Highlights of Climate Change Impact on Hydrologic Flows in Five Basins of Ethiopia

Climate changes is a global phenomenon exhibited by three prominent signals, that is: (1) global average temperatures are gradually increasing, (2) changes in global rainfall patterns and (3) rising of sea levels. The objective of this paper is review climate change impact on hydrologic flows of five basins of Ethiopia. This study reviewed climate change impact studies in five basins of Ethiopia: Blue Nile, Awash basin, Baro Akobo, Central Rift valley and Tekeze basin. Several studies approved that Ethiopia is vulnerable to climate change since the economy of the country mainly depends on agriculture, which is very sensitive to climate change. Water harvesting technology is very important to minimize climate change impacts.

Ethiopia. This study reviewed climate change impact studies in five basins of Ethiopia: Blue Nile, Awash basin, Baro Akobo, Central Rift valley and Tekeze basin.

THE IMPACTS OF CLIMATE CHANGE ON HYDROLOGIC FLOWS OF ETHIOPIA 3.1. Climate change impacts on hydrologic flow of Blue Nile basin
The study conducted on Lake Tana basin using different Global Climate Model (GCM) showed that there was a significant decline in mean annual flow for the different time periods. This study added the investigation on the possible impact of climate change on the annual changes in actual ET, soil moisture, surface runoff, and groundwater for the periods of 2046-2065 and 2080-2100. The results indicated that AET increases considerably, on the other hand, soil moisture showed little change (between 0% and -2% decreases). Groundwater flow and surface runoff projected reduced for future periods (Setegn et al., 2011). Other study conducted Lake Tana subbasin to predict the impact of climate change on water resource using different GCM. The study predicts a reduction of annual runoff by 12.6%-18.2% for future periods (Tarekegn and Tadege, 2006). The other study conducted on this catchment to investigate about impact of climate change on hydrological behavior. The study showed that in summer season, (June -September) in which the basin receives above 70% of the rainfall amount, the precipitation increased by about 2.6 and 5.7% for ECHAM and IPSL, respectively. At the end of the 21st century ECHAM and IPSL project again an increase in precipitation by about 3.5 and 5.8% respectively. And this study summarizes, the precipitation changes by small amount or show a significant decrease in some of the summer months like June when the basin receives more than 70% of the total precipitation. The significant rise in winter precipitation for both future periods could indicate that there would be also seasonal shift in precipitation due to the changing climate. And this study added potential evapotranspiration is expected to increase for future periods. At the end of the 21st century potential evapotranspiration is expected to increase in all months of the year. The minimum and maximum increase in potential evapotranspiration is 3.9% and 15% in January and December respectively (Enyew et al., 2014).
The other study conducted on the Upper Blue Nile River Basin (UBNRB) showed that there might be an increase in mean annual precipitation for three time windows (2030s, 2050s and 2080s) under all scenarios in the range of 2.1% to 43.8% under the A2a and RCP8.5 scenarios respectively. At 2080s, the maximum relative change of mean annual precipitation is expected to be 43.8% under RCP8.5 scenario, while the minimum relative change of mean annual precipitation to be 2.1% at 2050s (Mekonnen and Desse, 2016).
The study conducted on Megech catchment of Blue Nile showed that the mean monthly precipitation shows both increasing and decreasing trend for both A1B and B1 scenario for future period . It is likely to be increased in the range of +6.7% to +34.5% and +11.0% to +38.89% while it is projected to be decreased in the range between -1.14% to -31.88% and -1.6% to -36.42% for A1B and B1 scenarios respectively (Abebe and Kebede, 2017).

climate change impacts on hydrologic flow of Awash basin
The study conducted on Lake Haik of Awash basin showed that water level fluctuation is higher. The study approved that the water level became decrease through time (Tadess et al., 2011). Workiye (2009) observed that the water level of Lake Haiq was appreciably lower at the time of their visit in January 1969 than in May 1938(Tadese et al., 2011 According to MoWR (2000) water storage capacity of Lake Haiq was 1034×10 6 m 3 but in 2007 the storage capacity reduced to 988.7×10 6 m 3 Molla et al., 2007). Other study conducted by Abi (2016) showed that the annual average flow volume showed an increasing trend and the maximum increment was reached up to an average of 14.90% from 2017 to 2100.

