-
Académie d'Aix Marseille Université d'Avignon et des Pays de
Vaucluse
THESE
Pour obtenir le grade de docteur de l'Université d'Avignon et
des Pays de Vaucluse
Ecole Doctorale : 380, Sciences et Agronomie
Discipline: Sciences de la Terre (Earth sciences)
Spécialité: Hydrogéologie (Hydrogeology)
METHODES ISOTOPIQUES ET GEOCHIMIQUES POUR L'ETUDE D ES EAUX
SOUTERRAINES
ET DE L'HYDROLOGIE DES LACS : CAS DU BASSIN DU NIL BLEU ET DU
RIFT ETHIOPIEN
Environmental isotopes and geochemistry in investigating
groundwater and lake hydrology: cases
from the Blue Nile basin & the Ethiopian Rift (Ethiopia)
Présenté par (by)
SEIFU KEBEDE
Présentée et soutenue publiquement
le 10 décembre 2004
JURY M. Edmunds Professor Université d’ Oxford, Center for Water
Research Rapporteur J.L. Michelot CR, HDR Université de Paris Sud,
LHGI Rapporteur Y.Travi Professeur Université d'Avignon, Labo.
Hydrogéologie Directeur de Thèse T. Alemayehu Asso. Professor Addis
Ababa University, Geology Department Examinateur K. Rozanski
Professor University of Krakow, Dept. Nuclear & Env. Physics
Examinateur P. Aggarwal Doctor Head, Isotope Hydrology Section,
IAEA Examinateur B. Blavoux Professeur Université d' Avignon, Labo.
Hydrogéologie Examinateur
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Résumé On utilise les isotopes de l’environnement (δ18O, δD,
δ13C, 3H) et l’hydrogéochimie pour étudier le fonctionnement
hydrologique des eaux souterraines et des lacs sur des secteurs
sélectionnés en Ethiopie. Il s’agit de la dépression de l’Afar, du
Rift Ethiopien et du bassin du Nil Bleu. On s’intéresse tout
d’abord à la relation entre le climat et la composition des eaux
météoriques. Les conclusions obtenues sont ensuite utilisées pour
l’étude des eaux souterraines et des lacs. La variation saisonnière
de δ18O et δD des eaux de pluie sur l’Ethiopie est principalement
sous la dépendance du mouvement saisonnier de la ZITC, des origines
des masses d’air, et des trajectoires associées, de l’humidité
atmosphérique. Une fois que l’humidité issue des principales
sources (Océans Indien et Atlantique ou évaporation continentale)
atteint les reliefs éthiopiens, la composition isotopique de la
pluie est modifiée par les effets locaux d’altitude, de température
et de masse. Un exemple typique est donné par l’appauvrissement en
δ18O de 0.1 ‰ par 100 mètres lorsque les masses humides se
soulèvent le long du versant ouest des montagnes éthiopiennes.
Toutefois, aucun de ces effets isotopiques ne paraît avoir une
influence prédominante sur la variation spatiale ou temporelle de
la composition isotopique des eaux météoriques. C’est pourquoi, la
thèse recommande de considérer l’ensemble de ces effets qui peuvent
s’opposer ou s’ajouter, plutôt que de mettre en valeur un seul
effet, lorsqu’on interprète les signaux isotopiques (dans les eaux
météoriques actuelles ou les archives isotopiques
paléohydrologiques). L’identification de différents mécanismes de
recharge pour les trois secteurs (Plateau Nord Ouest, Rift
Principal et dépression de l’Afar) constitue un des principaux
résultats. Le taux de fractionnement du à l’évaporation, avant la
recharge, est le plus élevé dans l’Afar et le plus faible sur le
Plateau Nord Ouest. Dans l’Afar la principale source de recharge
provient des bras morts de cours d’eau partiellement évaporés ou
d’écoulement de crues en provenance des escarpements qui bordent la
dépression ou de la plaine de l’Awash. En couplant les méthodes
géochimiques et isotopiques, ce travail précise également les
mécanismes de recharge des eaux souterraines, leur temps de
résidence et leur évolution géochimique dans le bassin supérieur du
Nil Bleu. Bien que les basaltes du Cénozoïque soient le principal
aquifère, plusieurs systèmes hydrogéologiques ont pu être
identifiés et décrits sur la base des données hydrogéochimiques.
Par ailleurs, dans deux secteurs (linéament volcanique de Yerer
Tulu Welel -YTVL et graben du lac Tana-GLT) le dioxyde de carbone
d’origine profonde joue un rôle important pour le contrôle de
l’évolution chimique des eaux souterraines du type NaHCO3 avec un
TDS élevé. Le bassin du Nil Bleu était autrefois considéré comme
une région avec un système hydrogéologique simple constitué
d’aquifères de roches cristallines L’application de la méthode du
bilan isotopique à quelques lacs éthiopiens sélectionnés montre que
la méthode est plus performante en comparant l’état hydrologique
des lacs et en calculant les flux d’eau souterraine autour des
lacs. On propose d’utiliser une droite d’évaporation hypothétique
locale comme référence pour comparer les compositions isotopiques
(actuelles ou anciennes) et obtenir ainsi des informations
hydrologiques immédiates. Mots clés: isotopes de l’environnement,
effets isotopiques, recharge en eau souterraine, bilan de lac, Nil
Bleu, Rift Ethiopien, Ethiopie
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Abstract This work uses environmental isotopes (δ18O, δD, δ13C,
3H) and geochemistry in groundwater and lake hydrological studies
of selected sites from Ethiopia. The sites are the Afar Depression,
the Main Ethiopian Rift and the Blue Nile Basin. The thesis first
investigates the relationship between the seasonal and spatial
variations in the isotopic composition of Ethiopian meteoric waters
and the Ethiopian climate. It then makes use of this understanding
in the groundwater and lake hydrological studies. The seasonal
variation in δ18O and δD compositions of Ethiopian rainfall waters
are mainly influenced by the seasonal drifting of the ITCZ and
associated changes in sources of moisture or associated changes in
moisture trajectory. Once the moisture mass from the major sources
(Indian, Atlantic or continental) reaches the region, its δ18O and
δD compositions is modified by the local altitude effect, the
temperature effect and the amount effect. A clear example is the
0.1 ‰ per 100 meter depletion in δ18O as moisture mass moves upward
over Ethiopian mountains facing west. The relation between spatial
variation in mean air temperature and the spatial variation
isotopic composition of meteoric waters has the form: δ18O = 0.21
Tair (°C) - 6.5. However, none of the isotope effect seems to
dominate the other in influencing the spatial and temporal
variation in isotopic composition of meteoric waters. Therefore,
the thesis recommends that when one interprets the isotope signals
(in modern meteoric waters or in paleo isotope record from
archives) from the region one should consider the interplay of all
effects that reinforce or cancel each other rather than singling
out one isotope effect. One of the major results of the thesis is
the identification of differences in ground recharge mechanisms of
the three sectors (North Western Plateau, the MER, and Afar
Depression) of the study region. The degree of evaporative
fractionation prior to recharge is the highest in Afar Depression
and the lowest in the NWP. In Afar the major source of groundwater
recharge is from 'incompletely' evaporated losing streams or flush
floods converging towards the Afar Depression from the bordering
escarpments and from infiltration by Awash flood plain water. By
coupling the isotopic and the geochemical methods this thesis also
shows groundwater recharge mechanisms, its subsurface residence
time and its geochemical evolution in the Upper Blue Nile Basin.
Although the Cenozoic basalt is the principal aquifer in the upper
Blue Nile Basin, multiple geochemically recognizable groundwater
bodies/layers have been identified. This allows to describe
different hydrogeological systems. Furthermore in two zones (the
Yerer Tulu Welel Volcanic Lineament-YTVL and the Lake Tana
Graben-LTG) carbon dioxide from deeper sources plays an important
control on geochemical evolution of the high TDS NaHCO3 type
groundwaters. The Blue Nile basin was previously considered as a
region with simple hydrogeology underlain by crystalline aquifers.
The isotope balance study of selected Ethiopian lakes shows that
the isotopic lake balance method is more powerful in comparing the
hydrological status of lakes and in computing groundwater flux
around lakes. The thesis proposes a hypothetical local evaporation
line for Ethiopia as a reference with which the isotopic
composition (present or ancient from archives) of any lake could be
compared to gain rapid hydrological information. The technique
developed in this thesis has wider application in wet land and
interconnected lake system studes including the analysis of degree
of lake interconnectiveiy and wetland interconnectivity. The method
is also used to quantify groundwater flux around lakes. Key words:
environmental isotopes, isotope effects, groundwater recharge, lake
water balance, Blue Nile Basin, Ethiopian Rift, Ethiopia
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INTRODUCTION GENERALEINTRODUCTION GENERALEINTRODUCTION
GENERALEINTRODUCTION GENERALE
i.i.i.i. Objectifs et démarche utilisésObjectifs et démarche
utilisésObjectifs et démarche utilisésObjectifs et démarche
utilisés La présence de nombreux lacs, dépôts lacustres, et d’un
flux de chaleur du à l’amincissement de la
croûte dans le Rift d’Afrique de l’Est, a suscité de nombreuses
investigations scientifiques depuis le
début des années 1970. La majorité de ces études ont réalisé des
mesures isotopiques et chimiques,
avec comme objectif d’évaluer le potentiel des ressources
géothermiques (Gonfiantini et al., 1973;
UNDP, 1973; Scholes et Faber, 1976; Craig et al., 1977; IAEA
projets en cours depuis 1994), ou pour
l’analyse des changements environnementaux (Lamb et al., 2002),
ou encore pour comprendre la
dynamique des fluides crustaux dans la vallée du rift est
africain (Darling, 1996). Quelques travaux
indépendants ont utilisé les isotopes de l’eau pour étudier les
interactions eaux de surface – eaux
souterraines (Darling et al., 1996; Chernet, 1998; Mckienze et
al., 2001; Ayenew, 1998; Kebede et al.,
2002, Gizaw, 2002, les projets AIEA:
http://www-naweb.iaea.org/napc/ih/tcs_list_region.asp ) et la
climatologie actuelle (Rozanski et al., 1996). Ces travaux ont
fourni une banque de données utile en
particulier dans la vallée du rift éthiopien et la dépression de
l’Afar.
