used in r 1 η computation because they differ by more than 2σ from the mean values of the group.

-18 ii) For 8 stations with 5 0 ranging from -3 to -5¾©: ã = -0.0ΐ49,σ, = 0 . 0 0 3 6.O =

I η

0017; Β = -0.75, = 0.88, = 0.42. For the same reason as for Barbados,the values

f Uaupès have not been included in the computations of this group. 18

iii) For 1 6 stations with δ 0 < -5%o: ã =-0.0178, a] = 0.0076,σ η =0.0024;b= -1.53,

= 1.67, σ η = 0.53. ι ι -18 These results show that there is a hint also for |aj to increase as δ 0 decreases

is in principle it should. However, the differences between mean values of groups are

rather small in comparison with their variance, and therefore the level of significance

is always below 9 0 ¾ . In addition, according to theory, assuming that condensation of

the atmospheric vapour is produced by adiabatic cooling, the slope of the second group

and that of the third group should be respectively about 1.3 and 1.8 times greater in

absolute value than the slope of the first group, while the values of a reported above

indicate rather ratios equal to 1.2 and 1.4. ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^

On the isotopic composition ... INSTITUTO NACIONAL DE

PESQUISAS DA AMAZÔNIA I N P A - C O D C

125

Among the various processes which might lower the absolute value of theslopeaof

the relationship between the mean monthly values of 6 and of the amount of precipitation

recycling of vapour through evapotransplration is probably one of the most important.

This process in fact tends to smooth down the isotopic variations of precipitation by

recycling into the atmospheric water vapour with a rather constant isotopic composition

equal or related to that of groundwater and of surface water bodies, which in turn

should be similar to the mean isotopic composition of precipitation. The attenuation of

the Isotopic variations of precipitations due to recycling of water vapour in the Ama­

zonian basinwas shown by Dall '01 io e t a l . (1979)and by Salati e t a l . (1979) on the basis

of a multibox model assuming a westward flux of vapour.

Another reason to observe absolute a-values lower than those predicted theoreti­

cally, is that the model based on a Rayleigh type condensation process is incorrect,

because it implies that the vapour phase is only present in the system, and the liquid

formed is continuously removed. In reality, the two phases coexist in the clouds,and

rains remove only a part of the liquid water present.

In principle, the slope a should also be different for island and coastal stations

with respect to inland stations. Such a difference, however, does not become apparent

from the data of Table 2 .

3.2 C o r r e l a t i o n o f S o u t h American s t a t i o n s w i t h Bel em

The mean monthly 6 values of precipitation of most South American stations show

parellel variations. Table 3 includes the values of the slope a and of the intercept

b for the reduced major axis linear correlation between various stations and Belem. The

correlations are generally good, with the exception of those of Cayenne, which are not

significant, and those of Brasilia, which are rather weak (at least for oxygen-18).This

confirms that the precipitation has the same seasonal trends and obeys meteorological

mechanisms which are to a considerable extent similar in all tropical stations south of

the equator.

From Table 3 it can be seen also that all the coastal stations show a value of

the slope a < 1 , that is the seasonal variations of <5 are smaller than at Belem. On the

contrary, the seasonal variations are larger in inland stations, where the values of a

are always greater than 1 . This indicates the peculiar situation of Belem, which has a

behaviour intermediate between those of coastal and of inland stations,correspondingalso

to its geographical position at about 100 km from the ocean.

The values of a for the inland Amazonian stations are somewhat surprising.because

the recycling of water vapour by evapotranspirat ion should rather reduce the seasonal

variations of the precipitation isotopic composition. This is in competition, however,

with the effect continentality, which tends to increase the seasonal δ-variations.

RESUMO

São discatiÁOÁ aZgumas tendências e características da composição isotópica dak

precipitações nas estações tropicais. Em ilhas pequenas do Pacífico, as estações apresentam variação de acordo com a latitude, com valores-8 menores entre 15°N e 15oS e variações maiores a latitudes mais elevadas. Os isótopos pesados, nas estações interioranas tias, apresentam-se empobrecidos com relação às estações costeiras, porém algumas vezes esse efeito continental é bastante complexo, como por exemplo na África. A média mensal de valores -8 indica uma sensível correlação com a quantidade de precipitação, porém as variações em declive não indicam uma dependência clara da média a longo prazo do valor-8 como era de se esperar teoricamente. Nas estações da América do Sul, de acordo com as estações do ano, há uma correlação entre as variações das médias mensais dos valores-8 os quais são maiores nas estações mais para o interior devido a continentalidade. São também discutidos os possíveis efeitos da reciclagem do vapor dágua proveniente da evapotranspiração.

On the isotopic composition 127

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Gonfian

precipitation (in mm).

Station a+o n correlation significance coefficient level %

slope intercept

America & Western A t l a n t i c

Veracruz -0,0101+0,0020 -O.98+O.29 1 1 -O.845 99 San Salvador -0,0072+0,0026 -4.99+O.52 8 -0 .757 95 Barbados -O.O423+0,0090 +3 .62+0 .95 12 -O.830 99.9 Howard APB -0.0303+0.0060 +0.38+0.91 1 1 -0 .862 99.9 Cayenne -0.0042+0,0014 -0.3053.43 12 -O.696 98 üaupès -O.O365+O.OO8O +5.04+1.94 12 -0 .822 99.9 Belém -0.0136+0,0022 +1.58+0.51 12 -O.893 99.9 Hanaus -0.0186+0 .0029 -0.96+0.54 12 -O.90I 99 .9 Fortaleza -0.0070+0.0011 -0.82+0.16 12 -0 .902 99.9 fatal -0.0101+0.0021 -0.29+0.23 12 -0,846 99.9 Porto Velho -O.OI55+O.OO7O -2 , 2 3+1.32 12 -0 ,579 95 Salvador -0.0012^3.0018 -1 . 2 8+0.33 12 -0.204 n . s . Cuiabá -O.0294+O.OO9I -0 .18+1.07 1 1 -0 .731 98 Brasilia -0.0178^).0052 - 1 . 1 1 + 0 , 8 7 9 -0.794 98 Rio de Janeiro -O.OI8I+O.OO39 - I . 8 5 5 3 . 4 O 10 -0.827 99.9

