Understanding Temporal and Spatial Variation of Stable Water Isotopes in
Precipitation over Upper Blue Nile River Basin
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 δ18O 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 δ18O 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‰, -11.73‰, 0.04‰) in Addis
Ababa, (5.26‰, -12.74‰, and -2.52‰) in Entoto Hill and (4.08‰, -9.65‰,
2.41‰) in Debremarkos respectively. The δ2H- δ18O relationships, monthly
weighted d-excess variation in the Basin revealed the temporal variation
of δ18O 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 δ18O as good alternative for
hydrological and hydrologeological investigations when needed.