Les études antérieures ont permis de préciser les sources de
recharge et l’hydrogéochimie dans les
environs d’Addis Abeba (Gizaw, 2002), la dynamique des eaux
souterraines autour des lacs de la
vallée du rift éthiopien (Ayenew, 1998), les interactions eaux
de surface – eaux souterraines dans le
rift (Darling et al., 1996; Chernet, 1998; Chernet et al.,
2001), les sources de pollution (Mckenzie et
al., 2001; Reimann et al., 2003 ) et les ressources géothermales
dans le rift éthiopien et la dépression
de l’Afar (Craig et al., 1977; Darling, 1996). Ces études
dispersées n’apportent cependant pas une
vision générale des variations spatiales et temporelles des
signaux isotopiques et de leurs contrôles
climatologiques et hydrologiques.
La principale question que l’on doit poser avant d’utiliser les
isotopes de la molécule d’eau pour
l’étude des eaux souterraines ou de l’hydrologie des lacs est,
évidemment : quelle est la composition
du signal d’entrée i.e. la composition de l’eau d’alimentation,
que l’on va suivre dans le système. Ceci
nécessite une connaissance précise de la variation spatiale des
isotopes de l’eau et de leur relation avec
les facteurs climatiques ou non climatiques. Cette connaissance
est la base du traçage des eaux
souterraines et de l’étude isotopique des lacs, et elle donne
une relation isotope – climat actuel qui peut
servir de référence pour interpréter les archives
paléo-isotopiques.
La plupart des données isotopiques obtenues jusqu’ici en
Ethiopie concerne des surfaces limitées soit
aux environs du rift éthiopien, soit dans la dépression de
l’Afar et occasionnellement sur les
escarpements bordant le rift. Ces deux régions dépendent d’un
régime météorologique complexe où
les deux moussons (Indienne, Atlantique), la topographie,
beaucoup d’autres courants atmosphériques
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(comme les Jets tropicaux d’Est, l’air froid et sec d‘Arabie) le
fractionnement du à l’évaporation et
l’orographie interagissent et jouent un rôle important dans la
détermination de la composition
isotopique du 'signal' d’entrée. Déterminer les relations
isotope – météorologie est ainsi complexe et
constitue un sujet important d’investigation des ressources en
eau et de leur variabilité.
Par ailleurs, les données isotopiques fiables étaient
apparemment inexistantes sur le Plateau Nord
Ouest Ethiopien. Beaucoup d’études antérieures indiquent un
écoulement des eaux souterraines en
direction du rift depuis les reliefs adjacents. Cependant, on
sait peu de choses sur l’hydrogéologie,
l’hydrogéochimie et la composition isotopique des eaux
souterraines des plateaux adjacents. Ainsi, le
mécanisme de transfert des eaux souterraines du plateau vers le
rift n’est pas clair.
L’examen des variations spatiales des isotopes de l’eau ou
l’analyse des transfert d’eau souterraine
depuis le plateau vers le rift ne sont pas les seuls objectifs
qui nous ont conduit a travailler en partie
sur le Bassin supérieur du Nil Bleu (Le Plateau Nord Ouest) dans
cette thèse. Cette étude a démarré à
la suite de la réalisation du « Master Plan » (BCEOM, 1998). Ce
travail fournit une vision d’ensemble
de l’état des eaux souterraines et les données physiques de base
des aquifères régionaux. Il a laissé
beaucoup de questions à résoudre sur les eaux souterraines du
bassin du Nil bleu (définition des
aquifères principaux, les origines de la recharge, l’écoulement
souterrain, les relations eau de surface –
eau souterraine, la qualité de l’eau souterraine. Ceci nous a
conduit à utiliser les isotopes et
l’hydrogéochimie pour examiner l’origine des eaux souterraines
dans le Bassin supérieur du Nil bleu.
En Ethiopie (limite nord de la ZITC et donc grande vulnérabilité
vis-à-vis des conditions climatiques)
ce n’est pas seulement l’évaluation des ressources en eau qui
est utile, mais également sa variabilité .
Plusieurs études existent sur la relation climat/variabilité des
ressources en eau en Ethiopie. Les
travaux sur les ressources en eau et la variabilité climatique
de l’échelle millénaire à l’échelle
saisonnière ont été soigneusement répertoriés par Nyssen et al.
(2004). Au Quaternaire et à
l’Holocène, la région a subi des variations dramatiques de la
pluie, du niveau des lacs et du climat en
général. La région a également subi une fluctuation majeure des
variations inter annuelle et
saisonnières de la pluie au cours de la dernière décennie.
Savoir comment le climat a varié dans le
passé peut être utilisé comme référence pour ses variations
futures. Toutefois, comment interpréter le
paléoclimat à partir des archives sédimentaires fait encore
l’objet de nombreuses investigations
globales. Dans les régions au climat complexe comme l’Ethiopie,
la calibration entre la relation
actuelle climat/hydrologie et la composition isotopique des lacs
devrait fournir une information utile
pour mieux interpréter les isotopes des archives isotopiques.
Par ailleurs, la calibration hydrologie-
isotope-climat sur quelques lacs actuels sélectionnés est une
approche pratique pour examiner d’autres
lacs ou réservoirs peu connus.
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Tous ces éléments nous ont finalement conduit à organiser le
travail suivant quatre objectifs
interdépendants, qui utilisent le même type de données. Les
principaux objectifs sont:
• Fournir ou améliorer le schéma général des variations
temporelle et spatiale des compositions
isotopiques et leur contrôle météorologique, dans les régions
centre et nord de l’Ethiopie.
• Examiner les mécanismes de la recharge dans les trois régions,
à savoir : le Plateau Ethiopien
Nord Ouest, Le Rift Ethiopien Principal et la Dépression de
L’Afar.
• Examiner la géométrie des aquifères, la circulation des eaux
souterraines, leur recharge, et leur
potentialité dans le bassin du Nil Bleu, en utilisant les
techniques chimique et isotopique, et
enfin,
• préciser les relations entre la composition isotopique de
l’eau des lacs actuels et les
caractéristiques hydrologiques climatiques et hydrographiques de
lacs éthiopiens sélectionnés
(y compris les lacs du bassin du Nil Bleu).
ii. Approche (méthodologie)ii. Approche (méthodologie)ii.
Approche (méthodologie)ii. Approche (méthodologie)
Pour atteindre ces objectifs, cette thèse utilise les isotopes
de l’eau et la géochimie des solutés. Pour
étudier un système à l’aide des isotopes, on a besoin du signal
d’entrée et du signal de sortie pour le
caractériser. Ceci implique que l’application de la méthode
dépend du type de système. Cette thèse
tout en s’attaquant aux objectifs, testera aussi la pertinence
de la méthode (en particulier l’application
des isotopes à l’étude du bilan des lacs) sous le climat de
l’Ethiopie et d’autres conditions spécifiques
au site comme la salinité de l’eau des lacs et
l’hydrographie.
Isotopehydrogeology
Modernlake isotope
water balance
Modern isotopeclimate
relations
Paleoclimate
modern
calibration/modeling
Gw tracing
Composition of input signal
Lake groundwater link
Modern
calibration/modelling
proxy
Input signal
proxy
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Démarche logique, suivie et proposée par la thèse, pour l’étude
des ressources en eau en Ethiopie. Les résultats de ce travail
tendent à montrer l’importance des systèmes hydrologiques
continentaux actuels pour calibrer et modéliser les paléoclimats en
Ethiopie en suivant la logique indiquée sur ce diagramme.
iii.iii.iii.iii. Organisation de la thèse Organisation de la
thèse Organisation de la thèse Organisation de la thèse
Le mémoire comporte cinq parties. Une courte introduction et un
résumé des résultats précèdent les
parties II et III ; elle est suivie par des articles
scientifiques (soumis ou à l’impression).
La première traite du premier objectif :- donner une meilleure
vision de la composition isotopique
(δ18O, δD, 3H) des eaux météoriques en Ethiopie, et des facteurs
qui commandent leur variations
spatiale et temporelle dans le cycle de l’eau. On s’intéresse en
particulier à la variation saisonnière de
δ18O et sa relation avec la ZITC, l’influence « feed back » de
la surface du sol sur la composition
isotopique des précipitations en Ethiopie, la variation spatiale
des isotopes de la molécule d’eau et son
contrôle. Cette première partie confirme que la composition du
signal isotopique fourni par les
précipitations en Ethiopie est influencée à la fois par des
processus à grande échelle (eg. déplacement
saisonnier de la ZITC et l’apport associé de son humidité) et
les processus qui interviennent à la
surface du sol (ré-évaporation à partir des bassins continentaux
étendus ou activité locale de
convection de vapeur, effet « d ‘ombre » sur la pluie,
évaporation en cours de chute, effets
orographique etc. ).
Dans la seconde partie, on caractérise la composition isotopique
des eaux souterraines du Rift
Ethiopien, du Plateau Nord Ouest et de l’Afar, et on discute
ensuite ces caractéristiques pour les trois
régions. Les mécanismes de recharge dans ces trois secteurs
importants d’Ethiopie (Plateau Nord
Ouest, Rift Ethiopien Principal et dépression de l’Afar) sont
comparés. On montre que la recharge des
eaux souterraines est rapide, que les trajectoires de
l’écoulement sont courtes, et que le fractionnement
du à l’évaporation avant la recharge est peu important sur le
Plateau Nord Ouest alors qu’il constitue
un processus important en Afar. Les eaux souterraines du Rift
Ethiopien Principal présentent des
propriétés intermédiaires.
