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\begin{document}

\title{Understanding Temporal and Spatial Variation of Stable Water Isotopes in
Precipitation over Upper Blue Nile River Basin}



\author[1]{Bisrat Cholo}%
\author[2]{Seifu Kebede}%
\author[3]{Kei Yoshimura}%
\author[4]{Tadesse  Kenea }%
\affil[1]{Arba Minch Water Technology Institute}%
\affil[2]{College of Natural and Computational Sciences}%
\affil[3]{University of Tokyo}%
\affil[4]{Arba Minch University}%


\vspace{-1em}



  
  \date{May 23, 2024}


\begingroup
\let\center\flushleft
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\selectlanguage{english}
\begin{abstract}
Stable isotopes in precipitation and vapor are a powerful tool for
tracing the origin of moisture and mixing processes. This paper
discusses time and space variation of \selectlanguage{greek}δ\selectlanguage{english}18O in precipitation and
controlling features over upper Blue Nile Basin using data from GNIP,
observed data in 2014 and simulated data by AGCM. IsoGSM simulation in
precipitation was verified with observation. The \selectlanguage{greek}δ\selectlanguage{english}18O variation shows
clear seasonality with the lowest 18O values in August and dry season,
and enriched 18O in spring, June and September. Spring sample is
enriched compared to summer, and assumed to be related with moisture
sources. More enriched isotopes in spring and lower d-excess could be
related to the source of air masses in short travel path from North
Indian Ocean, Mediterranean and Red sea while summer rain is depleted
with larger d-excess could be related to longer travel path of moisture
from south Indian Ocean with mixing of potential evaporated moisture
from open surface and transpired moisture from Congo vegetation and also
from Gulf of Guinea. The isotopic statistics of three stations shows
maximum, minimum and average value of (8.23%0, -11.73%0, 0.04%0) in Addis
Ababa, (5.26%0, -12.74%0, and -2.52%0) in Entoto Hill and (4.08%0, -9.65%0,
2.41%0) in Debremarkos respectively. The \selectlanguage{greek}δ\selectlanguage{english}2H- \selectlanguage{greek}δ\selectlanguage{english}18O relationships, monthly
weighted d-excess variation in the Basin revealed the temporal variation
of \selectlanguage{greek}δ\selectlanguage{english}18O in precipitation is essentially shaped by the source of the
moisture and spatial differences is due to Rayleigh rainout effect along
the moisture trajectory. The source of moisture is primarily controlled
by the north south movement of ITCZ within the Basin. The study
recommends the use of model simulated \selectlanguage{greek}δ\selectlanguage{english}18O as good alternative for
hydrological and hydrologeological investigations when needed.%
\end{abstract}%



\sloppy


\textbf{Understanding Temporal and Spatial Variation of Stable Water
Isotopes in Precipitation over Upper Blue Nile River Basin}

Bisrat Elias Cholo*, \textsuperscript{2}, Seifu Kebede
Gurmesa\textsuperscript{1}, Kei Yoshimura\textsuperscript{3, 4}and
Tadesse Tujuba Kenea\textsuperscript{, 2}

\textsuperscript{1}School of Earth Sciences, Addis Ababa University,
Addis Ababa, Ethiopia

\textsuperscript{2}Faculty of Meteorology and Hydrology, Water
Technology Institute, Arba Minch University, Arba Minch, Ethiopia

\textsuperscript{3}Institute of Industrial Science, University of Tokyo,
Tokyo, Japan

\textsuperscript{4}Atmosphere and Ocean Research Institute, University
of Tokyo, Kashiwa, Japan

\textbf{Corresponding Author} \emph{: Bisrat Elias Cholo, Arba Minch,
Ethiopia email: bisratelias27@gmail.com}

\emph{Abstract}

Stable isotopes in precipitation and vapor are a powerful tool for
tracing the origin of moisture and mixing processes. This paper
discusses time and space variation of \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O in
precipitation and controlling features over upper Blue Nile Basin using
data from GNIP, observed data in 2014 and simulated data by AGCM. IsoGSM
simulation in precipitation was verified with observation. The
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O variation shows clear seasonality with the
lowest\textsuperscript{18}O values in August and dry season, and
enriched\textsuperscript{18}O in spring, June and September. Spring
sample is enriched compared to summer, and assumed to be related with
moisture sources. More enriched isotopes in spring and lower d-excess
could be related to the source of air masses in short travel path from
North Indian Ocean, Mediterranean and Red sea while summer rain is
depleted with larger d-excess could be related to longer travel path of
moisture from south Indian Ocean with mixing of potential evaporated
moisture from open surface and transpired moisture from Congo vegetation
and also from Gulf of Guinea. The isotopic statistics of three stations
shows maximum, minimum and average value of (8.23%0, -11.73%0, 0.04%0) in
Addis Ababa, (5.26%0, -12.74%0, and -2.52%0) in Entoto Hill and (4.08%0,
-9.65%0, 2.41%0) in Debremarkos respectively. The \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H-
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O relationships, monthly weighted d-excess
variation in the Basin revealed the temporal variation of
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O in precipitation is essentially shaped by the
source of the moisture and spatial differences is due to Rayleigh
rainout effect along the moisture trajectory. The source of moisture is
primarily controlled by the north south movement of ITCZ within the
Basin. The study recommends the use of model simulated
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O as good alternative for hydrological and
hydrologeological investigations when needed.

