3.3.3. Shalla Sub-Basin

The response of this sub-basin to the analysis in the model indicates a stronger range of variations in its water balance components. However, the Shalla sub-basin has a lower annual runoff amount than the Ketar and Meki sub-basins (Figure 5c). However, the changes in annual runoff vary between −21.9% and +32.8% from the baseline data simulation outputs. The average annual changes in WY vary from −10.1% to +12.0% because of the impacts. The changes in ET vary from +7.8% to +15.1%. The detail annual variations in percentage for each CSc and each component in each sub-basin are indicated in Table 8. ET increases significantly between June and September for all RCP projections. ET is the largest component, and most of the rainfall turns into ET. Because of the high ET and the small runoff, the entire sub-basin is characterized as a water-scarce region. The WY result for the Shalla sub-basin was moderate for all the CSc. Compared to other previous studies (for example Ayenew, 2007; Gadissa, et. al., 2018), Shalla has small WY output, but in the analyses conducted in this study, the sub-basin yielded a relatively higher amount [5,32]. The difference could possibly be due to its complex hydrogeologic setting that needs to be verified in further studies. However, there is agreement on the fact that its surface water availability will be depleted due to the high ET and the low Q occurrences.

The projected monthly average values of each of the water balance components in each sub-basin with their respective baseline monthly average output values for each of the scenarios are presented in Figure 6. It indicates that the hydroclimatic impacts in the future in the CRVB are very high. The baseline data outputs are indicated with yellow rings around their graphs.

**Figure 6.** Monthly average values of 30 years of surface runoff (Q), water yield (WY), and evapotranspiration (ET) in the Ketar, Meki, and Shalla sub-basins for different climate scenario simulations in relation to the baseline data simulation outputs.

#### **4. Discussion for Water Management Options**

From the projected analyses of the impacts of climate change in the model, the major water balance components such as surface runoff and water yield are mainly expected to decrease, and evapotranspiration is projected to increase in the sub-basins. This will have an impact on the increasing demands for agricultural water in the sub-basins. Seasonal shifts in the patterns of the projected water balance distributions were also observed. Therefore, water management strategies that help mitigate the impacts should be identified and applied. Their application might help to face the food security challenge caused by the water shortage that would occur due to climate changes.

Based on the resulting projected water balances, agricultural water management in the Ketar sub-basin should, in the future, focus on the time modification of farm operations, and on water harvesting to store excess water occurring in the unusual months. Scarcity of water for agriculture is inevitable from the analyses (Figure 5a). Therefore, water saving, and water use optimization must be sought and applied in the future. The WY is the major water balance component of the Ketar sub-basin in all the scenarios, and its enhancement together with conservation, will make the basin rich enough in water to curb the impacts of climate change. In addition, irrigation water supply scheduling based on the modified climate pattern is the recommended method of agricultural water management for the Ketar sub-basin.

High water losses through ET in the Meki sub-basin can be mitigated by water management interventions such as crop mulching, farm operations during minimum evaporation seasons, favoring minimum tillage to reduce soil evaporation, selecting crops that are more resistant to high levels of evaporation, favoring efficient irrigation water application, and introducing regular soil and water conservation practices to reduce the high seasonal runoff and ET. In the Meki sub-basin, water harvesting and storage during periods of high runoffs can also reduce water scarcity during peaks in demand. High runoff management and protection infrastructures are also inevitable as there will be untimely and repeated higher runoff expected beyond the usual baseline trends, as per the analysis.

The high ET rates and low runoff makes the Shalla sub-basin a water-scarce region. The water scarcity problem in the sub-basin should be mitigated by improving WY via yield enhancement approaches that also help to reduce evaporation losses. These include soil and water conservation to improve subsurface storage, crop selection, farm operation scheduling based on the new climate pattern and minimum tillage to reduce soil evaporation, and the selection of highly ET-resistant crop varieties. Investigating afforestation for controlling ET losses, and controlled farm operations are also very crucial. Furthermore, inter-basin water transfers are recommended for adapting to the impacts on the sub-basin.

