The impacts of climate change on hydrological processes of Gilgel Gibe catchment, southwest Ethiopia

future time horizons are consistent with other researchers’ ranges and falls within the IPCC’s range. Numerous research on climate change in Ethiopia have suggested that temperatures are likely to rise across the country’s various regions. However, the degree of change varies accord- ing on the downscaling methods and kinds of climate models. According to the Global Facility for Disaster Reduction and Recovery [63] analysis on the country profile for climate risk and adaptation, the mean annual temperature is expected to rise by 1.1 to 3.1 degrees Celsius by the 2060s and by 1.5 to 5.1 degrees Celsius by the 2090s. According to the analysis by [61], the entire upper Blue Nile basin’s average annual temperature will rise by 1.5˚C, 2.6˚C, and 4.5˚C between the years 2011 and 2040, 2041 and 2070, and 2071 and 2100, respectively. According to [64] study’s on the entire Nile Basin, which used an average of 11 GCMs, the yearly average temperature change between 1950 and 1999 and 2010 to 2039 will range from 0.91 to 1.9 Fig 12. Seasonal projected change in mean maximum temperature in Gilgel Gibe catchment for 2041–2070 and 2071–2099 under RCP4.5 and RCP8.5. https://doi.org/10.1371/journal.pone.0287314.g012 PLOS ONE The impacts of climate change on hydrological processes of gilgel gibe catchment PLOS ONE | https://doi.org/10.1371/journal.pone.0287314 June 28, 2023 18 / 28

degrees Celsius. Furthermore, [57].’s study on the Blue Nile, which used 17 GCMs, predicted that temperatures will rise by 2 ˚C and 5 ˚C. 3.2.3 Impacts of climate change. Changes in rainfall and temperature were used to pre- dict the impacts of climate change on water balance components of the catchment. Conse- quently, the projected changes in mean annual precipitation and temperature, under the two RCPs, caused a significant variation in the projected water balance components of Gilgel Gibe catchment. Reduced surface flow, groundwater, and overall water yield are predicted as a result of the decline in precipitation and rise in temperature, according to SWAT models for the intermedi- ate term and far future Table 6. Changes in temperature and PET are correlated positively. In turn, this led to a probable rise in evapotranspiration and evaporation, which may have a sig- nificant impact on the decline in water yield. Evaporation is a factor in the future decline of the surface runoff since the forms of water are prone to losses due to temperature changes. This is expected considering the warming trends of temperature. Generally, the interactions of precipitation, temperature, and evapotranspiration can be used to describe how climate change has affected the hydrology of a catchment. Reduced rain- fall lowers surface runoff, while rising temperatures enhanced an increase in evapotranspira- tion that would have caused the runoff to decline. In this regard, the correlation between precipitation and the simulated surface runoff, groundwater and total water yield is positive,

while the relationship between the temperature and hydrological process are positive only for PET. However, the relationship between the precipitation and temperature with the hydrologi- cal process varies with the RCPs and period of the scenario. Changes in precipitation, the expected decline in groundwater and total water yields, for instance, is larger under both RCPs for the period 2071–2100. However, under RCP4.5, the reduction in surface runoff is only more pronounced during the years 2071–2099 (-14.71%) (Fig 14). Fig 11 illustrates how the decline in surface water under RCP8.5 is greater throughout the period 2040–2070 (-12.55%). Fig 13. Seasonal projected change in mean minimum temperature in Gilgel Gibe catchment for 2041–2070 and 2071–2099 under RCP4.5 and RCP8.5. https://doi.org/10.1371/journal.pone.0287314.g013 PLOS ONE The impacts of climate change on hydrological processes of gilgel gibe catchment PLOS ONE | https://doi.org/10.1371/journal.pone.0287314 June 28, 2023 19 / 28

This demonstrates that changes in precipitation is not the only factor affecting surface water. This result was in line with previous studies on the Nile River. In the upper Nile, [65] found that hot, dry years are occurring more frequently due to rising regional temperatures. The spatial distribution of the hydrological process under the projected climate change for RCP4.5 and RCP8.5 is presented in the Figures. The catchment’s surface runoff, groundwater, and water yield were all reported to have decreased due to climate change, but the yields in each sub-basin varied. While temperature changes are projected to have indirect effects, spa- tiotemporal patterns and magnitudes of precipitation are predicted to directly disrupt the hydrological process. In both scenarios throughout the study projections taken into consider- ation, the upstream of the catchment is often anticipated to have the greatest decline in surface runoff, groundwater, and water yield. Under the RCP4.5 and RCP8.5 scenarios, the projection for the decline in surface runoff and water yield is greater in the intermediate than in the far future. In both scenarios, the southwest is where surface runoff is expected to decline at the slowest rate over the intermediate term and the far future, followed by the northeast. The southwest corner has a higher rate of groundwater and water yield decline. The highest rise in PET is shown by RCP8.5 around the outlet of the catchment in the mid future projection. In both scenarios, it was observed that the catchment’s surface runoff had decreased as a