Keywords: IsoGSM; Deuterium Excess; Upper Blue Nile;
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H, \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O, moisture source

\textbf{Introduction}

The stable water isotopes (\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O and
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H) are important tracers of the source of moisture
and mixing processes (Bonne et al., 2014; Dansgaard, 1964; Delattre et
al., 2015; Kebede and Travi 2012; Yoshimura et al., 2004), largely used
to reconstruct continental palaeoclimates (Joussaume et al., 1984;
Dansgaard et al., 1969) and used as a natural tracer for hydrological
cycles (Dansgaard, 1964; Jouzel, 2003). Oxygen isotopes
(\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O) in precipitation are strongly influenced by the
water source and trajectory history and its variation could be used to
reconstruct the past atmospheric circulation (Burnett et al., 2004;
Okazaki et al., 2015). The knowledge of spatial and temporal
distribution of the stable water isotopes (\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O and
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H) in a big river Basin is useful for investigating
hydrological, hydrogeological and paleoclimate information.

Recently, numerous hydrological and hydrogelogical applications rely on
water isotope tracers to find out the geospatial origin of water, the
type of geological materials, the mixing of geological materials with
water. However, mapping the isotopic distribution of rainwater and/or
surface water large River Basins are difficult due to lack of finer
resolution data in space and time. The monthly weighted isotopic data
from some GNIP stations only is an accessible for different isotopic
studies now. Addis Ababa station is the only IAEA/WMO station with
monthly weighted isotopic data of precipitation near to the upper Blue
Nile River Basin. Other stations have only short -term rainfall isotopes
were occasionally measured. Identifying isotopes in meteoric water of
large spatial coverage in the world and big river Basin like the upper
Blue Nile River is difficult. A simple interpolation or statistical
regression methods was used in some areas (e.g. Bowen, 2003) to estimate
isotopic distributions from the available information to understand the
spatial information.

Mapping and modeling actual isotopes in meteoric waters (and
paleoclimate) is useful for hydrological studies due to the growing
ability and potential of modeling isotopic GCMs and the increasing
availability of data. Recently, there is a focus that reflects a growing
appreciation of unique information that water isotope data contribute to
the analysis and understanding of hydrological cycle which is difficult
in other techniques. Simulation of isotopes in precipitation by isotopic
GCMs was first emerged during 1980s by coarse version of models
(Joussaume et al., 1984; Jouzel et al., 1987) and subsequent
developments have included sensitivity studies and simulations of
isotopes. A variety of current isotope incorporated GCMs (Hoffmann et
al., 1998; Mathieu et al., 2002; Noone and Simmonds, 2002; Schmidt et
al., 2005; Yoshimura et al., 2008; Tindall et al., 2009; Risi et al.,
2010) incorporate the complex fractionation processes during water phase
changes in large scale air masses transport and provide three
dimensional isotopic distributions which could be a good alternative to
understand the spatial and temporal distribution of isotopes in large
river Basins. However, AGCMs (which are only forced by the observed sea
surface temperature) may not reproduce short term isotopic variability
and limited spatial resolution and the imperfect representation of
atmospheric circulation (Hoffmann et al., 2000).

The deuterium excess in water, defined as \selectlanguage{greek}δ\selectlanguage{english}D-8*\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O
(Dansgaard 1964) is able to study and understand water movement in the
atmosphere--biosphere--lithosphere interfaces (Gat et al., 2000; Gat and
Airey, 2006;), diagnostic tool for recognizing the contribution of
evaporated moisture into the atmospheric water (Cui et al., 2009; Gat
and Matsui, 1991; Ingraham and Taylor, 1991, Gat et al., 1994) and used
to identify the origin of vapor source in precipitation (Rozanski et
al., 1992). It can fingerprint evaporation more reliably than the
independent stable water isotopes (\selectlanguage{greek}δ\selectlanguage{english}D and \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O)
variation as the second may sometimes lead to wrong conclusion that no
evaporation has occurred (Barnes and Turner, 2000). Accumulation of
recycled moisture into atmospheric water through evapotranspiration adds
a considerable role in the hydrological cycle (Matsui et al., 1983;
Rindsberger et al., 1983). The mixing of water from open surface water
evaporation and transpired moisture from plants in the atmospheric air
masses characterizes with different isotopic nature. Transpiration
returns the precipitated waters essentially unfractionated to the
atmosphere that does not change in the d- excess value and resulting
atmospheric water becomes enriched in heavy isotopes and evaporation
from an open water surface fractionates the isotopes of hydrogen and
oxygen will result in a higher d-excess value than 10%0 (Cui et al.,
2009; Dansgaard 1964; Gat et al., 1994). The d-excess values for
continental precipitation events issued from oceanic moisture are close
to 10%0 (Craig, 1961; Dansgaard, 1964). Variation of isotopic composition
of Meteoric water depends on fractionation during phase changes from
vapor to liquid or ice and vice versa (Dansgaard, 1964). The continental
loss of moisture content by condensation of vapor during advection of
air mass as it moves away from its source is dominant factors of
fractionation in hydrological cycle (Yurtsever, 1975). However, the
influence of mixing of evaporated water from open water bodies and/or
transpired moisture from plants (Gat and Matsui, 1991), and storm
trajectory (Friedman et al., 1992; Lawrence et al., 1982) was
demonstrated to change isotopic signature of precipitation events. The
seasonal oscillation of \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O and \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H
in precipitation reflects the evaporative effect in rain droplets
enhanced by meteorological conditions, the rain out effect through the
moisture trajectory or the mixing of different air masses in moisture
trajectories. The variations of \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H and
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O of Meteoric water have been explained with the
Global Meteoric Water Line since (Craig 1961).