La troisième partie se rapporte au troisième objectif. En
utilisant les connaissances acquises dans la
première partie, elle essaye de donner un schéma amélioré des
ressources en eau du bassin du Nil bleu.
On utilise essentiellement l’hydrochimie et l’hydrologie
isotopique pour atteindre ces objectifs. Deux
principaux bassins, structuralement déformés, et présentant une
évolution chimique et une
hydrodynamique homogènes, ont été identifiés. Ce sont celui de
la zone de l’alignement volcanique de
Yerer Tullu Welele et le graben du lac Tanna. Les isotopes de la
molécule d’eau associés à quelques 3H, le Carbone -13, et
l’hydrochimie précisent les contrôles sur l’évolution chimique des
eaux
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souterraines dans le bassin. Dans ce chapitre on essaye de
quantifier le potentiel en eaux souterraines à
partir d’une approche physique simple.
La quatrième partie se rapporte au quatrième objectif: -
calibrer la relation entre la composition
isotopique des lacs et le climat régional. On utilise la méthode
du bilan isotopique (δ18O et δD) dans
les études de bilan hydrologique des lacs. La Droite
d’Evaporation Locale hypothétique sous les
conditions climatiques de l’Ethiopie a tout d’abord été calculée
et les compositions isotopiques de
quelques lacs sélectionnés lui ont ensuite été comparés. La
comparaison de la composition isotopique
modélisée des lacs avec la composition isotopique mesurée
fournit un moyen rapide de classification
des lacs en : lacs à flux de sortie dominant, à évaporation
dominante, à diminution de volume ou eau
souterraine dominantes. La même approche peut aider à obtenir
des informations sur les facteurs non
climatiques (comme les effets hydrographiques, les effets de
lacs en série, ou les effets de salinité) qui
influencent le régime isotopique des lacs et ainsi des sédiments
utilisés comme archives.
La cinquième partie présente une synthèse du travail. Elle
résume les principaux résultats, et les
perspectives que l’on peut en tirer pour le futur. Elle permet
d’esquisser les avantages et les limites de
l’utilisation des méthodes isotopiques sous les conditions
climatiques de l’Ethiopie et son contexte
géologique.
Figure i. Localisation de quelques sites importants
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9
iv. Localisation de la zone d’étude, toponymie, régionsiv.
Localisation de la zone d’étude, toponymie, régionsiv. Localisation
de la zone d’étude, toponymie, régionsiv. Localisation de la zone
d’étude, toponymie, régions
Les sites étudiés(figure i) couvrent trois régions principales.
Ce sont le Rift Ethiopien Principal, la
Dépression de L’Afar et le Plateau Ethiopien Nord Est. La
station GNIP/IAEA se situe au centre de
l’Ethiopie à (2360 masl). Des données isotopiques sur les
pluies, couvrant une courte période, ont été
obtenues sur quatre stations dans la vallée du Rift (Sodo,
Awassa, Kofele, et Agermariam). Le Nil
Bleu prend sa source au lac Tana et draine le Plateau nord
ouest. Tous les cours d’eau, à l’exception de
l’Awash, s’écoulent depuis les reliefs centraux. Le lac de
cratère Bishoftu qui contient des varves
sédimentaires annuelles se trouve près d’Addis Abeba. Les champs
géothermiques (exemple le centre
géothermique d’Aluto Langano) et la zone des lacs constituent
deux singularités du Rift Ethiopien
Principal.
v. v. v. v. Définitions, symboles, notationsDéfinitions,
symboles, notationsDéfinitions, symboles, notationsDéfinitions,
symboles, notations
Isotopes de l’eau: L’eau est une molécule composée de deux
atomes d’hydrogène et d’un atome
d’oxygène. L’hydrogène possède deux isotopes stables 2H/D
(espèce rare généralement appelée
deutérium, 1H (espèce abondante) et un isotope radioactif 3H
(tritium). L’oxygène possède deux
isotopes stables 16O (abondant) et 18O (rare). Ces isotopes se
combinent pour former quatre types
d’eau : H216O (le plus abondant), D2
16O, H218O et D2
18O (plus rares. La composition de l’eau en ces
isotopes varie dans les systèmes hydrologiques en fonction des
conditions physiques et d’autres
processus chimiques ou biologiques prévisibles.
Rapports isotopiques: la composition de l’eau en ces isotopes
s’exprime généralement par le rapport
de l’isotope lourds sur l’isotope léger (18O/16O, D/H). Dans la
mesure où l’isotope lourd est très rare, le
rapport est un nombre très petit. Ces fractions ne sont pas
facilement utilisables pour des opérations
mathématiques simples.
La notation delta pour mille: Les rapports isotopiques de la
molécule d’eau sont généralement
comparés au rapport isotopique d’une eau standard de rapport
isotopique connu. Le « Standard Mean
Ocean Water » (Vienna-SMOW, VSMOW) est le standard le plus
largement utilisé. Ainsi, les
abondances en 18O et D s’expriment comme un rapport en notation
delta pour mille (parts pour mille,
‰) différences relative au standard. Les nombres obtenus sont
entiers et utilisables avec des
opérations mathématiques simples.
( ) 1000*1)16/18(
)16/18( .18
−=
SMOW
Ech
OO
OOpourmilleOδ et ( ) 1000*1
)/(
)/( .
−=
SMOW
Ech
HD
HDpourmilleDδ
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10
L’Excès en deutérium (d-excess) est équivalent à δD-8δ18O d’une
eau météorique donnée. L’excès
en deutérium moyen de l’ensemble des précipitations à l’échelle
globale est de 10. L’Excès en
deutérium initial dans les pluies s’écarte de 10 en relation
avec les conditions d’évaporation à l’origine
de la vapeur et de l’influence de la vapeur continentale. Ainsi,
l’Excès en deutérium est souvent utilisé
comme marqueur d'origine de la vapeur dans une région
donnée.
Enrichissement/Appauvrissement ou enrichi/appauvri: Cette
terminologie est utilisée pour
comparer les compositions en δ18O et δD de différents types
d’eaux météoriques dans une région
donnée. Les eaux qui contiennent de forts δ18O et δD par rapport
aux autres eaux de la région sont
souvent considérées comme 'enrichies', Les eaux avec de faibles
δ18O et δD comme appauvries. Deux
exemples simples : les volumes d’eau évaporée sont enrichis en
isotopes lourds du fait de
l’évaporation, ou les eaux souterraines sont souvent appauvries
par rapport à la composition isotopique
de la pluie locale du fait d’une recharge sélective.
La DEMG (GMWL): Sur un diagramme δ18O-δD la vapeur qui se forme
à partir de l’évaporation des
océans, les eaux qui se forment par condensation de la vapeur
océanique, ou les eaux souterraines sur
les continents directement rechargées par les pluies sans
modification majeure, ou les eaux des rivières
isotopiquement non modifiées (à une échelle globale), se
regroupent sur une droite de pente 8 et de
décalage à l’origine de 10. Cette droite s’appelle la Droite des
Eaux Météorique Globale ou droite de
Craig.
La DEML (LMWL): Les eaux météoriques ne se situent pas toujours
sur la Droite Météorique. En
fonction des condition évaporatoires sur la surface de l’Océan
et des sources d’humidité, localement
une déviation par rapport à cette droite peut exister. La droite
que l’on obtient à partir de la
composition des eaux météoriques dans une région donnée est
appelée Droite des Eaux Météoriques
Locales. Le diagramme des eaux de pluie d’été non évaporées sur
Addis Abeba donne une droite : δD
= 8δ18O +15. La totalité des pluies mensuelles donne la relation
: δD = 7.2δ18O +12.
La DEL (LEL): Les eaux météoriques sujettes à l’évaporation
(lacs, rivières, mares, etc.) vont subir
un fractionnement isotopique du fait de la perte d’eau sous
forme de vapeur. Les eaux évaporées ont
tendance à se situer sur une droite qui s’écarte de la Droite
des eaux Météorique mondiale. La droite
qu’elles ont tendance à former (sur un diagramme δ18O-δD) est
appelée Droite d’Evaporation Locale.
La pente de cette droite se situe entre 3.5 et 6 et dépend de
l’humidité locale.
L’effet d’altitude ou le pseudo effet d’altitude : Lorsqu’une
masse d’air humide s’élève le long
d’une barrière montagneuse, la température de l’air humide tend
à se refroidir adiabatiquement.
Beaucoup de facteurs peuvent provoquer l’appauvrissement en
isotope lourd avec l’altitude. On peut
citer la température. La condensation qui est causée par la
chute des températures suivant
l’augmentation de l’altitude conduit à un appauvrissement en
isotope lourd. L’effet Rayleigh est une
autre cause. Quand une masse d’air humide est contrainte à
s’élever les isotopes les plus lourds
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11
tendent à être retirés préférentiellement de la vapeur par les
gouttes d’eau. Ceci produit un
appauvrissement en isotope lourd avec l’altitude. L’effet
d’altitude est souvent observé aussi sur le
versant sous le vent d’une montagne. Le pseudo effet d’altitude
est souvent confondu avec l ‘effet
d’altitude. Dans les deux cas il y a appauvrissement en isotope
lourd avec l’altitude. Le pseudo effet
d’altitude est provoqué par un enrichissement par évaporation
des gouttes de pluie au cours de leur
chute sous le nuage. Il est souvent observé dans les vallées ou
sur le versant sous le vent des chaînes
de montagnes. Cet enrichissement par évaporation, différent de
l’effet initial de Rayleigh dans le
nuage, provoque aussi une diminution de l’excès en deutérium,
marquant ainsi clairement cette
situation.
