Abstract:
Climate change is a topic of discussion worldwide including Ethiopia. The present study
assessed the possible impacts of climate change on runoff in the upper Birbir sub basin
which is located in 8°48′17′′ and 9°22′19′′ N and 34°C49′39′′and 35°C37′27′′E; western
Ethiopia. CORDEX-Africa data outputs of HadGEM2-ES for upper Birbir sub basin were
selected under representative concentration pathways (RCP4.5& RCP8.5) scenario to make
future projections in two time frames: mid-term (2041-2070) and long-term (2071-2100).
After assessment of data quality; Bias, Coefficient of Variation and correlation were used to
evaluate the systematic error of rainfall amount, degree of rainfall variability and the
relationship of model outputs and observed datasets in addition to graphical plots. Trend
detection was evaluated by Mann-Kendall trend test and sen’s estimators. Hargreaves
method was used to estimate current and future evapotranspiration. HBV model was selected
to simulate future stream flow for the projected climate variables . Coefficient of
determination (R
2
), Nash-Sutcliffe Efficiency (NSE), Relative Volume Error (RVE) and Root
mean square error observation standard deviation ratio (RSR) were statistical tools used for
performance evaluation of rainfall-runoff model (HBV) with a result of 0.82, 0.81, 5.70%
and 0.11during calibration period. The observed annual rainfall shows upward trend except
for Yubdo station, and temperature shows warming trend. Future projections of runoff will
be expected to decrease in winter and spring while increase in summer and autumn, mainly
followed the rainfall projected pattern. In mid-term, the annual average runoff projection
will be expected to decrease by 2.05% and 1.35% under RCP4.5 and RCP8.5, respectively.
In long-term, it will be expected to decrease by about 0.03% a nd 2.0% under RCP4.5 and
RCP8.5 over upper Birbir sub basin.
Keywords: Upper Birbir sub basin, Climate change, HBV, Trend detection, RCP and Runoff.