Global distribution of mean annual \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O interpolated
from Global Network of Isotopes in precipitation (GNIP) stations (Bowen
and Wilikinson, 2002) shows more enriched \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O in
tropics and particularly in east Africa and Ethiopia. The high
enrichment of \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O in rainwater and the larger
d-excess value in Addis Ababa first recognized by (Dansgaard, 1964) and
related it with partial recycling of rain droplets and this irregular
nature of stable water isotopes in Addis Ababa precipitation was also
acknowledged by (Sonntag et al., 1979; Joseph et al., 1992; Rozanski et
al., 1993; Darling and Gizaw, 2002; Hailemichael et al., 2002; Levin et
al., 2009). The reason of high enrichment of precipitation isotopes is
due to the effect of contribution of continental moisture in the sources
of rainfall trajectory and which could be from the Congo vegetation
(Sonntag et al., 1979; Rozanski et al., 1993; Kebede 2004 and Levin et
al., 2009). More recently, Kebede and Travi 2012 broadly disclosed that
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O anomaly is a real anomaly which is attached to
sources of moisture and local meteorological processes with widespread
data sets of meteoric waters in Ethiopia.

This study describes the temporal variation of stable water isotopes
(\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H and \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O) in precipitation,
their spatial variability signal and controlling features of their
variation with time over upper Blue Nile Basin of Ethiopia. The stable
water isotopes (\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H and \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O) data
from observed precipitation isotopes in the upper Blue Nile River Basin,
Global monitoring data of water isotopes from Global Network for
Isotopes in Precipitation (GNIP) in Addis Ababa stations and
Isotope-incorporated Global Spectral Model (IsoGSM) (Yoshimura et al.,
2008) simulations in observational stations was used.

Precipitation samples were collected in three stations (Addis Ababa,
Entoto Hill and Debremarkos) and analyzed by Liquid Water Isotope
Analyzer in the school of Earth Sciences laboratory of Addis Ababa
University, Ethiopia and the GNIP data of IAEA/WMO station rainfall
isotope record at Addis Ababa (2360 masl) since 1961 has been used. The
IsoGSM was dynamically downscaled and interpolated using 0.125\selectlanguage{ngerman}° spatial
resolution and a forcing time step of 6 hours in the upper Blue Nile
River Basin. The 6 hourly simulations of precipitation
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O in the grid point of two observational stations
(Addis Ababa and Debremarkos) in the year 2014 were converted to daily
and monthly \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O. The daily and monthly variability of
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O, the scatter relationship between
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H and \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O relationships and
Deuterium- Excess variations was used to realize the features
controlling temporal and spatial variation of heavy isotope composition
in precipitation. The daily and monthly reproducibility of precipitation
isotopes by IsoGSM were verified by observational samples in two
stations (Addis Ababa and Debremarkos) in 2014 and Entoto Hill
observational samples were used only for assessing spatial variations
with the other two stations and also observing an altitude effect with
Addis Ababa. Addis Ababa station and Entoto are near each other except
variation in altitude.

IsoGSM is an isotope incorporated AGCM with the spectral nudging
techniques was applied toward the atmospheric analysis using the
NCEP/DOE Reanalysis (Kanamitsu et al., 2002). Spectral nudging
techniques were believed to better represent large circulation fields as
the short term isotopic variation in precipitation is more controlled by
large scale moisture transport (Yoshimura et al., 2003; Yoshimura et
al., 2004). The IsoGSM was evaluated with satellite data (Yoshimura et
al., 2011) and observational data (Okazaki et al., 2015) and showed good
performance with little inconsistency which could be related to the
assumption of no land surface fractionation. The synthesis was made
based on combining observed rain isotopes, IsoGSM model simulations and
GNIP data. The temporal and spatial variation of \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O
in the study area distinguishes the dominant atmospheric processes that
affect the isotopic composition of precipitation locally.

\textbf{Data and Methods}

\textbf{Study Area and General Weather Conditions}

Precipitation events analyzed were collected at three sites (Addis
Ababa: 9.01\textsuperscript{0}N, 38.73\textsuperscript{0}E, altitude:
2355m; Entoto Hill: 9.12\textsuperscript{0}N, 38.77\textsuperscript{0}E,
altitude: 2634m and Debremarkos: 10.34\textsuperscript{0}N,
37.72\textsuperscript{0}E, 2446m) during rainy season of 2014. In total
of 139 rainfall events were collected in Addis Ababa (Mar 13, 2014 - Oct
10, 2014), 82 events in Entoto Hill (Jul 12, 2014 - Oct 12, 2014) and 80
events in Debremarkos (Jul 12, 2014 - Oct 12, 2014) were collected in
the upper Blue Nile River Basin. The location of sampling site is shown
in Fig. 1. Precipitation events were collected within 24 hrs intervals
using a standard rain collector which prevents evaporation once the rain
passed through the tube. The volume of 24 hours composite precipitation
sample was transferred into 1.5 mL glass bottles with tight cap, and
then the rain collector was dried with a clean dry cloth before
returning it to its collecting position. Collected samples were kept in
a cool and dark place. Upper Blue Nile River is upstream of part of Nile
River which starts flow in northern Ethiopia to 1,400 km to the Sudanese
capital Khartoum from Lake Tana 1,800 meters above sea level in the
Ethiopian Mountains. Precipitation occurs in summer season (June through
September), in the mid spring season (February or March through May) and
the remaining precipitation occurs in the dry season (October through
January or February). More than 80\% of annual flow in the Blue Nile
Basin results from the summer season and is concentrated between July
and October as shown in Fig. 1(c). The average annual rainfall varies
between 1200 and 1800 mm/yr ranging from an average of about 1000
mm/year near the Ethiopia/Sudan border to1400 mm/yr in the upper part of
the basin (Conway 2000).