Tritium et Unité Tritium: Le tritium est un des isotopes de
l’hydrogène. Il est radioactif avec une
demi vie de 12.26 ans. Il est produit dans l’atmosphère par le
bombardement cosmique de l’azote 14N
+ n => 3H+ 12C. La concentration en tritium des eaux
s’exprime T/H. Ceci correspond à une fraction
minuscule. Une méthode alternative consiste donc à utiliser
l’Unité Tritium (UT). Le rapport T/H =
10-18 correspond à 1UT. Il est souvent utilisé pour dater les
eaux souterraines jeunes. A l’heure actuelle
en Ethiopie, dans les eaux de pluie exemptes de pollution
nucléaire (cosmogéniques) les
concentrations en tritium se situent entre 5 et 10 UT
vi. Abbreviations utilisées dans la thèse
NWP- The North Western Ethiopian Plateau SEP- The South Eastern
Ethiopian Plateau MER- The Main Ethiopian Rift GNIP- Global Network
for Isotopes in Precipitation of the IAEA BCL- Bishoftu Crater
Lakes YTVL- The Yerer Tulu Welel Volcanic Lineament LTG- The Lake
Tana Graben JJAS- June-July-August-September CLEL- Calculated local
evaporation line
vi. Reférences
Ayenew, T. 1998. The Hydrogeological system of the lake district
basin, central Main Ethiopian Rift. Phd thesis, ITC publication
Number 64, the Netherlands, 200p. Battistelli, A., Yiheyis, A.,
Calore, C., Ferragina, C., Abatneh, W., 2002. Reservoir engineering
assessment of Dubti geothermal field, Northern Tendaho Rift,
Ethiopia. Geothermics, 31: 381–406 BCEOM, 1999. Abay River Basin
integrated master plan, main report, Ministry of Water Resources,
Addis Ababa. Chernet, T., 1998. Etude des Mechanismes de
mineralisation en fluorure et elements associes de la region des
lacs du rift Ethiopien. Ph.D. Thesis, Avignon, France. Chernet ,
T., Travi, Y., Valles, V., 2001. Mechanism of degradation of the
quality of natural water in the lakes region of the Ethiopian Rift
Valley. Water Reser.35, 2819–2832. Craig, H., Lupton J.E.,
Horowiff, R.M., 1977. Isotope Geochemistry and Hydrology of
geothermal waters in the Ethiopian rift valley. Scripps Institute
of Oceanography, University of California report, 160p. Darling,
G., Gizaw, B., Arusei, M., 1996. Lake-groundwater relationships and
fluid-rock interaction in the East African Rift Valley: isotopic
evidence. J. African Earth Sci. 22, 423-430. Darling, WG., 1996.
The Geochemistry of fluid processes in the eastern branch of the
east African rift system, Ph.D thesis, British Geological Survey,
UK, 235p. Gizaw, B., 2002. Hydrochemical and Environmental
Investigation of the Addis Ababa Region, Ethiopia. Ph. D
dissertation, Faculty of Earth and Environmental Sciences
Ludwig-Maximilians-University of Munich, 157p.
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12
Gonfiantini, R., Borsi, S., Ferrara, G. and Panichi, C., 1973.
Isotopic composition of waters from the Danakil Depression
(Ethiopia). Earth and Planetary Science Letters 18: 13-21. IAEA TC
projects ETH8005 ETH8006, ETH8 007 (1995 to present) . Ongoing and
completed projects conducted by International Atomic Energy Agency,
the Ethiopian Science and Technology Commission and the Ethiopian
Geological Surveys, Various unpublished and expert visit reports,
isotope data, etc; Ethiopian Geological Survey, Addis Ababa,
Ethiopia. Kebede, S., Lamb,H., Telford,R., Leng, M. and Umer, M.,
2002. Lake-Groundwater relationships, oxygen isotope balance and
climate sensitivity of the Bishoftu Crater Lakes, Ethiopia.
Advances in Global Change Research, 12: 261-275. Lamb, H., Kebede,
S., Leng.M.J., Ricketts, D., Telford, R., Umer, M., 2002b. Origin
and stable isotope composition of aragonite laminae in an Ethiopian
crater lake. In: Odada, E., Olago, D. (Eds.), The East African
Great Lakes Region: Limnology, Palaeoclimatology and Biodiversity,
Advances in Global Research Series. Kluwer Academic Publishers,
Dordrecht. McKenzie, J., Siegel, D., Patterson, W., McKenzie, J.,
2001. A geochemical survey of spring water from the main Ethiopian
Rift Valley, southern Ethiopia: implication for well head
protection. Hydrogeol. J. 9, 265-272. Nyssen,J., Poesen, J.,
Moeyersons, J., Deckers, J.,Haile, M., Lang, A., 2004. Human impact
on the environment in the Ethiopian and Eritrean highlands—a state
of the art. Earth sciences reviews, 64: 273-320. Reimann, C.,
Bjorvatn,K., Frengstad, B., Melaku, Z., Tekle-Haimanot, R.,
Siewers, U., 2003. Drinking water quality in the Ethiopian section
of the East African Rift Valley, part I: data and health aspects.
The Sci. Tot. Env. 31, 65-80. Rozanski, K., Araguas-Araguas, L.,
Gonfiantini, R., 1996. Isotope patterns of precipitaion in the East
African Region. In: Johnson, T.C., Odada, E. (Eds). The
Climatology, Palaeoclimatoloy, Paleoecology of the East African
Lakes, Gordon and Breach,Toronto, pp. 79-93. Schoell, M., Faber,
E., 1976. Survey on the isotopic composition of waters from NE
Africa. Geologisches Jahrbuch. 17, 197-213. UNDP, 1973. Geology,
geochemistry and hydrology of hot springs of the East African Rift
system within Ethiopia., UNDP report DD/SF/ON-11, N.Y.
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13
PARTIE I
Composition isotopique des eaux météoriques en Ethiopie
(Plateau NW, Afar et Rift Ethiopien Principal)
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14
Introduction
Les concentrations en δ18O et δD des eaux météoriques en
Ethiopie, ainsi que leur variations spatio-
temporelles sont commandées par l’interaction d’une grande
variété de facteurs. Parmi les plus
importants on notera ; a) les facteurs d’échelle continentale ou
globale tel le déplacement saisonnier de
la ZITC (Zone Inter Tropicale de Convergence) et les
déplacements des masses humides qui lui sont
associées ; et b) les facteurs locaux ou régionaux qui
influencent ou modifient les compositions
isotopiques. Les facteurs locaux comprennent l’effet d’altitude,
l’effet de l’évaporation locale, l’effet
d’ombre (pseudo altitude) sur les versants « sous le vent, et
l’action de la vapeur ré évaporée depuis le
sol. Dans ce chapitre on va :
1. Discuter brièvement les mécanismes à l’origine de la pluie,
qui ont une influence sur le
marquage isotopique des eaux en Ethiopie,
2. discuter la relation entre la variation des teneurs
isotopiques mensuelles des pluies et le
déplacement saisonnier de la ZITC,
3. essayer de comprendre l’origine de la composition isotopique
des eaux météoriques en
Ethiopie en comparant avec des régions similaires en
Afrique,
4. proposer des hypothèses relatives à l’importance de
l’orographie sur la composition des eaux
météoriques en Ethiopie,
5. décrire les variations spatiales des teneurs en isotopes de
l’eau et leur relation avec les
trajectoires des masses d’air et le climat local
(température/précipitation/évaporation),
6. discuter les avantages et les contraintes liés à
l’utilisation des isotopes de l’eau pour les études
hydrologiques en Ethiopie,
7. présenter des données préliminaires sur la chimie des eaux de
pluie pour essayer d’identifier
les sources de vapeur.
L’analyse de la relation entre les variables climatiques et la
composition isotopique des eaux
météoriques en Ethiopie servira de base à: a) la compréhension
des mécanismes de recharge et pour le
traçage du transfert des eaux souterraines depuis le Plateau
Nord Ouest (NWP) vers le Rift Ethiopien
Principal (MER) et la dépression de l’Afar (Partie II); b)
l’évaluation des ressources en eau souterraine
des aquifères hydrogéologiquement peu connus du bassin du Nil
Bleu (Partie III); et, c) estimer le
bilan isotopique de quelques lacs éthiopiens représentatifs
(Partie IV).
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15
1) Climate and rainfall derivation in Ethiopia
There is a general agreement that the Ethiopian rainfall regime
is under the influence of the Indian and
the Atlantic Ocean monsoons (Griffits, 1972, Gemechu, 1977). The
flow of the monsoon moisture to
the region is controlled by the seasonal migration of the Inter
Tropical Convergence Zone (ITCZ).
In summer (June July August September- JJAS) the ITCZ is located
in northern Ethiopia and the
region is under the influence of the southwesterly and southerly
monsoon flows (figure 1). The
southwesterly and southerly flows bring moisture from three
sources including, low level moisture
which is pulled from the Congo vegetation basin, from the
Atlantic Ocean and partly from the
Equatorial Indian Ocean (Hemming, 1961; Suzuki, 1967; Gemechu,
1977; Camberlin, 1997; Nyssen et
al., 2004). Open continental water bodies (such as tropical
lakes and Lake Victoria) may also play an
important role in feeding the low-level southwesterly flows
(Kebede, 1964; Camberlin, 1997; Okeyo,
1992). The NWP and the Rift Valley including Afar get rainfall
during this time.
Between October and March the ITCZ is located south of Ethiopia.
This results in northerly flow of
dry and cold air from the Arabian continent. The coldest
temperature is recorded in the highland
region during this time. The same southward flow brings some
moisture from Arabian Sea and
Northern Indian Ocean to the eastern lowlands bordering the
South Eastern Plateau (SEP) producing
rainfall in October, November and December (Gemechu, 1977).
In spring (March and April-MA) the ITCZ is moving northward
crossing Ethiopia. This results in
northeasterly and easterly moisture flows. These bring the
spring rain to the region from the Northern
Indian Ocean. Only the southe eastern plateau (SEP) and the
southern and eastern sectors of the north
western plateau (NWP) are influenced by this moisture. In the
lowlands bordering Sudan and the
central sector of the NWP the influence of the North Indian
Ocean moisture is absent or is very weak
(see histogram in figure 1). Seventy five percent of annual
rainfall in the NWP and in the MER occurs
during summer (JJAS) when the Inter-Tropical Convergence Zone
(ITCZ) is located north of Ethiopia.
