Thesis Open Access

ASSESSMENT OF CLIMATE CHANGE IMPACT ON INFLOWS TO KOKA RESERVOIR IN UPPER AWASH SUB BASIN

SALIH DURI ABDULAHI


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    <subfield code="a">&lt;p&gt;Climate change impact and its variability on stream flows are now becoming one of&lt;br&gt;
the significant challenges. Hence evaluation of the impact and the trends of the change&lt;br&gt;
is very important at watershed level. The aim of this study was to evaluate the impacts&lt;br&gt;
of climate change on inflows to Koka reservoir in upper awash sub basin using HBV&lt;br&gt;
model. The climate model variables (precipitation and temperature) were obtained&lt;br&gt;
from Coordinated Regional Downscaling Experiment (CORDEX) from Regional&lt;br&gt;
Atmospheric Climate Model Version2.2 (RACMO22T) under representative&lt;br&gt;
concentration pathway (RCP4.5) and (RCP8.5) scenarios. Bias correction was applied&lt;br&gt;
to climate variables before transferred to hydrological model (HBV) to simulate&lt;br&gt;
discharge. For model performance evaluation; Coefficient of determination (R2), Nash&lt;br&gt;
Sutcliffe Efficiency (NSE) and percent bias (PBIAS) were used during calibration (2002-&lt;br&gt;
2010) and validated (2011-2015) period. The result of calibration and validation (R2,&lt;br&gt;
NSE and PBIAS) are 0.88, 0.78 and -6.72 and 0.88, 0.83 and -12.51 respectively. Stream&lt;br&gt;
flow was simulated for two consecutive periods (2021&amp;ndash;2040) and (2041&amp;ndash;2060) for both&lt;br&gt;
scenarios and compared with the base period (1996&amp;ndash;2015) to explore impact of climate&lt;br&gt;
change on inflows. The result obtained shows that overall mean annual flow is projected&lt;br&gt;
to decrease by 11.7 and 18.1% under RCP4.5 and RCP8.5 scenarios respectively. For&lt;br&gt;
the period (2021-2040) flow expected increase in October under both scenarios. The&lt;br&gt;
increment will be 113.2% and 115.5% under RCP4.5 and RCP8.5 respectively while the&lt;br&gt;
maximum reduction will be expected in March (84.6%) under RCP4.5 and in January&lt;br&gt;
(69.5%) under RCP8.5. Similarly for the period of (2041-2060) the maximum increment&lt;br&gt;
will be in October under both scenarios while the maximum reduction projected in&lt;br&gt;
January under both scenarios. Flow duration curves indicate that, the probability of&lt;br&gt;
occurrence of high flow (Q5) and median flow (Q50) are likely to be more in the future&lt;br&gt;
under both scenarios. However, low flow (Q95) is predicted to decrease under both&lt;br&gt;
scenarios. High flow (Q5) will be vary from (35.6% to 55.9%) and median flow (Q50) vary&lt;br&gt;
from (37.2% to 85.8%). Low flow (Q95) expected to decrease by 78.3% under RCP4.5&lt;br&gt;
and 71.8% under RCP8.5. The monthly and seasonal variations are expected to be&lt;br&gt;
more relative to annual variations. During summer and autumn season, flow expected&lt;br&gt;
to increase while decrease in winter and spring in the study area under both scenarios.&lt;br&gt;
As the result of high increment in flow in certain months and high reduction in the&lt;br&gt;
other months, flood and drought may be expected in future in the basin. Overall there&lt;br&gt;
will be a variations of flow month to month and season to season. Therefore, it is&lt;br&gt;
important to consider this variation of flows to Koka reservoir to frame appropriate&lt;br&gt;
guidelines for planning and management&lt;br&gt;
&amp;nbsp;&lt;/p&gt;</subfield>
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