The movement of Inter Tropical Convergence Zone (ITCZ) is controlling
the flow of the monsoon rainfall and seasonal rainfall cycle in the
Northern Ethiopia (Camberlin, 1997). It oscillates an extreme northward
of 15\textsuperscript{0}N in July and an extreme southward of
15\textsuperscript{0}S in January along with the zones of maximum of low
pressure and maximum temperature in Africa annually. ITCZ activates
southwesterly and southerly moisture trajectories during summer (JJAS)
and moving northward crossing Ethiopia in spring (MAM) and initiating
northeasterly and easterly wind trajectories. In summer, the air masses
are usually traveling from the Indian Ocean, the Gulf of Guinea and the
region to the north of Ethiopia and converge above the Ethiopian
highlands (Korecha and Barnston 2007; Segele and Lamb, 2009; Viste and
Sorteberg, 2011). The Lagrangian method of moisture identification
(Viste and Sorteberg, 2012) showed westerly/easterly trajectories to the
west and southwest of Ethiopia are the dominant and reliable moisture
transport into the northern Ethiopian highlands with moisture from Congo
Basin, the Gulf of Guinea and from the Indian Ocean. Westerly anomalies
above Central Africa increase the amount of moisture entering Ethiopia
with air coming from the Gulf of Guinea, while easterly anomalies reduce
this contribution.

Moisture identification during July and August demonstrated a variety of
moisture trajectories sources: flow from the Gulf of Guinea, flow from
the Indian Ocean, and flow from the Mediterranean region across the Red
Sea and the Arabian Peninsula and air coming from the south. The air
entering the northern Ethiopian highlands through the African continent
to the south comes from the Indian Ocean, regardless of its
southwesterly entry direction suggesting the source from Atlantic ocean
in July and August, and while the moisture transport is reduced from
Central Africa whether from an Atlantic direction or Indian Ocean in
June and September. In a spring a source of air masses is from the
Northern Indian Ocean and either from Mediterranean and/or red sea.
Between October and March the ITCZ is located in south of Ethiopia and
the region is under influence of northerly flow of dry and cold air from
the Arabian continent. The atmospheric condition affecting the transport
from the north are less well understood, but may be as relevant as those
in the southwest and also the contribution and importance of moisture
transport from the locally evaporated moisture (Viste and Sorteberg,
2012).

\textbf{Rainfall amount and Isotope Data Sources}

Temporal and spatial variation of stable water isotopes
(\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O and \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H) were used from
observational samples of precipitation in 2014, data from the
International Atomic Energy Agency-Global Network of Isotopes in
Precipitation (IAEA-GNIP) for Addis Ababa (1961-2009) and IsoGSM
simulations. Long-term measurements of available precipitation amount in
observational stations were obtained from National Meteorological Agency
of Ethiopia.

\textbf{Laboratory Analysis}

Oxygen-18 and deuterium measurements of observational precipitations
have been carried out by LGR Laser Isotope Instrument of Stable isotopes
in the laboratory of School of Earth Sciences, Addis Ababa University.
Isotopic analyses were reported in \selectlanguage{greek}δ \selectlanguage{english}units in per mill relative to
Vienna Standard Mean ocean water (VSMOW) according to the following
equation:

\selectlanguage{greek}δ \selectlanguage{english}(%0) = ( \(\frac{\text{Rsample}}{\text{Rvsmow}}\) -- 1) X 10\textsuperscript{3} Where R
refers to\textsuperscript{18}O/\textsuperscript{16}O
or\textsuperscript{2}H/\textsuperscript{1}H ratios. The analytical error
is \selectlanguage{ngerman}± 0.2 \selectlanguage{english}%0 for \textsuperscript{18}O, and +- 2 \selectlanguage{english}%0 for\textsuperscript{2}H.

\textbf{Isotope-enabled Global Spectral Model (IsoGSM)}

IsoGSM is isotope enabled the spectrally nudged atmospheric GCM toward
wind and temperature fields from the National Centers for Environmental
Prediction (NCEP)--Department of Energy (DOE) Reanalysis 2 (R2)
(Kanamitsu et al., 2002) in addition to being forced by sea surface
temperature (SST). Spectral nudging technique with reanalysis datasets
(e.g., horizontal winds from NCEP reanalysis) has been applied to some
AGCMs to constrain large-scale atmospheric circulations and the dynamic
fields were close to observations.

The model uses T62 horizontal resolution (about 200 km) and 28 vertical
levels, and the temporal resolution of the output is 6 hourly. The
convection scheme is the relaxed Arakawa--Schubert scheme (Moorthi and
Suarez, 1992). Nudging the model winds by ERA-40 reanalysis improved the
simulation of temperature and \selectlanguage{english}?\textsuperscript{18}O at middle latitudes
(Risi et al., 2012) and the simulated isotopic fields are also more
accurate compared to observations from small to large scale time scales.
The model reproducibility for daily to interannual timescales was
evaluated by comparing it with the precipitation isotope ratio and the
vapor isotopes ratio by (Yoshimura et al., 2008) and from satellite
measurements (Yoshimura et al., 2011).