result of climate change. The upstream of the catchment is frequently expected to have the Table 6. Changes of water balance components under a climate change. RCM RCP Time Period Surface Runoff (%) Groundwater flow (%) Water Yield (%) PET (%) CCLM4-8 RCP4.5 2041–2070 -7.61 -9.34 -8.92 16.01 CCLM4-8 RCP4.5 2071–2099 -14.96 -13.83 -14.01 19.02 CCLM4-8 RCP8.5 2041–2070 -12.65 -9.89 -10.58 16.69 CCLM4-8 RCP8.5 2071–2099 -9.57 -16.08 -13.76 22.67 HIRHAM5 RCP4.5 2041–2070 -7.79 -9.32 -8.78 16.15 HIRHAM5 RCP4.5 2071–2099 -14.64 -13.98 -14.03 19 HIRHAM5 RCP8.5 2041–2070 -13.87 -9.91 -10.99 16.79 HIRHAM5 RCP8.5 2071–2099 -9.96 -16.02 -13.94 22.78 RACMO22T RCP4.5 2041–2070 -7.14 -9.11 -8.19 16.05 RACMO22T RCP4.5 2071–2099 -14.59 -13.99 -14.04 19.01 RACMO22T RCP8.5 2041–2070 -13.72 -9.48 -10.31 16.68 RACMO22T RCP8.5 2071–2099 -9.31 -16.27 -13.51 22.69 RCA4 RCP4.5 2041–2070 -7.46 -9.73 -8.89 16.12 RCA4 RCP4.5 2071–2099 -14.85 -13.91 -14.02 19.02 RCA4 RCP8.5 2041–2070 -13.67 -9.63 -10.05 16.48 RCA4 RCP8.5 2071–2099 -9.34 -16.03 -13.71 22.47 CRCM5 RCP4.5 2041–2070 -7.84 -9.76 -8.82 16.07 CRCM5 RCP4.5 2071–2099 -14.48 -13.96 -14.07 19.01 CRCM5 RCP8.5 2041–2070 -13.92 -9.99 -10.82 16.75 CRCM5 RCP8.5 2071–2099 -9.51 -16.05 -13.78 22.69 REMO2009 RCP4.5 2041–2070 -7.55 -9.45 -8.73 16.09 REMO2009 RCP4.5 2071–2099 -14.72 -13.92 -14.01 19.02 REMO2009 RCP8.5 2041–2070 -12.56 -9.78 -10.54 16.68 REMO2009 RCP8.5 2071–2099 -9.52 -16.08 -13.75 22.67

Ensemble RCP4.5 2041–2070 -7.56 -9.44 -8.72 16.08 Ensemble RCP4.5 2071–2099 -14.71 -13.93 -14 19.01 Ensemble RCP8.5 2041–2070 -12.55 -9.79 -10.55 16.67 Ensemble RCP8.5 2071–2099 -9.53 -16.09 -13.74 22.66 https://doi.org/10.1371/journal.pone.0287314.t006 PLOS ONE The impacts of climate change on hydrological processes of gilgel gibe catchment PLOS ONE | https://doi.org/10.1371/journal.pone.0287314 June 28, 2023 20 / 28

the catchment and worsen water stress downstream (Fig 17). In general, the increase in temperature and decline in annual and seasonal precipitation in both scenarios could be the cause of the catchment’s decreased average annual flow. The possi- bility of reduced surface water, groundwater, and total water yield in the catchment could have an impact on the availability of water resources in the catchment and exacerbate water stress downstream. The acceleration of the increasing water abstractions could also contribute to the further decline in water resources besides the climate change pressure. Reduced soil moisture, which is necessary for plant growth and groundwater storage, may result from the increase in PET (Fig 18) brought on by rising temperatures and a decline in precipitation. This may indicate decreased water availability for agricultural production, which will become a chronic problem for the farmers in the catchment whose only source of Fig 14. Impacts of climate change on water balance. https://doi.org/10.1371/journal.pone.0287314.g014 PLOS ONE The impacts of climate change on hydrological processes of gilgel gibe catchment PLOS ONE | https://doi.org/10.1371/journal.pone.0287314 June 28, 2023 21 / 28

income is agriculture. Additionally, a rise in temperature increased PET, which in turn raised the requirement for irrigation. Similar findings were supported by research on climate change in Africa, namely the sub- Sahara, which showed a range of warming trends in the inland subtropics, a recurrence of high hot events, an increase in aridity, and changes in rainfall [66]. The effects of climate change, according to [66], have a considerable impact on the region’s undernourishment, infectious diseases, vulnerability to rain-fed agriculture, and flash flooding catastrophes. The results also support the findings of [57], which indicated that the Nile river is predicted to decline between the years of 2040 and 2070 and again between 2070 and 2099 as a result of decreased precipitation and higher evaporation rate. In their study on the effects of climate change on the Tekeze river basin’s water sources, [67] also predicted that surface runoff would decrease by 13% and 14%. The decline of the expected surface runoff under both RCPs due to climate change was also confirmed in the study by [62] using the ensemble of five GCMs. 4. Conclusion and management options This study undertook an analysis of the effects of climate change on the hydrological processes within the Gilgel Gibe catchment. Specifically, the study evaluated the impact of potential Fig 15. Spatial patterns of the changes in surface runoff due to climate change. https://doi.org/10.1371/journal.pone.0287314.g015 PLOS ONE The impacts of climate change on hydrological processes of gilgel gibe catchment

