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Variations of the stable isotopic compositions of rainfall events from the Cameroon rain forest, Central Africa R. Njitchoua a , L. Sigha-Nkamdjou b , L. Dever a, * , C. Marlin a , D. Sighomnou b , P. Nia b a Laboratoire d’Hydrologie et de Ge ´ochimie Isotopique, Universite ´ Paris-Sud, F-91405, Orsay Cedex, France b Institut de Recherches Ge ´ologiques et Minie `res, Centre de Recherches Hydrologiques, B.P. 4110, Yaounde ´, Cameroon Received 15 April 1999; accepted 23 June 1999 Abstract This paper discusses the factors controlling the variations of heavy isotope contents of 57 rain events collected throughout the rain forest regions of Cameroon, Central Africa, during the great rainy season (August–November) of 1994. Our data display a wide range of values, from 11.69 to 210.80‰ for 18 O and from 123.2 to 285.5‰ for 2 H. The data follow closely the global meteoric water line suggesting that rain formation processes occurred under isotopic equilibrium conditions between both the condensate and the corresponding vapour. This also indicates that the observed heavy isotope contents have not been altered by evaporation during the descent of raindrops to the ground. The low heavy-isotope contents observed for some precipitation events are assumed to be controlled by the amount of rainfall and/or by the low condensation temperature. The former effect is related to the northward passage of the intertropical convergence zone (ITCZ) over the region under study, whereas the second effect is linked to high altitudes of condensation. Moreover, the latitudinal variations of the weighted mean 18 O contents indicates that in coastal areas (0–230 km inland from the Atlantic coast) the stable isotopic compositions of precipitation are controlled by a continuous extraction from the atmospheric reservoir of vapour as rains according to the Rayleigh distillation model, whereas at distances . 230 km, recycling of continental moisture is thought to have influenced the isotopic compositions of rains. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Rain events; Stable isotopes; Deuterium; Oxygen-18; Cameroon; Moisture recycling; Africa rain forest 1. Introduction This paper discusses the factors controlling the variations of deuterium and oxygen-18 contents in present-day rain events collected throughout the Cameroon rain forest region, in Central Africa (Fig. 1). The knowledge of these factors is required when using heavy isotopes of the water molecule for hydrological, hydrogeological and hydroclimatologi- cal studies. In the tropics, the regional mapping of the distribution of stable isotope contents of precipitation events is of great importance not only for the use of isotope data as input function for hydrological systems, but also for describing atmospheric circula- tion patterns of air masses and the recycling of conti- nental moisture. In contrast to the Amazon rain forest, where several studies in this topic have been carried out (Salati et al., 1979; Gat and Matsui, 1991; Marti- nelli et al., 1996), isotope data for rainfall events from the African rain forest areas are extremely limited. Journal of Hydrology 223 (1999) 17–26 0022-1694/99/$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0022-1694(99)00087-6 * Corresponding author. Tel.: 1 33-169-15-6423; fax: 1 33- 169-15-4917. E-mail address: [email protected] (L. Dever)

Variations of the stable isotopic compositions of rainfall events from the Cameroon rain forest, Central Africa

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Page 1: Variations of the stable isotopic compositions of rainfall events from the Cameroon rain forest, Central Africa

Variations of the stable isotopic compositions of rainfall eventsfrom the Cameroon rain forest, Central Africa

R. Njitchouaa, L. Sigha-Nkamdjoub, L. Devera,* , C. Marlina, D. Sighomnoub, P. Niab

aLaboratoire d’Hydrologie et de Ge´ochimie Isotopique, Universite´ Paris-Sud, F-91405, Orsay Cedex, FrancebInstitut de Recherches Ge´ologiques et Minie`res, Centre de Recherches Hydrologiques, B.P. 4110, Yaounde´, Cameroon

Received 15 April 1999; accepted 23 June 1999

Abstract

This paper discusses the factors controlling the variations of heavy isotope contents of 57 rain events collected throughout therain forest regions of Cameroon, Central Africa, during the great rainy season (August–November) of 1994. Our data display awide range of values, from11.69 to210.80‰ for18O and from123.2 to285.5‰ for2H. The data follow closely the globalmeteoric water line suggesting that rain formation processes occurred under isotopic equilibrium conditions between both thecondensate and the corresponding vapour. This also indicates that the observed heavy isotope contents have not been altered byevaporation during the descent of raindrops to the ground.