The other 25% of rainfall occurs in spring (MA) when the ITCZ is
still passing over Ethiopia
northwards.
Locally, the elevated terrain of the Ethiopian plateau
influences the orographic enhancement of rainfall
(Camberlin, 1997). Although the mountains in the Ethiopian
plateau act generally as moisture
enhancement they also lead to extremely complex pattern of
rainfall, temperature and aridity (figure 2)
over the region with pockets of humid climates alternating with
arid ones within a few tens of
kilometers (Nicholson, 1996). At the regional scale, the MER and
the Afar depression which are
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16
located in the leeward side of both the summer and the spring
monsoons and they are characterized by
arid to semiarid climate owing to the capture of moisture by the
mountainous areas bordering them
from East and West.
Figure 1. Seasonal drifting of the ITCZ and its influence on
rainfall regime of Ethiopia. Histograms show monthly rainfall
distribution starting from January. The western most sector of
Ethiopia gets only the JJAS rainfall the central sector is
characterized by bimodal rainfall distribution getting the March
April rainfalls and the JJAS rainfalls with a break in May and
June. The eastern sector of Ethiopia gets its main rainfall from
March to May and in October. The figure in the right shows the
elevation map of Ethiopia and the mountains bordering the rift
valley and Afar. The lower graph indicates the different air flow
pattern over Africa in July and January. The east west arrow in
July circulation indicate the direction of the AEJ. The north south
dashed line (July) is the Congo convergence zone of the Indian and
the Atlantic Ocean monsoons. Figures from Telford (1998) and
Nicholson (1996).
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17
Figure 2. Ethiopian mean annual rainfall (above) in mm and mean
annual temperature map (below) in °C. The Afar rift and the MER get
lower amount of rainfall owing to capture of moistures from the
Indian Ocean and the Atlantic Ocean by the mountains bordering
them. The temperature is also higher in Afar and the MER. There is
no direct relation between rainfall amount and elevation in the
NWP. In the SEP and the Rift rainfall increases with elevation.
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18
The mountains also influence the local convective activities;
they influence the local rainfall
distribution and the timing of rainfall in the day. In the
Northern plateau clouds are formed at the end
of the morning because of evaporation and associated convection.
In Eritrean highland (just north of
Ethiopia) for example 80% of daily precipitation in summer
occurs between 12 and 16 hours (Krauer,
1988). The same diurnal distribution of rainfall is common in
the NWP particularly in September. This
convective nature of rainfall explains why rainfall amounts are
locally extremely variable in Ethiopia
(Nyssen et al., 2004) particularly in the NWP.
The configuration of the mountains also influences the spatial
variations in mean annual temperature.
The mean annual air temperature of the NWP is 16°C compared to
35°C in the Afar (figure 2). The
coldest temperature region is located in the arid mountains in
the NWP bordering Afar. The annual
rainfall in NWP ranges between 1000mm and 2000mm while in the
Afar it is less than 250 mm/year.
Rainfall and temperature in the MER is intermediate between the
NWP and the Afar.
Despite its location between the Sahel belt and the Equatorial
Africa there are some characteristics that
make the climate regime of the northern Ethiopia distinct.
Compared to similar latitude regions of
Sahelian Africa the Ethiopian region gets prolonged and higher
rainfall amounts due to orographic
enhancement. Furthermore, because of its proximity to the Indian
Ocean, the Ethiopian highland gets
part of its moisture from the Indian Ocean unlike the western
Sahel which gets its rain predominantly
from the Atlantic Ocean. The low-level westerly flows also
traverse a vast expanse of vegetated basin
in the Congo before they reach the Ethiopian highland. This
makes the Ethiopian highland to get part
of its moisture from continental sources.
Compared to the tropical eastern Africa, the NWP gets much of
its rainfall during the Sahel summer
(JJAS) and the little rains during March and April. The tropical
eastern Africa gets much of its
moisture from March to May and little rains from October to
December (Camberlin and Okoola, 2003).
The differences in rainfall distribution and amount among the
three sectors of Africa are associated to
the position of the ITCZ. The position of the ITCZ in turn
influences the exact location of the source
of moisture and the trajectories that moisture laden airs follow
before they reach these regions. In
addition to the low-level Atlantic Ocean monsoon and the Indian
Ocean monsoon, there are many
other airflow patterns at different altitude and from different
directions over the Sahel and East Africa.
These include the Tropical Easterly Jet and the African Easterly
Jet in the upper level1. The isotopic
composition of rains associated with the Indian and Atlantic
Ocean monsoons are relatively well-
studied (Rozanski et al., 1996; Taupin et al., 2000, HAPEX-Sahel
project
1 Following the East West Line bordering the ITCZ an east west
moisture flow exists in upper atmosphere. This east west zonal flow
is called the AEJ. In
sub tropics it is called the TEJ. It is to be noted that the TEJ
is different from the Indian Ocean monsoon which flow towards the
ITCZ at low levels.
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19
http://directory.eoportal.org/pres_HAPEXSAHEL.html) while the
Easterly Jets and their significance
in influencing the isotope regime of Sahel rainfalls were also
mentioned by some authors (eg. Joseph
et al., 1992; Hailemichael et al., 2002). The following section
of this thesis and Taupin et al. (2000)
show the easterly jets are less important in influencing the
isotopic composition of Ethiopian and Sahel
rains respectively. The influence of the Jets on airflow pattern
is beyond the scope of this thesis.
2. The isotope data
As the MER and the Afar contain numerous lakes and high
geothermal flux, they have been the
subjects of paleo-hydrological, paleo-climatological and
geothermal studies since the second half of
20th century. These studies have produced hydrogeochemical and
environmental isotope data. Recently
the International Atomic Energy Agency (IAEA) through its
Technical Cooperation (TC) projects is
conducting isotope hydrological studies in Ethiopian Rift and
Adjacent plateaus.
No previous stable isotope data has been apparently available
from the NWP until the recently
gathered and analyzed over 200 samples for δ18O, δD, δ13C, 3H
and hydrochemistry for this thesis. The
majority of the previously collected data is compiled and used
with the new data for analyzing the
relation between spatial isotope variations and climatic factors
that control these variations.
The isotopic composition of over 1000 groundwater wells, 60
rivers samples, 100 cold springs, 100
lakes, and 133 geothermal springs were compiled (CD included)
and used in the analyses of spatial
variations in terms of moisture sources and local climatic
conditions. Rainfall isotope data for Addis
Ababa station was downloaded from the IAEA/WMO/GNIP data base
(http://isohis.iaea.org) and were
analyzed to understand the relationship between temporal
(seasonal or long-term) variations in δ18O-
δD and climate variables.
Some of the previous works that were utilized to compile isotope
data include IAEA-TC-projects
(1996- an ongoing project) Gizaw (2002), Kebede et al. (2002a),
Kebede et al. (2002b), McKenzie et
al.( 2001) Beyene (2000) Ali (1999) Travi and Chernet (1998)
Chernet (1998) Ayenew (1998)
Rozanski et al. (1996) Darling (1996) Darling et al. (1996)
Fontes et al. (1980) Craig et al.( 1977)
Schoell and Faber (1976) Gonfiantini et al. (1973) UNDP (1973)
etc.
3. Results and Discussion
3.1. Seasonal variation in isotopic composition of Ethiopian
rainfall waters
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20
Isotopic composition (δ18O, δD and 3H) of rainfall has been
measured somewhat regularly at an
IAEA/WMO station at Addis Ababa (2300masl, 16°C mean annual
temperature, 1260mm/yr longterm
mean annual precipitation) since 1965. Short-term rainfall δ18O
and δD compositions were
occasionally measured at few other Ethiopian rift valley
stations.
The short-term δ18O and δD compositions of rainfalls from the
MER (see location map i) show a
similar pattern of seasonal variation and comparable values of
δ18O and δD composition to those of the
Addis Ababa rainfalls. Box 2 (Appendix 1) gives the tritium
content of the Addis Ababa rainfalls and
estimates the missing data.
There is a good relation between the seasonal variation in
isotopic composition of Ethiopian rainfalls
and the seasonality in Ethiopian climate. The δ18O and d-excess
of the Addis Ababa rainfalls and the
two-year rainfall isotope data from MER stations are
characterized by a notable seasonal variation
(though not as pronounced seasonal variation as in temperate and
high latitude regions) (figure 3).
The summer rainfalls waters are relatively depleted in δ18O and
they have higher d-excess than the
spring rainfalls reflecting typical characteristics of the Sahel
rains. The weighted average δ18O and δD
composition of the summer rainfall waters of the Addis Ababa
IAEA station is -2.5‰ in δ18O, -5‰ in
δD, and 15 in d-excess. The spring rainfalls have a weighted
mean composition of +1‰ in δ18O,
+20‰ in δD and 10 in d excess.
In a δD-δ18O plot (not shown) the monthly rains of Addis Ababa
is defined by the relation: δD =
7.2δ18O + 12. A similar plot on non-evaporated summer rains at
Addis Ababa has the relation: δD =
8δ18O + 152. The Maximum-Minimum-Average plot (figure 4) shows
that, the most depleted δ18O
were recorded in the dry season when the northerly dry and cold
winds reach the Ethiopian highland.
2 Since the summer moisture is the main water available for
runoff, recharge and lake inflows this work uses the relation δD
=8δ18O +15 as the
Local Meteroic Water Line (AAMWL). This is more meaningfull than
the LMWL constructed from the annual rains since the small rains do
not produce major runoff, recharge or lake inflows.