For this study, the IsoGSM was dynamically downscaled and interpolated
using 0.125deg spatial resolution and a forcing time step of 6 hours in
the upper Blue Nile River Basin. The 6 hourly simulations of atmospheric
precipitation were converted to daily and monthly \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O
for Addis Ababa and Debremarkos for evaluation with observation. Isotope
processes were incorporated in the IsoGSM based on (Joussaume et al.,
1984): isotopic fractionation takes place whenever phase transition
occurs. Most fractionation can be assumed to occur at thermodynamic
equilibrium, except for three particular cases: surface evaporation form
open water, condensation from vapor to ice in supersaturation conditions
under -20\textsuperscript{0}C, and evaporation and isotopic exchange
from liquid raindrop into unsaturated air. The model assumes no isotopic
fractionation of evapotranspiration fluxes from the land surface
(Yoshimura et al., 2008).

\begin{enumerate}
\tightlist
\item
  \textbf{Results and discussions}
\item
  \selectlanguage{greek}\textbf{The temporal variations δ\textsuperscript{18}O}\selectlanguage{english}
\item
  \selectlanguage{greek}\textbf{Monthly variability of δ\textsuperscript{18}O}\selectlanguage{english}
\end{enumerate}

The monthly variations of \textsuperscript{18}O contents of
precipitation collected at Addis Ababa, IsoGSM simulation and amounted
weighted \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O from GNIP data in Addis Ababa (Fig. 2)
and monthly variations of \textsuperscript{18}O contents of
precipitation in Debremarkos (Fig. 3) is given. The bar graphs in both
graphs represent the monthly average of rainfall during observational
sampling time (2014) and long year mean (1964-2005). The association
between monthly weighted observed \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O with model
simulated (Fig. 2 and Fig. 3) shows suitability of model simulation in
capturing seasonal pattern of precipitation isotopes in two stations
with slight underestimation. The simulated \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O
demonstrates better suitability and small gap in Debremarkos compared
with Addis Ababa except in September. A strong correlation of monthly
weighted simulation against observation for existing
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O in Addis Ababa (0.88) and Debremarkos (0.6) was
found. A possible reason of a small bias in simulation could be related
the IsoGSM incapability of capturing the local recycling effects as
IsoGSM assumes no isotopic fractionation over the land surface. Even if
the results of the simulation show slight gap in peak enriched isotopes
of June and September and underestimation in Addis Ababa precipitation,
it captures the monthly-scale variability reasonably well. This shows
that the model is a good alternative to use for hydrological and
hydrogeological investigation with minor bias adjustment which could be
considered to be land surface fractionation.

The annual rainfall starts either in February/March and stops in October
with two rainy seasons; small rains in spring (MAM) and huge rainfall in
summer (JJAS) in the Upper Blue Nile River Basin. The rainfall pattern
shows high amount of rainfall in July, August and September in the
Basin. The general pattern of monthly weighted variation of observed and
simulated \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O in Addis Ababa (Fig. 2) and Debremarkos
(Fig. 3) show seasonal variation for both simulation and observation
with a relatively more enriched \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O in June and
September and more depleted at the heart of the great summer rainy
season (August) and dry season. The annual heavy oxygen isotopes pattern
in Addis Ababa shows slight increasing in \textsuperscript{18}O values
from onset of rainfall in March until June and declining its composition
until the lowest values in August, and then increasing in heavy oxygen
composition until the end of rainfall in October and declining its heavy
oxygen composition in dry season which starts in November. The simulated
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O (May -- September, 2014) and observed
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O (July -- October, 2014) in Debremarkos shows as
similar pattern as Addis Ababa except the depletion of heavy oxygen
isotopes starts in October. Such a clear correspondence between the
degree of isotope enrichment in spring (FMAM) and depletion of
precipitation progressively in summer (JJAS) rainy season reflects
presumably the influence of the south north movement of ITCZ. The south
north movement of ITCZ may trigger the variability of the wind direction
which brings moisture either in the northern hemisphere: Mediterranean
Sea/ red sea or Indian Ocean with time.

High enrichment of stable water isotopes in the area could be related to
the mixing of transpired moisture from Congo Basin which is a relatively
enriched and not fractionated than oceanic moisture sources. The monthly
weighted \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O variation of precipitation (Fig. 2 and
Fig. 3) shows high amount of rainfall in July and September accompanied
with high isotopic enrichment. This shows little impact of rainfall
amount in isotopic signature. Isotopic signature in annual cycle is
governed by the mixing of moisture sources and variation of moisture
trajectories accompanied with the movement of ITCZ. The
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O values in September for both Addis Ababa and
Debremarkos (Table 1 and Table 2, respectively) show high amount of
rainfall accompanied with more enriched \textsuperscript{18}O and low
amount of rainfall accompanied with depleted \textsuperscript{18}O in
October. This shows that the monthly isotopic variability in the upper
Blue Nile River Basin is not related with the amount effect.

The reason of relative enrichment of spring rain with respect to summer
could be the moisture travel path. In spring, ITCZ is moving northward
crossing Ethiopia and initiating northeasterly and easterly wind
trajectories in which moisture travels shorter distance from North
Indian Ocean, moisture from the North either Mediterranean or red sea.
In summer, ITCZ is in Northern hemisphere and trigger southwesterly and
southerly moisture trajectories which usually bring the air masses
traveling longer distance from the south Indian Ocean, the Gulf of
Guinea and the Congo Basin and converge above the Ethiopian highlands.
The relative depletion of \textsuperscript{18}O could be related to
degrees of rainout as ordered by the temperature along the air mass
trajectory. It is controlled by the source of moistures and the mixing
processes in the moisture trajectory which varies with season. The south
north movement of ITCZ is controlling the progressive increase of flow
of air masses and seasonal moisture sources in the Northern Ethiopia.
The heavy oxygen isotope \textsuperscript{18}O composition is relatively
enriched in spring rainfall than summer as shown in (Fig. 4, Table 2 and
Table 3). The amount weighted mean of \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O from GNIP
data (1961 -- 2009) as plotted (Fig. 4) shows a relative enrichment of
the heavy oxygen isotope, \textsuperscript{18}O in spring (FMAM) than
summer (JJAS) in Addis Ababa except in 1987. This could be related to
the difference of moisture with seasons in addition to the low amount of
rainfall during spring. This results in a progressive depletion of heavy
isotopes from spring to summer particularly (June to August) with
respect to the movement of ITCZ and the lowest \textsuperscript{18}O
values being observed in August and slightly enriching in September and
October. The moisture travels long distance from south Indian Ocean and
mixes with moisture from Gulf of Guinea and the Congo Basin.