The low heavy-isotope contents observed for some precipitation events are assumed to be controlled by the amount of rainfalland/or by the low condensation temperature. The former effect is related to the northward passage of the intertropicalconvergence zone (ITCZ) over the region under study, whereas the second effect is linked to high altitudes of condensation.Moreover, the latitudinal variations of the weighted mean18O contents indicates that in coastal areas (0–230 km inland from theAtlantic coast) the stable isotopic compositions of precipitation are controlled by a continuous extraction from the atmosphericreservoir of vapour as rains according to the Rayleigh distillation model, whereas at distances. 230 km, recycling ofcontinental moisture is thought to have influenced the isotopic compositions of rains.q 1999 Elsevier Science B.V. All rightsreserved.

Keywords:Rain events; Stable isotopes; Deuterium; Oxygen-18; Cameroon; Moisture recycling; Africa rain forest

1. Introduction

This paper discusses the factors controlling thevariations of deuterium and oxygen-18 contents inpresent-day rain events collected throughout theCameroon rain forest region, in Central Africa (Fig.1). The knowledge of these factors is required whenusing heavy isotopes of the water molecule for

hydrological, hydrogeological and hydroclimatologi-cal studies. In the tropics, the regional mapping of thedistribution of stable isotope contents of precipitationevents is of great importance not only for the use ofisotope data as input function for hydrologicalsystems, but also for describing atmospheric circula-tion patterns of air masses and the recycling of conti-nental moisture. In contrast to the Amazon rain forest,where several studies in this topic have been carriedout (Salati et al., 1979; Gat and Matsui, 1991; Marti-nelli et al., 1996), isotope data for rainfall events fromthe African rain forest areas are extremely limited.

Journal of Hydrology 223 (1999) 17–26

0022-1694/99/$ - see front matterq 1999 Elsevier Science B.V. All rights reserved.PII: S0022-1694(99)00087-6

* Corresponding author. Tel.:1 33-169-15-6423; fax:1 33-169-15-4917.

E-mail address:[email protected] (L. Dever)

Page 2: Variations of the stable isotopic compositions of rainfall events from the Cameroon rain forest, Central Africa

Fontes and Olivry (1977) studied the isotopiccomposition of precipitation events from the MountCameroon region, and established an altitude gradientof 20.16‰ per 100 m for oxygen-18. Loehnert(1988) and Mbonu and Travi (1994) studied thedeuterium and oxygen-18 contents for precipitationfrom the south-western and north-central parts ofNigeria and reported the local meteoric lines forthese areas.

This study is based on 57 rainfall eventscollected throughout the Cameroon rain forestduring the great rainy season (August–November)of 1994. Extended within the latitudes 18300 and5800N and the longitudes 108200 and 158500E, thisregion covers a large part of south Cameroon (Fig.1). It is formed of tablelands of not more than200 m in elevation in its coastal part (0–200 kminland) and of 600–800 m in elevation at inlanddistance.200 km. The whole area is covered bydense moist forest vegetation and is crossed byseveral rivers. Among these rivers, the Sanaga,the Nyong and the Ntem rivers flow westwards

to the Atlantic ocean, whereas the Dja, theNgoko and the Boumba rivers flow eastward tosupply the Congo river. The rainfall regime overthe Cameroon rain forest is mainly governed bythe monsoon, which generally accompanies thenorthward migration of the intertropical conver-gence zone (ITCZ). The Cameroon rain forest ischaracterised by two rainy seasons, namely thewinter rainy season from March to June and thesummer rainy season from August to November(Fig. 1). Throughout the region, the annual rainfallvaries from nearly 3000 mm at Kribi in the vici-nity of the Gulf of Guinea to approximately1300 mm at Moloundou. Another important hydro-climatological feature to be noticed in this regionis the influence of dense forest vegetation on thewater cycle. Approximately 60–75% of precipita-tion falling over the African rain forest wererecycled as continental moisture (Monteney andCasenave, 1989). In the study region, evapotran-spiration represents about 80% of the annual rain-fall (Sigha-Nkamdjou, 1993).

R. Njitchoua et al. / Journal of Hydrology 223 (1999) 17–2618

Fig. 1. Map of the study area showing the location of the sampling sites and the monthly variation of precipitation at each site.