Climate change impacts on hydrologic flow of Rift valley basins
The impact of sedimentation and climate variability on the hydrological status was investigated by Belete (2013). The author showed correlation between the water level variability and the occurrences of the El Niño-Southern Oscillation (ENSO) phenomenon, and approved that the water level of Lake Hawassa became rise due to climate variability impacts.
The other study conducted at Bilate watershed in the Ethiopian Rift valley basin using GCM and SWAT out puts, the results showed that average total seasonal flow shows decreasing pattern in month of March to September for A2a scenario but for B2a scenario it is almost constant. But in months of October to February there might be an increasing pattern of average total seasonal flow in both scenarios (2020s, 2040s and 2070s) (Tekle, 2014).

Climate change impacts on hydrologic flow of Genale-Dawa basin
The research conducted on Weyb watershed in Genale-Dawa basin using GCM and SWAT model showed that, precipitation will increase in the dry season and decrease in the wet season (the main rainy season) for both A2a and B2a scenarios. The overall increase in mean annual precipitation for both A2a and B2a scenarios (Shawul et al., 2016). In all future time series, for the months of November and December, the projected rainfall amounts could increase with large magnitude compared with the baseline period, for both A2a and B2a scenarios. But for the rainiest months of the year, August and September, the model shows a decrease in each future time series for both A2a and B2a scenarios. For the months of January, February, March, and April, the projected rainfall amount increased from the baseline period except for January and February 2080s which presented a decrease for both A2a and B2a scenarios. On the other hand, for the months of May, June, July, and October, the projected rainfall amount in all future time horizons shows a negligible change from the baseline period for both A2a and B2a scenarios. Future mean annual precipitation with a range of 909.01 mm to 1163.47 mm for years 2040 and 2061, respectively, for A2a scenario, and 954.85 mm to 1136.57 mm for years 2042 and 2090, respectively, for B2a scenario. It shows substantial variability of total average annual precipitation from year to year (Shawul et al., 2016).

Climate change impacts on hydrologic flow of Tekeze basin
The mean annual precipitation in the future increases over Tekeze basin. The change in mean annual precipitations varies from + 32 to +40% for mid rang RCP4.5 and high level RCP8.5 scenario over all future time periods. In all future time periods, Kiremt season (June-September) precipitation showed an increasing trend and varies from + 21 to + 48%, but in Belg season (February-May) precipitation amount decreased and varies from -15 to -52% under both RCP4.5 and RCP8.5 climate scenarios. The Bega season (October-January) showed a mixed trend where precipitation decreased up to -23% in 2050s under RCP4.5 and -39.5% in 2020s (2011RCP4.5 and -39.5% in 2020s ( -2040 for RCP8.5 scenarios. On the other hand, precipitation increases from + 5.7 to + 50% under both RCP4.5 and RCP8.5 climate scenarios (Fantaw et al., 2018).
In Tekeze basin, climate scenarios produced moderate increase in mean annual streamflow which is due to projected increased in precipitation. However, RCP8.5 climate scenario in 2080s (2070-2100) showed a decrease in streamflow due to high increased in projected temperature that leads to increase in evapotranspiration than increased in precipitation. The mean annual streamflow varies from 39 to 50% in RCP4.5 and 10 to 23% in RCP8.5 climate scenarios but decreased in 2080s (2070-2100) up to 23% under RCP8.5 climate scenarios (Fantaw et al., 2018).

Summary
Several studies conducted in Ethiopia strongly suggest that, water resource of the country respond to global warming in ways that will negatively affect the water availability and water supplies. The studies conducted in Upper Blue Nile basin, Awash Basin, Rift Valley basin, Genale-Dawa basin and Tekeze basin of Ethiopia showed that, the climate change has the sever impacts on hydrologic flows of the country. Due to erratic rainfall Journal of Natural Sciences Research www.iiste.org ISSN 2224-3186 (Paper) ISSN 2225-0921 (Online) Vol.10, No.3, 2020 distribution (because of climate change); declining the surface water, increased evapotranspiration, lower flows and river became decline in one season and highly increased in another season on the country. The decrease in the runoff volume will lead to the decrease in the inflow to the reservoirs consequently; longer period might be required to fill the reservoir (Best example is like koka reservoir). As result of the rise in temperature the rate of evaporation from the reservoir open water surface may increase and this may create the reservoir to fail to supply at least the required amount of demand because of its depletion or decrease in its storage volume and/or water level. On the other hand, climate change affects water resource through increasing runoff that leads to sedimentation and flooding. In addition, climate change affects hydrologic flow of Ethiopia through unhealthy distribution of rainfall that leads to drought in one season and flooding on another season. Water harvesting technology is very important to minimize climate change impacts.