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21
-5
0
5
10
15
20
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
δ18 Ο
an
d d
-exc
ess
0
50
100
150
200
250
300
350
400
Ra
infa
ll in
mm
Rainfall in mm Awasa Sodo Agermariam Kofele Addis Ababa
d-excess
Figure 3. Monthly variation in mean δ18O and d-excess of the
Addis Ababa and the MER rainfalls. d excess plot is for Addis Ababa
station. Sources of data: IAEA GNIP data base of Addis Ababa
station (1965-1998) for Addis Ababa; IAEA-TC Projecs (1998 to 1999)
for the four stations in the MER.. Rainfall histogram is drawn from
Addis Ababa rainfall (1900-2000).
The depletion in the summer rainfall relative to the spring
rainfall is related to the difference in source
of moisture and to local meteorological processes. The summer
rainfall (75% of rainfall in Addis
Ababa) is derived from the admixture of the Atlantic Ocean and
the South/Equatorial Indian Ocean
air masses (figure 1). The small variability in δ18O of the
summer rains (figure 4) and section 3.3
suggest either nearly constant ratio of contribution of the two
sources over the last 40 years (which is
unlikely since the Ethiopian rainfall amount has varied at least
by ±20% during this time (Conway,
2000) while the interannual variation in isotopes nearly remain
constant) or that one of the two
monsoons is the predominant source for Addis Ababa summer rains.
However section 3.2 will show
that the isotopic composition of the summer rain are in
agreement with the meteorological evidence
which states the Congo basin and Atlantic are important sources
of moisture in summer.
-10
-8
-6
-4
-2
0
2
4
6
8
Jan Feb Mar Apr May June Jul Aug Sept Oct Nov Dec
δδ δδ18 0
Figure 4. Monthly Maximum-Minimum-Mean plot of the δ18O ‰ of the
Addis Ababa rainfalls (1965-2002). Highly variable δ18O content
is observed in June and October which marks the northward and
south ward passage of the ITCZ over central part of Ethiopia.
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22
The spring rainfalls are the most enriched compared to the
summer rains. During this time, the
oceanic moisture reaches the area from Northern Indian Ocean.
The enrichment of the rainfalls during
spring time may be related to three factors, a) as the Ethiopian
highland is geographically closer to the
North Indian Ocean and the moisture that reaches the area
represents the initial stage of condensation
which did not undergo major rainout fractionation effect (Joseph
et al, 1992); b) the high temperature,
the low atmospheric humidity and the low amount of rainfall
during this time favors evaporation of
rainwater leading to enriched rainfalls and low d-exces; c) the
high sea surface temperature over
northe indian ocean favors the formation of enriched vapor
coming to Ethiopia.
The dry season (November to February) is characterized by
relatively enriched and low d excess
compositions. The enriched δ18O and the low d excess
compositions of this period reflect evaporation
while raining owing to low rainfall during this time.
Unlike the other Tropical East Africa3 which shows small isotope
variation during dry seasons
(Rozanski, et al., 1996: Rietti-Shati, et al., 2000), the Addis
Ababa rainfalls show small variation
during July and August (see length of the line in figure 4).
The highest variability in δ18O is observed in October and June.
The most plausible explanation for
this variability is related to the northward and southward
passage of the ITCZ over central Ethiopia.
This creates variable local atmospheric condition. Strong
convection and high altitude condensation
with in the air column associated with the front of the ITCZ
while it is moving north results in the
most depleted δ18O. In June where the ITCZ is not yet
established, small local convection produces
enriched rains. Likewise, in October if the ITCZ is not yet
moved southward in the proceeding month
strong convection within the air column and associated cold air
mass from the Arabian continent
would favor formation of depleted rains. When it is already
moved southwards evaporation of rain
while falling or local convective clouds results in enriched
rains.
Seasonal variation in deuterium excess is also influenced by the
source of moisture and the local
conditions. High deuterium excess is recorded in the summer
rainfall and in September. The mean
weighted d-excess increases continuously from about 10 at the
onset of the North Indian Ocean
monsoon in March to 16 at the end of the summer monsoon. The
continuous increase in d-excess
through the summer may indicate the continuous increment in the
recycled component of the local
moisture. The low d excess in the dry season rains reflect
evaporation of rainwater while raining and
subsequent evaporative enrichment. The high d-excess in the
summer rains also reflect that the rains
during this time are less evaporated while raining owing to high
rainfall amount and the saturated
atmosphere.
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23
In September increase in δ18O is not accompanied by decrease in
d excess. Two superimposed
processes may result in this characteristic. While the
enrichment reflects more involvement of local
moisture as the ITCZ is moving southwards, the high d excess
suggests the influence of both recycling
and type of rain which is often solid form (afternoon hail
storm). In solid precipitation isotopic
disequilibrium may cause high d-excess (Gonfiantini et al.,
2001)4.
The same climate-isotope relation in Addis Ababa rainfalls
explains the pattern of the seasonal
variation in rainfall isotopic composition of the short-term MER
stations. Figure 3 shows that among
all the months September register nearly identical δ18O
composition in all the stations perhaps
reflecting that the rains were formed under similar rainfall
formation mechanisms and from similar
sources. Since the summer monsoon is retreating at this time and
local afternoon convective storms are
replacing it, the rains isotopic composition reflects local
convection and moisture source from local
recycled moisture.
The summer rainfall δ18O variation in Ababa mirrors the
variation in Sahel rains compositions
(enriched at the beginning and depleted at the end of the
summer). The later as reported by Taupin et
al. (2002). The West African rains in Cameroon have also similar
pattern of variation in summer
(Njitchoua et al., 1999). However a slight difference exists
between Addis Ababa rains and the West
African rains after the end of the main rainy season. In Addis
Ababa the southward migration of the
ITCZ pulls the dry and cold Arabian air which favors formation
of depleted rains in October. The most
depleted compositions are also recorded in October. In west
Sahel, the ITCZ after the rainy season
pulls dry but warm air locally called 'Harmatan' from the
Sahara. This produces enriched and low d
excess rains.
Other notable feature of seasonal variation in δ18O is that most
depleted compositions are observed in
October. This corresponds not to the amount of rain but to the
physical condition associated with the
southward migration of the ITCZ and the penetration cold air
from Arabian continent. Furthermore the
small rainy season (March- April) have high δ18O than the dry
season rains (between October and
February). This implies on seasonal basis rainfall amount is not
the only factor that influences the
isotopic composition of the rains.
3 the East African Rainfall stations include: Ndola, Dar es
Salam, Kampala, Harare, Antananarivo, Entebbe 4'Ice formation, if
occurring, is supposed to take place by freezing the water droplets
without affecting the isotopiccomposition.However, the isotopic
fractionation in the subsequent vapour condensation on the ice
surface, deviates from the equilibrium value because the light
molecules H216O may be privileged for their higher diffusivity in
air. This effect tends to offset the thermodynamic equilibrium by
which the isotopically heavy molecules are preferentially fixed in
condensed phases, and may determine a significant increase of the
deuterium excess, because of the relatively small difference in
diffusivity coefficients between HD16O and H218O.' Gonfiantini et
al., 2001.
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24
3.2. Origin of δ18O-δD of Ethiopian meteoric waters and its
comparison with Sahel and East
Africa
As previously noted (Sonntag et al., 1979: Joseph et al., 1992;
Rozanski et al., 1996; Darling and
Gizaw, 2002) and as newly observed (part III and IV) the
Ethiopian rainfall waters (and pristine
meteoric waters in general) are somewhat 'unique' in their
isotopic compositions compared to the
Sahel and the East African rainfalls (meteoric waters)
compositions. The major observations are:
1) Despite the low mean annual temperature and the high altitude
location of Ethiopia, the
weighted mean annual isotopic composition of Addis Ababa
rainfalls does not show depletion
compared to other East African rainfalls (Rozanski et al.,
1996).
2) Groundwaters (or rains ) in Ethiopia are enriched compared
with the western Sahel (Joseph et
al., 1992).
3) There is an imbalance between the mean isotopic composition
of rainfalls and the isotopic
compositions of groundwaters in Ethiopia while in other East
African region the two show
comparable compositions1 (Darling and Gizaw, 2002; Gizaw,
2002).
4) The Ethiopian plateau groundwaters do not show altitude
commensurate depletion despite the
high altitude location of the region compared to the Sahel (part
III this work). However the
general pattern of seasonal variation in δ18O of summer rains is
similar in both regions.
5) The Addis Ababa summer rainfalls particularly the rainfalls
of the month of September (just
after the retreat of the summer monsoon) contain the highest
d-excess and relatively enriched
δ18O.
6) The general pattern of JJAS variation in δ18O and δD (figure
3) resembles that of the Sahel
rainfalls than the East African rains. The seasonal isotopic
variation of Sahel west Africa was
documented by various authors including Taupin, et al., 2002;
Taupin, 2000. Unlike the other
East African stations which show small isotope variability
during the dry seasons (Rozanski et
al., 1996), the Addis Ababa station shows small isotope
variability during main rainy season in
summer (figure 4).
7) Ethiopian Lakes are more enriched than other East African
Lakes of comparable
Evaporation/Inflow ratios (Part III).
At least three major hypotheses (original hypotheses by: Sonntag
et al., 1979 or Rozanski et al., 1996;
Joseph et al., 1992 and Darling and Gizaw, 2002) exist to
explain the enriched isotopic characteristics
of the Addis Ababa rainfall compared to the Eastern and Sahelian
Africa. These are:
1) Influence of the Congo vegetation or continental open water
bodies: Earlier (Sonntag et al.,
1979) suggested that moisture advection from transpiration by
the Congo vegetation basin
could influence the isotopic composition of rainfalls in north
and northeast of the basin.
Rozanski et al. (1996) relates the enriched δ18O of the December
to May rainfalls of Addis
Ababa to the mixing of transpired moisture from the Congo
vegetated basin to the December
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25
to May rain bearing moisture. Since transpiration is a
non-fractionating process it returns
enriched moisture to the atmosphere making the Ethiopian rains
enriched.