Page 3: Variations of the stable isotopic compositions of rainfall events from the Cameroon rain forest, Central Africa

R. Njitchoua et al. / Journal of Hydrology 223 (1999) 17–26 19

Table 1Isotopic composition of individual rainfall events from the rain forest regions of Southern Cameroon, Central Africa (w.a.v. refers to theweighted average value calculated using the formulad � Pn

1 Pidi =Pn

1 Pi in whichPi anddi are the rainfall amount and the isotopic compositionof samplei, respectively)

Dates Rainfallamounts (mm)

Oxygen-18 Deuterium (‰)vs. VSMOW

Deuterium excess�d � d2H28d18O�

Kribi (lat.: 2856N; long.: 985E; alt.: 18 m; distance from the coast: 1 km)20 Aug 94 05.5 20.68 12.5 7.913 Nov 94 16.0 21.05 12.5 10.914 Nov 94 31.3 21.88 21.8 13.2w.a.v. – 21.50 20.1 11.9Ebolowa (lat.: 2855N; long.: 11809E; alt.: 603 m; distance coast: 140 km)24 Aug 94 26.0 21.99 22.3 13.627 Aug 94 12.0 22.76 217.4 4.731 Aug 94 30.0 25.42 234.6 8.804 Sep 94 2.0 23.09 218.2 6.512 Sep 94 3.0 21.58 211.0 1.613 Sep 94 8.0 24.12 228.2 4.814 Sep 94 12.0 22.93 218.9 4.515 Sep 94 1.0 21.36 21.9 9.016 Sep 94 12.0 21.74 23.7 10.219 Sep 94 13.0 24.31 228.1 6.421 Sep 94 5.0 25.51 234.1 10.022 Sep 94 1.0 26.34 247.4 3.323 Sep 94 15.0 28.42 257.8 9.624 Sep 94 2.5 28.78 258.2 12.026 Sep 94 22.0 24.15 227.2 6.0w.a.v. – 24.12 224.7 8.3Sangme´lima (lat.: 2856N; long.: 11859E; alt.: 713 m; distance from the coast: 230 km)15 Aug 94 10.5 23.30 217.7 8.722 Aug 94 4.7 20.74 24.0 1.930 Aug 94 22.0 24.23 220.7 13.110 Sep 94 11.8 24.07 222.8 9.813 Sep 94 12.3 25.68 238.1 7.327 Sep 94 10.2 24.57 229.0 7.604 Oct 94 20.5 210.80 285.5 0.911 Oct 94 32.0 25.01 231.0 9.116 Oct 94 26.0 24.96 229.1 10.619 Oct 94 14.0 26.75 242.8 11.226 Oct 94 18.8 24.44 222.4 13.116 Nov 94 11.0 22.95 26.6 17.0w.a.v. – 25.15 231.6 9.6Somalomo (lat.: 3825N; long.: 12840E; alt.: 617 m; distance from the coast: 320 km)11 Aug 94 6.0 25.47 237.3 6.514 Sep 94 5.0 27.78 251.4 10.820 Sep 94 3.0 24.49 226.9 9.021 Sep 94 43.0 26.59 242.5 10.226 Sep 94 3.0 23.73 227.1 2.703 Oct 94 15.0 210.34 273.6 9.118 Oct 94 12.0 25.03 230.9 9.326 Oct 94 3.0 26.74 243.9 10.0

Page 4: Variations of the stable isotopic compositions of rainfall events from the Cameroon rain forest, Central Africa

2. Sampling and analytical procedures

Precipitation events analysed were collected duringthe summer rainy season (August–November) at sixsites (Kribi: 28560N, 98540E; Ebolowa: 28550N,118090E; Sangme´lima: 28560N, 118590E; Somalomo:38250N, 128400E; Abong-Mbang: 38580N, 138120Eand Moloundou: 28030N, 158130E) located followinga west-to-east traverse from the western Atlantic coastto some hundred kilometres inland. The location ofsampling sites is shown in Fig. 1. Samples werecollected immediately after each rainfall event inorder to minimise the alteration of heavy isotopesby evaporation. Parallel to samplings, the amount ofeach precipitation event has been also recorded.

Oxygen-18 and deuterium measurements have beencarried out at the Laboratoire d’Hydrologie et deGeochimie Isotopique, Universite´ Paris-Sud, France.Samples were prepared following the conventionalprocedures of Epstein and Mayeda (1953) for18Oand of Coleman et al. (1982) for2H. Results are

expressed in delta “d” notation in per million relativeto Vienna standard mean ocean water (VSMOW)according to the following equation:

d �‰� � Rsample

RVSMOW21

� �× 103 �1�

whereR refers to18O/16O or 2H/1H ratios. The analy-tical reproducibilities are 0.2‰ for 18O and^ 2‰for 2H.