A study conducted by Kassie et al. (2015) applied an effective fertilizer with irrigation water as an adaptation measure to climate change for the maize crop in the CRVB. The study assessed the potential impacts of climate change on maize yield and explored specific adaptation options under climate change scenarios for the CRVB of Ethiopia by mid-century. They used GCM, RCPs, and crop models to search for adaptation options. The climate change impacts in their study are consistent with our study results. Their adaptation option offsets the severe impacts of yield loss in the area due to the climate impacts [34]. Thus, the effective application of fertilizer while producing the maize crop in the region together with irrigation water is crucial. In addition, the positive effects of changing the planting date were indicated in their study in offsetting the severe climate impacts on the maize crop [34].

Amare and Endalew (2016) assessed the importance of farm mechanization in rural Ethiopia for smallholder farmers. In their assessment, they indicated that mechanized farming helps in reducing water loss at the farms [57]. The study results showed that water distribution efficiencies in irrigated farms have been improved in the study regions, including the CRVB. This may be achieved by incorporating land use planning in a manner that its water allocations and use efficiencies will improve, for instance, farm mechanization and land leveling to minimize water loss and enhance even distribution [57]. Therefore, extensive farm mechanization and land leveling works are recommended as a means to improve water use and reduce its loss in the sub-basins' irrigated farm fields. These will help to increase the resilience capacity of the CRVB to the impacts of future climate changes.

Adaptation to climate impacts via water allocation planning based on weather, soil, and ecological characteristics and social benefit priorities can also reduce the unnecessary loss that may occur due to misallocation and weather variabilities. For instance, the cropping pattern alternatives that favor better gain based on the rainfall patterns of the rift valley region were adopted by some farmers, as indicated in the study conducted by Belay et al. (2017) [2]. The farmers applied a method of using different crop varieties of maize during long rainy seasons and during short rainy seasons. This has improved the gain in the worst water shortage seasons in the region, as reported in [2]. Accordingly, preparing alternative plans for seasonal climate change conditions for agricultural production, and for water use plans that can mitigate the dual impacts of climate and environmental changes while maximizing the benefits during the worst climate seasons are thus necessary. Hence, the possible alternative plans and the locally adopted measures by the farmers should be further assessed, tested, and applied in the worst seasons in the CRVB and in similar regions in the country. The plans need to be based on reliable data and on studies carried out for particular areas. This study aims to contribute to such a knowledge helping in the creation process of such adaptation plans for the CRVB in Ethiopia.

In addition, Kifle and Gebretsadikan (2016), conducted an experiment on the controlled application of irrigation water for potato production in the water-scarce region of Tigrai in Ethiopia [58]. They found positive effects of controlled irrigation water applications on potato production without losses for the deficit application of water with proper timing as means to curb water shortage due to climate changes. One of the best adaptation options for agricultural water uses in the sub-basins is thus the introduction of controlled irrigation that applies the water resources efficiently and that applies only the required amount of water at the proper time for effective use of the crops [58]. Controlled irrigation also helps avoid seepage and salinity problems via water applications to the required depth [58]. In addition, selecting fast-growing, highly productive quality seeds will help to save the resource for other economic and social uses. Controlled irrigation is thus recommended as a mitigating strategy for water scarcity and for environmental challenges that would occur due to the impacts of climate change and population growth.

For the CRVB, Musie et al., (2020) used SWAT models to assess the water conditions of terminal lakes in the CRVB and water management adaptation options. They recommended avoiding pollution of water sources and conserving the terminal lakes from pollution damages, both from sedimentation and other environmental pollutants. Thus, controlling the water level of the lakes, avoiding water quality degradations due to industrial and environmental wastes, and improving the storage capacity in the sub-basins will favor better use of the resources during peaks in demand [20].

Climate-based integrated development and use plan for the utilization of water resources according to its economic and social benefits, while safeguarding environmental sustainability, should be further assessed, modeled, and applied for its equitable use in equilibrium in the closed CRVB. Moreover, considering the response of the sub-basins to hydroclimatic impact while planning water use is crucial.