While meteorological observations supports the mixing of vapor
from the Congo vegetation
and other continental water bodies (see section 1) to the
moisture laden westerly air coming
from Atlantic to Ethiopia in JJAS, how it mix with the maritime
moisture is not clear. It is not
clear also how the vapor from Congo vegetation basin influences
the December to May
Ethiopian rains. Between December and May the ITCZ is still in
southern Ethiopia and
Central Ethiopia is getting its moisture from the Indian
Ocean.
2) Influence of the North Indian Ocean via the TEJ/AEJ in the
summer rainfalls: Based on
isotopic composition of groundwaters along the Sahelian Africa
starting from Djibouti to
Senegal, Joseph et al., 1992 hypothesized that the African
Easterly Jet (AEJ) and the Tropical
Easterly Jet (TEJ) may transport an important amount of moisture
from Indian Ocean or from
the Arabian see westward across Sahel Africa in summer. They
observed that the
groundwaters in Western Shaelian African are more depleted in
δ18O than the groundwaters
and rainfall waters of Eastern African regions (Djibouti and
Ethiopian Highlands). According
to this hypothesis both the March-April (from Indian Ocean
monsoon) and the JJAS rainfalls
(from the Zonal flows) over Ethiopian highland represents the
first condensations stage of the
Indian Ocean or Arabian Sea moisture. This makes the Ethiopian
meteoric waters enriched
compared to meteoric waters of the West Sahelian Africa. The
latter receives rains from the
Zonal flows at the end of their condensation stage.
The question that follow this hypotheses are a) How far is the
sampled groundwaters
representative of the continental scale meteorological
processes? Local evaporation effect
prior to recharge seems for example an important hydrological
process in Djibouti and the
Afar Depression (the areas from where Joseph et al., have taken
groundwater samples) making
the Ethiopian groundwaters enriched; and, b)does the generally
known meteorologically based
monsoon flow patterns and the ITCZ drifts support this idea? It
is widely agreed that (at least
in Ethiopia) the summer monsoon comes from the Atlantic or the
Congo basin or from the
Southern Indian Ocean than from the Indian Ocean alone.
Furthermore recent closer
monitoring of event based rainfall isotope monitoring (Taupin et
al., 2000) shows the Indian
Ocean moisture is unimportant in the Sahel rains.
The next section of this thesis will show a clear West to East
flow of the summer monsoon
over the Ethiopian Plateau facing west as demonstrated by
continuous depletion of heavy
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26
isotopes eastward on the NWP. These exclude the importance of
the Easterly Jets, which
should normally deplete westward, as important moisture source
in Ethiopia.
3) The Addis Ababa anomaly?: Although the state of sampling
condition is not directly implied
as a cause of the isotopic characteristics of the Addis Ababa
rainfall, a recent work by Darling
and Gizaw (2002) shows that there is an imbalance in the
weighted rainfall isotopic
composition and the isotopic composition of 'unmodified'
groundwaters in Ethiopia.
According to these authors, in all stations in Eastern Africa,
except Addis Ababa, the
comparison of the weighted mean annual rainfall isotopic
composition with that of unmodified
groundwaters shows a comparable range5.
However, if a comparison were made between the weighted mean
summer rainfall (the rainfall
which is available for recharge and runoff in the NWP) isotopic
composition (δ18O = -2.5‰
and δD = -5 ‰) and groundwater isotopic compositions of the
Ethiopian plateau the problem
of 'imbalance' should not have existed. As will be demonstrated
in Part III, the Ethiopian
Lakes are enriched than other East African Lakes of similar
evaporation to inflow ratios
reflecting a general enrichment of the Ethiopian meteoric waters
feeding them. This shows
again the relative enrichment of Ethiopian meteoric waters.
Based on the new and the previous observations and the new
questions, the following lines of
approach and hypotheses can be used/made about the origin of the
isotopic composition of the
meteoric waters of the Ethiopian highland.
Approaches
• Observations have still to be improved. Comparisons between
the three sectors were often
made on short isotope records or on few water samples.
5
A.
B
-60
-40
-20
0
20
40
60
80
100
-10 -8 -6 -4 -2 0 2 4 6 8 10
δδδδ18O
δδ δδD
Low TDS cold groundwaters high TDS Na-HCO3 waters from YTVL and
LTG Lakes and rivers draining them GMWL
*
Ave rage Summer Rainfal lat Addis Ababa
*Ave rage March-Apri l Rainfal lat Addis Ababa
Figure A (Darling and Gizaw, 2002 and Gizaw, 2002) shows
imbalance between groundwater and annual average δ18O and δD of
rains figure B (part II this work) shows the groundwaters from NWP
plots around the average summer rains. The imbalance between rains
and groundwater composition occurs therefore only if the average
isotopic compositions of annual
rain are plotted against isotopic composition of
groundwaters.
-
27
• The rainfall isotopic composition of the Ethiopian highland
and that of eastern Africa should
be compared on seasonal basis than on the mean annual rains as
the two regions are not
always influenced by similar moisture trajectories. The offset
in the main rainy seasons
between the Northern Ethiopian highland and other east African
region indicate the two
regions are influenced by different moisture trajectories.
Therefore the difference in the mean
annual rainfall δ18O compositions of rains of the two sectors
should be compared not in terms
of condensation history but in terms of the kind of land surface
(mountains, open water
bodies, vegetation, desert etc) on the moisture pathways and in
terms of evaporation
conditions at source. If the effects of condensation along
moisture trajectory were to be
compared between Ethiopian and the East African stations one has
to do the comparison on
the March-April rains which are in phase in the two regions.
• Likewise, if comparison were to be made between the Sahel and
the Ethiopian meteoric
waters to understand the history of moisture trajectory, it
should be made on the isotopic
composition of the summer rains or on the isotopic composition
of pristine groundwaters
recharged out of the summer rains rather than on the mean annual
rains isotopic compositions.
Furthermore the differences in the nature of rainfall formation
(monsoonal, convective,
orographic, and frontal) along the different part of the Sahel
should be considered.
Hypotheses
• The Ethiopian March April rains are enriched than the eastern
African equivalent because
the former rains represent the initial stage of condensation for
the moist easterly flows from
the Indian Ocean. One would for example see the March-April
rainfalls in Kampala are more
depleted than the March-April Addis Ababa rains (Rozanski, et
al., 1996) showing northeast-
southwest depletion of δ18O along moisture trajectory.
Furthermore the Ethiopian region gets its rain from northern
part of the Indian Ocean while the
East African stations get their rain from southern and
equatorial Indian Ocean. Sea surface
temperature difference on the Indian Ocean could result in
differences in isotopic composition
of the easterly moistures. At Addis Ababa the March-April months
are preceded by long dry
period compared to the east African stations, which have wet
seasons in October and
December. Evaporative effect and isotopic exchange with the dry
atmosphere enriches the
Addis Ababa March-April rains compared to East African
stations.
• The fact that the Ethiopian meteoric waters do not show
altitude-commensurate depletion
compared to the Sahel meteoric waters can be related to a
variety of factors. These include a)
part of the summer rainfall in Ethiopia is derived from the
Indian Ocean monsoon which did
not undergo previous condensation while the influence of Indian
Ocean is minimal in western
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28
Africa; b) as already shown by meteorological evidences and as
hypothesized by Sonntag et
al., 1979; part of the summer monsoon in Ethiopia is fed by
continental open water bodies
and the vapor from Congo vegetation basin. The preceeding
discussions favor the second
factor.
• Since Ethiopia is uplifted compared to Sahel Africa,
orographic effect can play an important
role in lifting up the enriched ground level moisture derived
from local evapo-transpiration or
from the low-level westerly flows from Congo. This enriches the
maritime monsoon air mass
(Atlantic/Indian) and therefore the summer rains of Ethiopia.
The elevated terrain in Ethiopia
and the heating of the plateau can trigger and maintain local
convective activity.
The apparent lack of altitude commensurate depletion of the
meteoric waters of northern and
central Ethiopia compared to the Sahel could be therefore partly
the result of the maintenance
of the convection of ground level enriched/recycled moisture due
to orography. Enrichment of
local convective rains resulting from mixing of ground level
vapor is not uncommon in
western Sahel (Taupin et al., 2002). However, in the Sahel, this
type of rains happens
temporarily when the ITCZ fails to move north promoting local
convection and storms
(Taupin et al., 2002).
In this sense it can be hypothesized that in tropical warm
mountains, topography can play two
opposing roles. While the decrease in temperature with altitude
(and reduction of evaporation
effect) leads to depletion of the heavy water isotopes, the
lifting up of enriched ground level
moisture from evapo-transpiration by mountains enriches the air
mass in heavy isotopes. The
isotope altitude effect or the continentality effect is
therefore the balance between the two.
The lack of strong depletion of isotopes despite high altitude
(or with altitude) is not only
restricted to the Ethiopian mountains. Similar pattern of
isotope distribution in tropical
mountains is observed in Tibetan Plateau (Zhang et al., 2004;
Sugimoto personal
communication, IAEA groundwater conference 2003), and the Kenyan
mountains (Rietti-Shati
et al., 2000). The latter attributes the lack of strong
'altitude effect' and the enrichment of
meteoric waters at high altitude in Kenya to the influence of
recycling of local moisture.
Zhang et al. (2004) attributes the lack of isotope
continentality effect north of Himalayas to the
contribution of locally produced vapor in the mountainous
region.
The role of orographic convection or orographic clouds was not a
widely pronounced issue in
the literature. Recently Liotta, et al. (2004) demonstrated how
orography plays an important
role in changing the original isotopic mark (particularly d
excess) of precipitation in Sicily.