3. Results and discussion

3.1. Thed 2H–d 18O relationships

The stable isotopic compositions of all precipitationevents collected for this study are listed in Table 1.Values ford 18O andd 2H from 11.69 to 210.80‰and from123.2 to285.5‰, respectively. The weightedmean values are24.92‰ for d 18O and228.4‰ ford 2H. Within each sampling site, the heavy isotope

R. Njitchoua et al. / Journal of Hydrology 223 (1999) 17–2620

Table 1 (continued)

Dates Rainfallamounts (mm)

Oxygen-18 Deuterium (‰)vs. VSMOW

Deuterium excess�d � d2H28d18O�

02 Nov 94 14.0 22.80 29.6 12.809 Nov 94 11.0 21.76 24.7 9.414 Nov 94 10.0 20.10 2.5 3.3w.a.v. – 25.55 235.6 9.4Abong-Mbang (lat.: 3858N; long.: 13812E; alt.: 694 m; distance from the coast: 382 km)07 Aug 94 19.0 10.14 18.5 7.425 Aug 94 10.7 21.32 20.6 10.023 Sep 94 75.0 27.55 246.7 13.709 Oct 94 55.2 24.03 224.9 7.327 Oct 94 66.0 25.19 230.8 10.716 Nov 94 2.5 11.69 123.2 9.7w.a.v. – 24.98 229.3 10.6Moloundou (lat.: 2803N; long.: 158 13E; alt. 500 m; distance from the coast: 600 km)08 Aug 94 54.0 25.54 227.2 17.221 Aug 94 7.0 24.65 225.8 11.413 Sep 94 15.0 25.53 229.3 14.924 Sep 94 49.0 25.63 230.8 14.204 Oct 94 80.0 26.90 243.0 12.221 Oct 94 15.0 24.37 224.1 10.914 Nov 94 49.0 24.84 223.0 15.709 Nov 94 8.0 22.87 20.9 21.924 Nov 94 7.5 11.39 21.2 10.1w.a.v. – 25.48 229.4 14.4

Page 5: Variations of the stable isotopic compositions of rainfall events from the Cameroon rain forest, Central Africa

contents of precipitation show a wide range of values.The difference between extreme18O values range fromapproximately 7‰ (Ebolowa) to 10‰ (Sangme´lima).

In Fig. 2(A) depicting thed 2H–18O relationship forall the investigated rainfall events, the least squaresline gives the following regional meteoric water line(RMWL)

d2H � �7:92^ 0:21�d18O1�9:27^ 1:01� �2�

with a correlation coefficientr � 0:98. Quite identicalrelationships have been reported for precipitationfrom south-western and north-central parts of Nigeria(Loehnert, 1988; Mbonu and Travi, 1994). As indi-cated in Fig. 2(B)–(F), the slopes for the local meteo-ric water lines (LMWL) are quite identical and veryclose to 8. This suggests that: (1) the rain formationprocesses throughout the whole region occurred underequilibrium isotopic conditions (Dansgaard, 1964);

R. Njitchoua et al. / Journal of Hydrology 223 (1999) 17–26 21

Fig. 2. d 2H–d 18O relationships for rain events from the Cameroon rain forest, Central Africa: (A) at the regional scale; and (B)–(F) at the localscale.

Page 6: Variations of the stable isotopic compositions of rainfall events from the Cameroon rain forest, Central Africa

and (2) that the isotopic compositions of the investi-gated precipitation events have not been modified byevaporation during the descent of rain drops to theground. Under isotopic equilibrium conditions, thevariation in the isotopic composition of rains can begiven by the following logarithmic differential

equation (Merlivat and Jouzel, 1979):

ddp

11dp� a�a21� dFV1FV da

a�FV1aFL� �3�

in whichFV andFL are the ratios of vapour and liquid,

R. Njitchoua et al. / Journal of Hydrology 223 (1999) 17–2622

Fig. 3. Monthly variations of precipitation (open circles) and weighted meand 18O values (dark circles) at Sangme´lima, Somalomo andMoloundou.