\selectlanguage{greek}\textbf{Daily variability of δ\textsuperscript{18}O}\selectlanguage{english}

The validation of daily IsoGSM reproducibility of \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O
based on their grid points vs. observational sample of precipitation
isotopes in Addis Ababa (March 2014 to Oct 2014) (Fig. 5) and in
Debremarkos (July 2014 to Oct 2014 in Debremarkos) (Fig. 6) is given.
The IsoGSM reproduced reasonable \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O and well
captured the seasonal variation of isotopes in both Addis Ababa and
Debremarkos stations with slight underestimation of isotopic enrichment.
The daily variation shows high enrichment in June and September and more
depleted in August in Addis Ababa station and also the simulation and
observation of \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O in Debremarkos shows similar
pattern as Debremarkos. However, Addis Ababa samples show a relatively
more enriched \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O than Debremarkos samples. The
maximum, minimum, amount-weighted mean values and the difference between
extreme \textsuperscript{18}O in available observational data of daily
precipitation and simulated \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O of three
observational stations in the upper Blue Nile Basin (March -- October,
2014) is given (Table 3) shows clear seasonal variations of stable water
isotopes. The \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O is relatively more enriched in
Addis Ababa (mean value of \textbf{-0.45} %0) than Entoto Hill (mean
value of \textbf{-2.03%0} ) and Debremarkos (mean value of \textbf{-2.57}
%0).

\selectlanguage{greek}\textbf{The δ\textsuperscript{2}H - δ\textsuperscript{18}O relationships
and Deuterium- Excess}\selectlanguage{english}

The monthly deviations of \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O in precipitation of
Addis Ababa and Debremarkos (Fig. 2 and Fig. 3) and daily deviations
(Fig. 5 and Fig. 6, respectively) could be related to the seasonal
fluctuation of moisture trajectories and sources. A scatter relationship
between \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H and \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O from observed
samples of precipitation events during rainy season of 2014 was
calculated. The \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H and \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O
relationships from samples of 139 observed precipitation for Addis Ababa
(\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H = 7.83\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O + 12.38) plotted in
(Fig. 7 (a) and 80 observed precipitation for Debremarkos
(\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H = 8.23.\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O + 21.24) plotted in
(Fig. 7 (b)). Both stations show coefficients nearly 8 with large values
of d-excess greater than 10. The effect of high d-values (greater 10) in
rainfall and the reduced degree of evaporation from falling rain
droplets (coefficients nearly 8) are the effects on both data sites.
This reflects the influence of recycled continental moisture either from
open water bodies or transpired moisture in both sites. The influence of
recycling of moisture is more pronounced than evaporation from falling
rain droplets for both locations with greater influence in Debremarkos.
But relative comparison between two sites show more pronounced recycling
of moisture in Debremarkos than Addis Ababa, and slight evaporative
effects in Addis Ababa. The variation of these effects in relative to
two sites may be explained with the Rayleigh rain out effect. The
moisture from the source arrive Addis Ababa first and goes to
Debremarkos. The high d-excess in Debremarkos accompanied with relative
depletion of \textsuperscript{18}O in Debremarkos is due to rainout
effect through the moisture trajectory from Addis Ababa to Debremarkos.
More d-excess in Debremarkos is because of the mixture of more recycled
local moisture throughout the moisture trajectory from Addis Ababa to
Deberemarkos.

Monthly weighted mean deuterium excess (d = \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H -
8.\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O) in three observational stations (Table 4) and
monthly weighted mean \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H-\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O
relationships (Table 5) was calculated in observed precipitation events
to understand spatial variations in the study sites. The
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H and \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O and d-excess variation
show increasing in mixing of recycled moisture from March to July.
However, the linear regression of rainfall sample of August in Addis
Ababa is with coefficient of 7.84 and intercept 9.81 is nearly similar
with Global Meteoric Water Line and d-excess is nearly 10.22 reveal that
the moisture is nearly from oceanic source with initial condensation in
Addis Ababa. However, d-excess is increasing from Addis Ababa to Entoto
Hill, and Entoto Hill to Debremarkos. This shows the moisture recycling
is increasing from Addis Ababa to Debremarkos in the moisture trajectory
from Addis Ababa to Debremarkos with increasing d-excess and depletion
of isotopes is due to Rain out effect and mixing of recycled local
moisture in the trajectory. The rainfall of Addis Ababa seems an initial
condensation with relatively more enriched \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O with
Debremarkos, and the mixing of continental recycling of moisture is
increasing accompanied with depletion in \textsuperscript{18}O due to
Rayleigh rain out effect in the moisture trajectory to Debremarkos.

A variety of moisture trajectories in the upper Blue Nile River Basin
was reviewed. The trajectories of moisture sources are changing with
seasons which are controlled by the north south modulation of ITCZ. The
monthly fluctuation of \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O in precipitation, the
scatter relationships between \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H and
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O and deuterium excess variations with each month
was carried out (Table 4 and Table 5) shows the mixing of different
moisture sources which is changing with time. An isotopic fractionation
is the reason of temporal variation of \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O in
precipitation of any location which is due to evaporation of water from
ocean or land surface, rainout process during condensation in the
moisture trajectory, the mixing of different moisture sources in the
moisture trajectory. In total rainfall events of Addis Ababa collected
in the year 2014, 30 events are from spring (March, April and May) and
109 events was from summer (June, July, August and September) and
October. The \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H--\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O relationships
of rainfall sample in Addis Ababa were calculated for spring and summer
season (Fig. 8). The \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H--\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O
relationships for spring (\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H =
7.15.\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O + 12.13) show relatively more influenced
with evaporation of falling rain compared to summer
(\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H = 7.85.\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O + 14.10) samples.
This variation could be partially related to lesser amount of rainfall
and high solar radiation during spring than summer season (also confirms
the explanation by Kebede 2004). However, summer rainfall is relatively
more influenced by recycling of moisture from continental surface than
spring with greater d --excess and spring rainfall is better influenced
by evaporative effects. This study also confirms the importance of
moisture recycling demonstrated by (Seifu et al., 2012).