Africa & Eastern A t l a n t i c

Sao Tomé I s . -0.019I+O .0062 -I.O9+O.58 10 -0 .737 98 Entebbe -O.OI24+O.OO39 -0,67+0.52 12 - 0 . 7 1 2 99 Bar es Salaam -0.011053.0029 -0 ,83+0 .27 12 - 0 . 7 7 4 99 Malange -O.OI23+O.OO71 -2.40+0.90 9 -0 .549 90 St, Helena I s , -0.0135+0 .0046 -0.29+0.34 12 -0 .675 98 Tananarive -O.Oi74fD.OO48 -3.01+0 .61 1 1 - 0 . 7 7 1 99 Asia & Indian Ocean

Hong Kong -0.0136+0.0029 -I .92+O.54 12 -0 .828 99.9 Bangkok -O.O27I+O.OO51 -I .O7+O.64 1 1 -0 .871 99.9 Singapore -O.OI92+O.OO72 - 3 . 4 6 + 1 , 2 9 12 -O.648 95 Djakarta -O.OO70+O.OOI8 -3.92+0 .28 12 - 0 . 7 9 1 99 Diego Garcia - 0 . 0 1 3 4 ^ . 0 0 1 7 -O.25+O.37 12 -O.930 99.9 Australia & P a c i f i c

Midway I s , -0 .ooiT+p.0052 -1 .94+0 .53 12 -O.105 n . s . Hilo, Hawaii I s , - 0 . 0 0 4 5 5 3 · 0 0 1 8 -O.91+O.48 12 -0.641 95 Wake I s , -0.0212+0.0033 +O.I4+O.25 1 2 -O.90I 99.9 Johnston I s . -0.025053.0070 -0 .03+0 .47 12 -0.749 99 Guam I s * ^,0196+0.0024 +O.65+O.52 12 -0.936 99.9 Yap I s , -0,0178+0,0046 -O.85+I.O7 12 - 0 . 7 7 8 99 Trufc I s , -0,0140+0.0037 -0 .68+1 ,10 12 -0 .771 99 Djajapura -0,0064+0.0040 -4.69+O.81 12 -0 .459 90 Madang -0.0207¾ .0031 -1 ,05+0,86 1 2 -O.908 99.9 Darwin -0,0094+0.0019 -1 ,34+0 .38· 9 -O.884 99 Apia -0,0139+0.0025 -0,22+0 .62 1 2 -0 ,874 99.9 Rarotonga -O.OIO553.OO27 +O.58+O.65 12 - 0 ,782 99 Brisbane +43.000953.0056 - 3 . 6 5 5 3 . 5 9 12 0 ,051 n . s .

N.B. a is the slope ύδ/dP of the correlation, and b is the δ-intercept at Ρ = 0. η is the number of months in the year for which the regression has been computed. The level of significance is based on the t-test.

On the isotopic composition ... 131

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18 FIG. 5 . Histogram showing the 0 distribution in groundwater in the Northern tropical

belt of Africa. The figures in brackets are the mean annual precipitation ίιν mm. The bars show the range of isotopic composition of precipitation at each site given by the mean annual value and the mean value of the most rainy month (August). White areas represent modern groundwater (high tritium and/or high 14 ), dark grey represent old groundwater (low 14„) and light grey area in the Chad Basin represents the overlap of modern and old groundwater samples. (From Dray e t a l . , 1981) .

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References

Craig, Η. - 1961. Isotopic variations in meteoric waters. Science, 133:1702-1703.

Oall'Olio, Λ. ; Salati, Ε. Tatagiba de Azevedo, C ; Matsui, E. - 1979. Modelo de £ra-cionamento isotópico de água na bacia Amazônica (Primeira aproximação). Acta Amazô­nica, 9:675-687.

Dans^aard, W. - 1964. Stable isotopes in precipitation. Tellus, 16:436-468.

Dray, ».,· Gonfiantini, R.; Zuppi, C M . - 1983. On the isotopic composition of ground­water in Southern Sahara. In: "Palaeoclimates and Palaeowaters", p.187-199. IAEA, Vienna.

IAEA - 1984. Statistical Treatment of Environmental Isotope Data in Precipitation (compiled by G.M.Zuppi and K. Lewis-Goettler). Technical Reports Series n? 206, IAEA, Vienna.

:Salati, E.; Dall'Olio, Α.; Matsui, Ε.; Gat, J.R. - 1979. Recycling of water in the Amzon Basin: an isotopic study. Water Res. Research, 15:1260-1258.

WMO/OOM - 1974. CIimatoiogical Normals (Clino) for climate and climate ship stations for the period 1931-1960. WMO/OOM - n? 117. TP. 52, Geneva.

Yurtsever, Y. & Gat, J.R. - 1981. Stable isotopes in atmospheric waters. In: Gat, J.R. & Gonfiantini, R. eds. "Stable Isotope Hydrology - Deuterium and Oxygen-18 in the Water Cycle", p.103-142. IAEA, Vienna.

(Aceito para publicação era 20.11.84)

On the isotopic basin 1 3 9