Page 7: Variations of the stable isotopic compositions of rainfall events from the Cameroon rain forest, Central Africa

anda is the temperature-dependent isotope fractiona-tion factor between liquid and vapour. Under closedsystem conditions, all the condensate is kept in thesystem, while in perfectly open system conditions,the condensate is immediately removed from thesystem �FL � 0�: In this latter case, assuming aconstant temperature of condensation�da=a � 0�;Eq. (3) will thus correspond to the Rayleigh distil-lation equation (Dansgaard, 1953; Ehhalt et al.,1963):

dp � a�11d0�F �a21V 21 �4�

wherea is the actual fractionation,d 0 the initial dof vapour and �a the mean fractionation betweeninitial condensation and precipitation. Eqs. (3) and(4) show that the condensation temperature, theamount of rainfall removed from the atmosphericreservoir and the isotopic composition of theinitial water vapour are the primary factorscontrolling the isotopic composition of precipita-tions.

In this paper, variations of the isotopic composi-tions of rains will be examined with respect to thetemporal evolution in relation with the northwardmigration of the ITCZ, the amount of precipitation,and the distance of the sampling sites to the coast.

3.2. The temporal variations and the amount effect

Fig. 3 shows the monthly variations of18Ocontents of precipitation collected at Sangme´lima,Somalomo and Moloundou. The general patternsof 18O contents at these sites clearly show thelowest 18O values at the heart of the great rainyseason (October), and higher18O values at boththe beginning and the end of the great rainyseason (August and November,respectively). Sucha clear correspondence between the degree of isotopedepletion of precipitation and the progressive installa-tion of the rainy season reflects presumably the influ-ence of the ITCZ. Similar findings have been alsomade in several regions world wide showing a seaso-nal rainfall regime (Gat and Matsui, 1991; Rozanski etal., 1992; IAEA, 1992; Mathieu et al., 1993). Duringsummer, the northward passage of the ITCZ over theCameroonian rain forest is accompanied by a progres-sive increase of moisture coming from the westernAtlantic ocean through the Gulf of Guinea, togetherwith a substantial increase in the intensity of rainevents. This results in a progressive depletion ofheavy isotopes, the lowest18O values being observedfor the wettest month (Fig. 3) consistent with the so-called amount effect (Dansgaard, 1964). This effectdescribes the empirical finding that the stable isotope

R. Njitchoua et al. / Journal of Hydrology 223 (1999) 17–26 23

Fig. 4. Plot ofd 18O values as a function of the amount of rains for all the investigated rain events.

Page 8: Variations of the stable isotopic compositions of rainfall events from the Cameroon rain forest, Central Africa

ratios of rain decrease with the increasing amount ofprecipitation.

However, all the precipitation events collected forthis study are distributed following two distinct trendsas seen in Fig. 4 where the18O contents are plotted asa function of the amount of precipitation. Rain eventsfrom trend I present higher18O contents (from12 to27‰) as expected for warm monsoon rains of whichformation process does not involve low temperatureof condensation (low altitude condensation). For thiscluster of rainfall events, the heavy isotope depletionappears clearly to be influenced by the amount effect.In contrast, the lowest18O values of all the rain events

collected for this study have been registered for rela-tively light rains (Trend II in Fig. 4). Such low heavy-isotope ratios of precipitation could be explainedeither by the condensation of residual vapour of acloud system, or by the condensation at high altitudes(thus involving a low temperature of condensation) ofconvective showers generated during the passage ofthe ITCZ. The latter mechanism may explain thelightest 18O values of 210.34‰ (rain amount�15.0 mm) and210.80‰ (rain amount� 20.5 mm)registered at Somalomo (3/10) and Sangme´lima (4/10). In contrast, the former hypothesis may likelyexplain the18O contents of26.34‰ (1 mm) and of

R. Njitchoua et al. / Journal of Hydrology 223 (1999) 17–2624

Fig. 5. Spatial distribution of the weighted meand 18O values of rainfall events in Cameroon rain forest: (a) contour map showing the aerialvariation ofd 18O values; (b) plot ofd 18O values as a function of distance of sampling sites from the Atlantic coast.

Page 9: Variations of the stable isotopic compositions of rainfall events from the Cameroon rain forest, Central Africa

210.80‰ (2.5 mm) collected at Ebolowa (22 and24/9).