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29
One of the evidences for existence of the influence of the local
(or regional) continental
recycled moisture in the Ethiopian meteoric waters is the
d-excess composition of the Addis
Ababa rainfalls and that of the groundwaters of the NWP. High
deuterium excess is often
attributed to land surface-atmosphere interaction via moisture
contribution from evapo-
transpiration (Rietti-Shati et al., 2000; Gat et al., 1994).
Figure 5 shows how this process
produces high d-excess (and enriched δ18O). The Ethiopian
rainfalls (at Addis Ababa) are
characterized by high deuterium excess (>10) ranging from 10
in pre-summer monsoon and
increases up to 16 during the retreat of monsoon. The continuous
increase in d-excess starting
from the onset of rainfall in March up to the end of monsoon in
September may be partly
related to the continuous increase in the volume of locally
recycled moisture in the rainfalls.
Figure 5 (modified from Gat et al., 1994). The isotopic
composition of evaporated surface water (δw), the original water
body, soil moisture or leaf-water prior to evaporation (δp), and
the evaporated water vapor (δE) all plot along the same line called
the local evaporation line. The meteoric water that forms from the
condensation (δp) of the 'monsoon only' moisture (δ a) is separated
from δa by the enrichment factor (ε*). When the local evaporate
(δE) mixes with the monsoon vapor (δa) due to orographic lifting of
the evaporate. A new enriched vapor (δa') that plot in triangular
zone bounded by the evaporation line, the MWL and above the MWL is
formed. The rains that condense from this new vapor plot along a
new line parallel to the MWL but with a higher d-excess and
enriched δ18O rainfalls. The composition of the new vapor δa'
depends on the degree of mixing between the monsoon moisture and
the 'local convective moisture'. Higher amount of local moisture
produces isotopically enriched rainfall with high d excess.
Generally it can be said that while differences in sources of
moisture make the Ethiopian meteoric
waters to have different isotopic composition as compared to
eastern Africa; Orographically triggered
convection of local or low level moisture from Congo basin and
the Atlantic enriches the Ethiopian
summer moisture mass.
3.3. Independent geochemical evidence on source of moisture
Often as a complementary source of information in understanding
sources of moisture, rainfall
chemical composition was used in Western Africa (eg. Savenije ,
1995). The assumption is that
salinity decrease that is observed in inward continental regions
(and during the rainy season) compared
to the coastal regions (the start of rainy season) is cause by
continuous recycling of moisture along
moisture trajectory. This assumption however sounds simplistic
because the salinity decrease in
rainfall may be caused by wash out and continuous cleaning of
the atmosphere starting from the onset
of rain bearing system through its development and end. It
appears that elemental ratios such as Na/Cl,
-
30
or ratios of other elements tell more about changes in moisture
sources than salinity used alone.
Appendix I-2 discusses the chemical composition of weekly
monitored rainfall of the summer 2003
rainfalls of Addis Ababa.
The rainfall chemistry (box 1 in Appendix I) measured at Addis
Ababa shows a continuous decrease in
the salinity and all major ions starting from the onset of the
summer rainfall. It starts to increase again
around the end of the monsoon. As to what factor (a continuous
clean up of the atmosphere or a
continuous recycling of moisture) this is related is not clear.
However the Na/Cl of the September
rainfalls is different from the other summer month's ratio
reflecting differences in sources of moisture
and the nature of rainfall formation. There is a likelihood of
mixing of near surface moisture into
raising air mass during strong September afternoon convection.
The isotopic composition of
September rainfall is also relatively enriched and has the
highest d-excess reflecting isotope
disequilibrium during raindrop formation.
Comparison between the Ethiopian summer rainfalls and the summer
rains from Sahelian West Africa
(Goni et al., 2001) shows that the former are dilute in all ions
but keeps similar Na/Cl ratio. This
suggests similarity in sources of moisture for the two regions
but different trajectories over which the
moistures pass before they reach the regions. The furthest
distance of Ethiopia from the Atlantic could
explain the relatively dilute salinity of the Addis Ababa rains
compared to the West Sahel rains.
3.4. Long-term variation in isotopic composition of
rainfalls
This part attempts to show the existence of long-term trends in
δ18O and d-excess of the Addis Ababa
rainfall (Figure 6 and Figure 7). To see the long term, trend
analyses is made on seasonal basis. The
year is divided into four parts: the dry season (September to
February), the spring rainfalls (March and
April), the summer rainfall (June to September) and the month of
May (the transition between the
spring and the summer rains). Some notable features from the
figures are:
• The regular pattern of variation in δ18O and d-excess of the
Summer rainfall (with a slight
decreasing trend in the d-excess)
• Quasi- regular to regular pattern of δ18O and d-excess of
spring rainfall with no major
increasing or decreasing trend except some points that fall out
of the major trend.
• Highly irregular pattern of δ18O and d excess of the dry
season rains
• Enriched δ18O and low d-excess summer rains centered over the
mid 1980s, a period of lowest
rainfall in Ethiopia (the enrichment is related to local
evaporation effect due to low rainfall)
• Generally a decreasing trend in d excess of summer
rainfalls
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31
-8
-6
-4
-2
0
2
4
6
8
j-65
j-67
j-69
j-71
j-73
j-75
j-77
j-79
j-81
j-83
j-85
j-87
j-89
j-91
j-93
j-95
j-97
j-99
j-01
δδ δδ18 O
Automn Spring May Summer
Figure 6. Long-term variation in amount weighted δ18O of Addis
Ababa precipitation.
-10
-5
0
5
10
15
20
jan
v-6
5
jan
v-6
7
jan
v-6
9
jan
v-7
1
jan
v-7
3
jan
v-7
5
jan
v-7
7
jan
v-7
9
jan
v-8
1
jan
v-8
3
jan
v-8
5
jan
v-8
7
jan
v-8
9
jan
v-9
1
jan
v-9
3
jan
v-9
5
jan
v-9
7
jan
v-9
9
jan
v-0
1
D e
xces
s in
spr
ing
and
sum
mer
ra
infa
lls
Summer Rainfall Spring Rainfall
Trend in d-excess of summer rainfall Trend in d-excess of spring
rainfall
Figure 7. Long-term variation in deuterium excess of the Addis
Ababa rainfall.
These observations lead to the following general remarks:
• The trend in δ18O of the summer rainfalls are dominantly
linked to global or continental scale
processes with minor effect of local scale processes. The
absence of strong inter-annual
variability with in the summer or the spring rains may also
imply the persistence of source of
moisture over the last five decades. Since the rainfall has been
changing during the last
decades the persistence reflects that the summer monsoon has
predominantly one source rather
than two or more sources mixing at sub-equal proportion. Had
there been two or more sources
of summer moisture in Addis rainfalls the isotope trend would
have been likely irregular.
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32
• The irregular pattern of the dry season rainfalls isotopic
compositions are influenced by local
climatic factors during rainfall (such as the time of the day
where rainfall takes place, the
rainfall intensity, the temperature etc).
• The slight decrease in deuterium excess irrespective of nearly
constant δ18O over the last sixty
years may indicate the decrease in the percentage of the
contribution of recycled continental
moisture to the maritime air mass because of changes in land
surface characteristics such as
massive deforestation over Ethiopian highland (Ethiopian forest
cover diminished from 47%
of the land cover to 3% in the last 50 years). The isotope data
do not allow conclusive remarks
because over the recent years analytical techniques have
improved and the variations in
isotopic composition may also be related to improvement in
precisions in measurements.
3.5. Spatial variation in isotopic composition of meteoric
waters
The altitude, the temperature and the amount effects
On the windward face of a mountain, the δ18O and δD composition
of rainfalls decrease with
increasing altitude. This phenomenon is termed as the 'altitude
effect'. The altitudinal variation of
isotopic composition provides a suitable basis to trace source
of groundwater recharge. A moisture
mass that ascends a barrier results is fractionation of isotopes
leading to a depleted composition at
higher grounds.
Because of complexities in topography and circulation of
rainfall bearing moistures, and complexity in
local convective activities getting a single isotope altitude
gradient for Ethiopia sounds imprecise.
In the MER and the Afar Depression, for example rain-producing
moisture descends into the lowlands
from the adjacent highlands. Although in some localities of the
Rift and the Afar depletion of isotopic
composition with altitude is observed (eg. McKenzie et al.,
2001; Gizaw, 2003), the cause of such
depletion is not the traditional/conventional altitude effect.
The depletion of δ18O with altitude in a
leeward side of mountains, if present is often the outcome of
the pseudo altitude effect which is
equivalent to Foehn effect in meteorology. The pseudo altitude
effects are often the result of high
temperature in the leeward side of mountains and the likelihood
of evaporation that causes isotope
fractionation of rainfalls6.
In regions where rainfall isotopic record is not available
across an altitude gradient, low TDS or low
chloride or low temperature and modern groundwaters can be used
to calibrate the altitude isotope
relations.
6The implication of this in isotope groundwater tracing is worth
noting. Occasionally the moisture coming from the windward
direction a mountain passes into the lee-ward side leading to
occasional heavy rainfalls and flooding. This kind of
meteorological processes were observed in Afar and the Ethiopian
rift valley several time (Gemechu, 1977). This kind of strong
monsoon flow may lead to formation of depleted rainfall in a region
otherwise should receive enriched moisture. The occasional flood
forming monsoon events which are able to pass over the mountain
barriers may result in isotopically depleted groundwater in Afar
and Djibouti. This kind groundwater recharge sources have been
previously noted by Fontes et al. (1980) in Djibouti.
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33
Figure 8 uses groundwaters as proxies of rainfall isotopic
composition. The figure shows on the NWP
there is a -0.1 ‰ depletion of δ18O and in the MER the Afar in
general there is a δ18O depletion of -
0.14 ‰ per 100m. The -0.1‰ depletion in δ18O in windward face of
the NWP is consistent with many
tropical mountains including Cameroon (Njitchoua et al.,1999)
mount Kenya Rietti-Shati, et al. 2000),
Costa Rica (Lachniet and Patterson, 2000). A recently
proposed