Monthly weighted average d-excess (Table 4) and Linear Regression
between Hydrogen and Oxygen isotopes (Table 5) in the precipitation show
that the effect of evaporation in the rainfall during rain drops is
increasing from March to June, and addition of recycled moisture in the
rainfall is relatively increasing from March to July. Evaporative effect
is more pronounced in June and moisture recycling is pronounced in July.
The coefficients and d-excess value from the relationship between
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H and \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O during August is 7.84
and 9.8 respectively. This show that, rainfall in August is not that
much influenced by recycling as well as evaporation during rain drops.
The rain bearing moisture in Addis Ababa during August is nearly from
Oceanic source. However, it is recycling from Addis Ababa to Debremarkos
as shown with greater d-excess value in Debremarkos compared to Addis
Ababa and Entoto Hill. The d-excess variation also reveals the more
mixing of moisture from recycled source in September and then the
d-excess value drops in October.

\selectlanguage{greek}\textbf{The spatial variation signal of δ\textsuperscript{18}O and
Altitude Effect}\selectlanguage{english}

The weighted mean \textsuperscript{18}O values of rain events (July 2014
to October 2014) from three sampling sites (Table 6) as a function of
the inland distance (from south east to north west) in the upper Blue
Nile River Basin (Fig. 1) is given. The weighted
mean\textsuperscript{18}O decreases from South East sampling sites
(Addis Ababa) to North West sampling site (Debremarkos). This shows a
relative depletion of \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O from Addis Ababa to Entoto
Hill and Entoto Hill to Debremarkos without regarding their altitude
except in September and, decreasing in d-excess (as shown in Table 4)
from Debremarkos to Entoto Hill and Entoto Hill to Addis Ababa. A
relative depletion of weighted mean \textsuperscript{18}O from Addis
Ababa to Entoto Hill could be related to both altitude effect and
Rayleigh distillation effect since Entoto Hill and Addis Ababa are very
close each other (less than 6 kms in distance) and 279m altitudinal
difference.

\textbf{Discussions}

The appearance of inland gradients of \textsuperscript{18}O values
likely indicates that variations in the heavy isotope contents of rain
events are influenced either by different condensation mechanisms,
varying sources of moisture or Rayleigh distillation effect. The heavy
isotope depletion and increasing d-excess from Addis Ababa to Entoto
Hill and Entoto Hill to Debremarkos could be explained by continuous
removing of water vapour as rainwater by the Rayleigh distillation
mechanism accompanied with local continental moisture mixing in the
moisture trajectory farther during moisture transport from Addis Ababa
to Debremarkos. The spatial variation signal of this study indicates
that the moisture transport history and the degree of rainout in the
site may be more important factors explaining monthly variations and
spatial differences in \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O. The source of local
moisture is assumed to be either evapotranspiration of the Congo
vegetation and/or evaporation of surface waters mixed with oceanic air
masses.

This study shows the moisture bearing precipitable water in the Blue
Nile River Basin is a mixture of different sources changing with seasons
and months. The \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O of rain events show the great
importance of recycled moisture in the Basin which is influenced by
rainout effect and a little amount effect within the Basin. The rainfall
during spring is relatively affected by evaporation of rain droplets
while the summer rainfall is more influenced by mixing of recycled
moisture. In general, the rainfall sampled was admixture of
re-evaporated and transpired moisture from surface, evaporative
enrichment from falling raindrops and the local atmospheric condition
with high deuterium excess record in the summer rainfall with two peaks
of d-excess in July and September as also elucidated by Kebede 2004. The
more enrichment of moisture in the Basin could be a return of flux of
moisture from transpiration from Congo vegetation. Transpired moisture
is non-fractionating, and thus when it mixes with the moisture coming
from ocean sources the resultant vapor becomes enriched in heavy
isotopes and injects high d-excess waters in to the clouds.

\textbf{Conclusions}

This study elaborates the temporal variation of stable water isotopes
(\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H and \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O) in precipitation,
their spatial variability signal and controlling features of variation
with time over upper Blue Nile Basin using time series observational
isotopes, GNIP data and IsoGSM simulation. IsoGSM model simulation with
observed precipitation in Addis Ababa and Debremarkos shows a reasonable
reproducibility of \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O in daily and monthly weighted
timescales and clear seasonal pattern of isotopes and well captured
large scale circulation fields with slight bias which could be related
to the significance of land surface fractionation. IsoGSM assumed no
isotopic fractionation over the land surface. The model evaluation shows
the importance of land surface fractionation to have more precise
simulations of stable water isotopes. Simulations could be used for
mapping distribution of stable isotope in the River Basin for
hydrological and hydrogeological assessment and paleoclimate
reconstruction.