3.3. The spatial variation ofd 18O and thecontinentality effect

The weighted mean18O values of rain events fromeach sampling site are plotted in Fig. 5 as a function ofthe inland distance of the sampling sites from the Gulfof Guinea. Despite the very limited number of rainevents collected at Kribi (1 km from the Atlanticcoast), we assume in the following that the weightedaverage18O value of21.5‰ for this station is repre-sentative of the first precipitations generated as thewestern Atlantic moisture enters the continent. There-fore, the weighted average18O values decreases withthe increasing latitudinal distance of sampling sites,from 21.5‰ in the vicinity of the Gulf of Guinea toapproximately25.5‰ at Moloundou located some600 km inland. However, as seen in Fig. 5(b), thisobserved inland heavy isotope depletion is not linearbut defines two main trends. In the first trend (0–230 km inland), the weighted average18O valuedecreases from21.50 to 25.15‰ (Sangme´lima),therefore defining an inland isotopic gradient of21.58‰ 100 km21. In contrast, in the second trend(230–600 km inland), the weighted average18O valuedecreases from25.15 to25.48‰, defining a slightisotopic gradient of20.09‰ 100 km21. The appear-ance of two inland gradients for18O values likelyindicates that variations in the heavy isotope contentsof the investigated rain events are influenced either bydifferent condensation mechanisms and/or by varyingsources of moisture. The heavy isotope depletionobserved in the first trend (0–230 km inland) couldbe explained by continuous removing of water vapouras rainwater by the Rayleigh distillation mechanism.In contrast, the quite constancy of18O values observedfor the second trend (230–600 km inland) of Fig. 5 isthought to reflect the influence of recycled continentalmoisture upon the isotopic composition of rain eventsin this portion of the area. As in the Amazon basinwhere an inland18O gradient of20.08‰ 100 km21

has been reported (Salati et al., 1979; Gat and Matsui,1991), the source of local moisture is assumed to beeither evapotranspiration of the dense forest vegeta-tion which covers the whole Central African continent

and/or evaporation of surface water from rivers cross-ing the region.

The influence of moisture recycling on the isotopiccompositions of precipitation events from the Camer-oon rain forest can also be evidenced by the deuter-ium-excess (d-excess) parameter which had beendefined by Dansgaard (1964) asd � d2H28·d18O. Ithas been well established that thed-excess values forcontinental precipitation events issued from oceanicmoisture are close to110‰ (Craig, 1961; Dansgaard,1964). However, in a hydrological system wheremoisture recycling through evaporation and/or evapo-transpiration play a significant role in the water cycle,higher d-excess values (.10‰) have been reported(Gat and Carmi, 1970; Salati et al., 1979; Matsui et al.,1983; Rindsberger et al., 1983; Ingraham and Taylor,1986). In this study, the calculatedd-excess value foreach individual rain event ranges from about11 to122‰ (Table 1), with a weighted mean value beingof approximately111‰. Of the 57d-excess dataavailable, 16 were greater than110‰ with most ofthese (44%) being reported for rain events from themost inland station of Moloundou. This confirms theimportance of recycling of moisture in this area.

4. Conclusions

In this paper, we have examined the factors control-ling the variations of heavy isotope contents of preci-pitation events collected during the summer rainyseason of 1994 in the Cameroon rain forest. It hasbeen proposed that the observed isotopic composi-tions of all the precipitation events collected for thisstudy have been controlled by: (1) the continuousremoving of vapour as rains from the cloud systemfollowing a distillation mechanism of Rayleigh type;(2) the low temperature of condensation in relationwith rain formation processes taking place at rela-tively high altitude; and (3) the supply of recycledcontinental moisture to the atmospheric vapour reser-voir. It appears from our data that the influence ofRayleigh mechanism is dominant in a portion spreadbetween the Gulf of Guinea and about 230 km inland,whereas the recycling effect is visible at inlanddistances .230 km. However, these preliminaryconclusions must be validated by further investiga-tions which should take into account all the

R. Njitchoua et al. / Journal of Hydrology 223 (1999) 17–26 25

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precipitation events from the two rainy seasons of theyear, and whenever possible, supplemented bymeteorological information.

Acknowledgements

The authors are very grateful to the staff of theMeteorological service in Cameroon for their help inthe samplings. We also thank A. Coudrain-Ribsteinand L. Araguas-Araguas and J. Jouzel for their helpfulcomments and suggestions on the earlier version ofthis manuscript.

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