PLOS ONE | https://doi.org/10.1371/journal.pone.0287314 June 28, 2023 22 / 28

future climate changes using six Global Climate Models (GCMs), and their corresponding ensemble mean, through the application of a physically based distributed hydrologic model known as Soil and Water Assessment Tool (SWAT). The study utilized six Regional Climate Models (RCMs), and their ensemble mean, from the Coordinated Regional Climate Downscal- ing Experiment for Africa (CORDEX-Africa) to examine the potential implications of climate change on the Gilgel Gibe catchment. Notably, the RCM simulations indicated a decrease in precipitation levels for scenarios involving high and medium-low emissions, with the excep- tion of HIRHAM5. In terms of temperature, all RCM projections indicate an upward trend with varying magnitudes of change. To analyze the hydrological impacts of climate change, the ensemble mean of the RCMs was employed to mitigate the substantial variability inherent in precipitation and temperature forecasts by different RCMs. The use of ensemble RCMs was found to be a suitable approach for evaluating uncertainties in individual RCMs, despite the presence of inherent uncertainties in climate prediction. As a result of the anticipated decrease in precipitation and the increase in temperature, surface runoff, groundwater, and total water yield are all anticipated to decrease, while potential evapotranspiration is expected to increase. Overall, the water resources of the Gilgel Gibe catchment are exceedingly sensitive to changes in climate. Fig 16. Spatial patterns of the changes in groundwater due to climate change.

https://doi.org/10.1371/journal.pone.0287314.g016 PLOS ONE The impacts of climate change on hydrological processes of gilgel gibe catchment PLOS ONE | https://doi.org/10.1371/journal.pone.0287314 June 28, 2023 23 / 28

Furthermore, the Gilgel Gibe catchment exhibits significant temporal and spatial variability in temperature and precipitation, which manifests in heightened occurrences of hot and dry years, leading to severe water scarcity. Consequently, the catchment is highly susceptible to the impacts of climate change, such as warming and crop failures resulting from prolonged dry seasons. The observed decrease in runoff within the catchment has contributed to enhanced water security. The study’s analysis indicates that the SWAT simulations provide adequate esti- mates of the impacts of climate change in the Gilgel Gibe catchments. To enhance our under- standing of the present and future climate dynamics, urgent attention is required to improve the availability and quality of hydro-climatic data in the region. The results of this study have provided crucial insights into the impacts of climate change on the hydrological processes of the Gilgel Gibe catchment. These findings can inform the development of effective water resource management initiatives in the region. The rehabilita- tion of degraded sloping lands could enhance ground recharge and reduce surface runoff that contributes to soil erosion and sedimentation in the Gilgel Gibe I dam. Additionally, appropri- ate management strategies should be implemented to safeguard the Gilgel Gibe I hydroelectric dam in the catchment. Overall, these results underscore the imperative for regional develop- ment and collaboration to facilitate robust and climate-resilient management strategies and

mitigate the adverse effects of climate change on the catchment. Fig 17. Spatial patterns of the changes in water yield due to climate change. https://doi.org/10.1371/journal.pone.0287314.g017 PLOS ONE The impacts of climate change on hydrological processes of gilgel gibe catchment PLOS ONE | https://doi.org/10.1371/journal.pone.0287314 June 28, 2023 24 / 28

Author Contributions Conceptualization: Zewde Alemayehu Tilahun, Yechale Kebede Bizuneh, Abren Gelaw Mekonnen. Data curation: Zewde Alemayehu Tilahun, Yechale Kebede Bizuneh, Abren Gelaw Mekonnen. Formal analysis: Zewde Alemayehu Tilahun. Investigation: Zewde Alemayehu Tilahun, Abren Gelaw Mekonnen. Methodology: Zewde Alemayehu Tilahun, Abren Gelaw Mekonnen. Software: Zewde Alemayehu Tilahun, Abren Gelaw Mekonnen. Supervision: Yechale Kebede Bizuneh, Abren Gelaw Mekonnen. Validation: Yechale Kebede Bizuneh, Abren Gelaw Mekonnen. Visualization: Abren Gelaw Mekonnen. Writing – original draft: Zewde Alemayehu Tilahun. Writing – review & editing: Zewde Alemayehu Tilahun, Yechale Kebede Bizuneh, Abren Gelaw Mekonnen. Fig 18. Spatial patterns of the changes in Potential Evapotranspiration (PET) due to climate change. https://doi.org/10.1371/journal.pone.0287314.g018 PLOS ONE The impacts of climate change on hydrological processes of gilgel gibe catchment PLOS ONE | https://doi.org/10.1371/journal.pone.0287314 June 28, 2023 25 / 28

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