The temporal variation shows a clear seasonal variability of
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O in both simulated and observed precipitation in
the study area. The highly enriched value of \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O with
minimum (-9.65%0) in Entoto Hill and maximum (7.79%0) in Addis Ababa was
observed. Maximum, minimum and average of amounted weighted value of
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O, respectively shows (7.79%0, -7.39%0 and -045%0) in
Addis Ababa, (5.26%0 to -10.76%0, and -2.03%0) in Entoto and (4.08%0 to
-9.65%0, and 2.41%0) in Debremarkos. The peak enrichments of heavy
isotopes were observed in June and September and more depleted
precipitation sample in August and October. The temporal variation in
daily and monthly weighted variation of oxygen isotopes
(\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O) from both the model simulation and observed
sample of 2014 showed an increasing of heavy oxygen
(\textsuperscript{18}O) composition in precipitation from March to peak
enrichment in June, and then depletion starts until August and rises in
September. The rainfall in dry season starts in October is relatively
more depleted. The monthly analysis of d-excess showed continuous
increasing from the onset of rainfall in Addis Ababa in March from 10.24
%0 to July 15.44%0, and also high d-excess value of 20.02%0 and 22.39%0
respectively in Entoto Hill and Debremarkos was observed in July 2014.
The d-excess is relatively dropped in August for all three stations and
rises in September, and then drops again in October.

The monthly variability of \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O and d-excess and the
scatter relationship between \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H and
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O in the Blue Nile Basin of Ethiopia shows a strong
influence of moisture source and trajectory history in variation of
precipitation isotopes. The variation of d-excess shows the influence of
recycled moisture which is increasing from March to July, and drops in
August and rises in September. Recycling of moisture is pronounced with
a little evaporative effect in three stations. The
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O is relatively enriched in spring (FMAM) than
summer (JJAS) precipitation. The amount effect is not much pronounced in
affecting the monthly and seasonal variation of \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O
in the upper Blue Nile Basin. For example more enriched
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O accompanied with high amount of rainfall in
September is recorded for both Addis Ababa and Debremarkos. Spring
(March, April and May) precipitation is relatively better affected by
evaporative effect than summer (June, July, August and September)
precipitation. The linear regression of \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H versus
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O shows that, the precipitation water is more
affected by evaporation of rain droplets in June that is enhanced by low
amount of rainfall.

The heavy isotopic composition of precipitation in spring is relatively
enriched and low influence of recycled moisture compared to summer.
During March, April and May, the ITCZ is moving northward crossing
Ethiopia and the moisture in the Basin is shaped by northeasterly and
easterly moisture flows which brings enriched moisture from short paths
of Northern Indian Ocean, Mediterranean and Red Sea. During June, July,
August and September, the ITCZ is located in northern Ethiopia and the
region is under the influence of the southwesterly and south easterly
monsoon flows. These flows bring moisture with varying trajectories
paths from south Indian Ocean, Gulf of Guinea and Congo Basin. The
depletion of moisture in this season is due to varying paths of moisture
which is influenced by rain out effect. Moisture from low level
trajectory from south Indian Ocean through Turkana channel pulls the
transpired moisture from Congo vegetation Basin which is expected to be
enriched in heavy isotopes. The monthly variation of isotopic
composition in this season could be related to the variation of moisture
paths from south Indian Ocean. Isotopic signature of rainfall during
August show more depleted in \textsuperscript{18}O, low mixing of
recycled water and evaporative effect compared to other months. The more
depleted \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O with d-excess value nearly 10 clearly
show that the moisture is likely from south Indian Ocean through the air
parcels traveling long pathways to the west before turning
northeastward. The more depletion of water is due to rain out effect in
the trajectory of moisture. Between October and March the ITCZ is
located in south of Ethiopia and the region is under influence of
northerly flow of dry and cold air which is more depleted water sourced
from the Arabian continent and extra-tropical air masses.

More enriched \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O accompanied with d-excess (greater
than 10) in the upper Blue Nile River Basin could be related to mixing
transpired moisture from Congo vegetation and evaporated water from open
bodies through the moisture trajectory from Indian Ocean. The transpired
moisture is non-fractionating, and consequently when it mixes with the
moisture from ocean sources the resulting vapor becomes enriched in
heavy isotopes and associated evaporation from open water bodies bring a
high d-excess air masses. An association of monthly weighted isotopic
composition of precipitation data from July, 2014 to October, 2014 in
three sites; Addis Ababa, Entoto Hill and Debremarkos shows a relative
depletion of \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O from Addis Ababa to Entoto and
Entoto to Debremarkos. The spatial variation of stable water isotopes in
the Basin is controlled by the rain out effect. The depletion of
isotopes from Addis Ababa to Entoto Hill and Entoto to Debremarkos
accompanied with increasing value of d-excess. This shows that, moisture
is transported from Addis Ababa to north of the Blue Nile River Basin
and increasing of d-excess is due to rainout processes along moisture
trajectory.

In general, the study shows that the temporal isotopic composition of
precipitation in the upper Blue Nile River Basin is essentially shaped
by the source of moisture which is controlled by the south north
movement of ITCZ. The mixing of recycled continental moisture is
pronounced and shown in the isotopic composition of stable water
isotopes. The spatial variation of isotopic composition is influenced by
Rayleigh rainout effect along the moisture trajectory.

\textbf{Acknowledgements}

The authors acknowledge the IAEA projects for assisting observational
data collection and National Meteorological Agency of Ethiopia for
precipitation data.

\textbf{DATA AVAILABILITY}

The data used for Figures and Tables for this research article is based
on DEM and GIS Shape Files produced from Geological survey of Ethiopia,
the Global Network of Isotopes in Precipitation (GNIP) data was used
from (https://www.iaea.org/services/networks/gnip), the IsoGSM
simulation of stable water isotopes (\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H and
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O)from the website:

http://isotope.iis.u-tokyo.ac.jp/\textasciitilde{}kei/?IsoGSM1.

Rainfall data in the study area was collected from National
Meteorological Agency of Ethiopia (NMA) and observed stable water data
collected in three locations during rainy season in 2014 were used.

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