#### **5. Conclusions**

This paper investigated the impacts of future climate change on the major components of the water balance in the central rift valley basin in Ethiopia from the seasonal and spatial points of view. The evaluations are based on the magnitude of water yield, evapotranspiration, and surface runoff components change in relation to the baseline data outputs. Regional climate models (RCM) data in CORDEX—Africa were applied for the investigation. RCP data from the MIROC-RCA4 ensemble driving climate models were downscaled, bias-corrected, and used for the analyses. The methodology followed a calibrated Arc-SWAT modeling approach to search for basin-wide climate impacts on water resources and to indicate possible agricultural water management and adaptation strategies. The findings are solely based on model simulation outputs within the scope of its evaluations and error limitations.

Accordingly, the study identified a general decrease in water yield and surface runoff and a seasonal increase in evapotranspiration in the Ketar and Shalla sub-basins in both the near-term (2031–2060) and long-term (2070–2099) periods in comparison to the baseline period (1984–2010). However, all three water balance components projected were showing an increment in the Meki sub-basin for all the periods. The sub-basins were also found to be heterogeneous, and they showed variabilities in terms of their hydroclimatic reactions to the impacts of climate change even though they are in one endo hydrogenic region. In the sub-basins, some similarities were also found in the ways in which the pattern of the water balance components will be changed. However, the magnitudes of the impacts varied from sub-basin to sub-basin, between the RCPs, and between near-term and long-term periods due to the projected climate changes. These indicate that each of the sub-basin has a unique water balance environment.

The study also indicated the huge impacts of regional climate models (RCM) on surface components of the regional water cycle. These RCMs are a derivative of the Global Circulation Models (GCM).

The management interventions to mitigate the climate impacts should therefore be carried out according to the sub-basin water balance sensitivities while keeping the equilibrium in the closed CRVB water requirements. Finally, an investigated integrated watershed, agricultural water use, and farm management in the water–agriculture–land and climate nexus approaches following each sub-basin's climate responses, and other alternative resource management options for the closed CRVB must be determined and applied to cope with the hydroclimatic impacts.

The calibrated SWAT model has proved to be a useful tool for analyzing and identifying the temporal and spatial conditions of the water resources at a basin level under different climate change conditions in the CRVB. Therefore, further studies dealing with climate-based water resource management in combination with farming practices using the SWAT model would bring additional benefits.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/w15010018/s1, Table S1: Some physical properties of major soils in the CRV sub-basins; Table S2: Location of meteorological stations used for the analysis of the weather parameters in the CRVB; Table S3: SWAT land use code and their description.

**Author Contributions:** Conceptualization, L.A.T.; Methodology, L.A.T. and K.B.; Software, L.A.T. and K.B..; Validation, L.A.T., S.M. and K.B.; Formal Analysis, L.A.T.; Investigation, L.A.T. and K.B.; Resources S.M.; Data Curation, L.A.T. and K.B.; Writing—Original Draft Preparation, L.A.T.; Writing— Review & Editing, K.B.; Visualization, L.A.T., Svatopluk Matula; Supervision, Svatopluk Matula; Project Administration, S.M. and K.B.; Funding Acquisition, S.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was partly supported by the Czech National Agency for Agricultural Research; NAZV (project number QK1910086), and by the Czech University of Life Sciences Prague, Faculty of Agrobiology, Food and Natural Resources (project number SV21-20-21380).

**Data Availability Statement:** The source data and materials used in this study will be made available upon reasonable request from the corresponding author.

**Acknowledgments:** We acknowledge the financial support from Czech National Agency for Agricultural Research. We would also like to thank the Ethiopian Meteorological Agency, the Oromia Bureau of Agriculture and Natural Resources, the Ethiopian Ministry of Water Resources, the Ethiopian Geospatial and Information Institute, and the Ethiopian Ministry of Agriculture and Natural Resources for providing the data and for their support during data collection. We would also like to thank Robin Healey for his suggestions and comments relating to the text. We would also like to acknowledge the contribution of the anonymous reviewers who helped us to bring the manuscript into the present form.

**Conflicts of Interest:** We declare that we have no known competing financial interest or personal relationships that could appear to have influenced the work reported in this paper.

#### **References**


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