<p>In recent decades, climate change has become a major threat to agriculture, ecosystems biodiversity, and food security. Climate change and anthropogenic factors cause long-term variability in the meteorological parameters of a watershed, leading to changes in the hydrological cycle. Compared to other regions of the world, climate change is predicted to be of the highest risk in Africa due to economic, demographics, education, natural, and governance factors. The long-term change in hydro-climatic variables is expected to have negative impacts on developing countries such as Ethiopia, where more than 80% of the livelihoods depend on agriculture, mainly rain-fed agriculture. Water resources in Ethiopia are highly vulnerable to climate and hydrological variability due to their topography and humaninduced factors, such as land degradation, high population density, and water management practices. The change in temperature and precipitation have a direct influence on water resource availability and hydrologic processes like evapotranspiration and runoff. Consequently, the variability and change in water availability affect agriculture, industry, and urban development. Climate change is already causing a significant challenge in Ethiopia by affecting water availability and increasing the frequency of natural disasters such as droughts and floods. In addition to climate change, land degradation, deforestation, and population growth are the major challenges that have an adverse effect on water availability, agriculture, and food security in Ethiopia. Understanding the long-term variability and change in climate variables and identifying the dynamics of environmental change is fundamental for the development and planning of sustainable and efficient environmental management systems and strategies. Furthermore, implementation and adaptation of appropriate climate change interventions require adequate knowledge of location-specific perceptions towards changes and challenges as well as opportunities. Even though several studies have been undertaken concerning hydrology and climate change in different parts of Ethiopia, little attention has been given to the hydro-climate variability and impacts of climate and environmental change in the Awash River Basin (ARB). Moreover, the influence, extent, and severity of the climate and environmental changes on water availability, irrigation, and agriculture have not been fully studied and quantified. Therefore, this study aimed to investigate the long-term hydro-climate variability and land-use dynamics and to predict future climate change and its impact on water resources using remote sensing, GIS, hydrological, and climate models in the ARB of Ethiopia. This study has five research chapters.</p> <p>The first research chapter attempted to characterize, quantify and validate the variability and trends of hydro-climatic variables in the ARB of Ethiopia using graphical and statistical methods. The rainfall and streamflow trends and their relationships were evaluated using the regression method, Mann–Kendall (MK) test and correlation analysis. The analysis focused on rainfall and streamflow collected from 28 and 18 stations, respectively. This research identified that about 85.7% and 75.3% of the rainfall stations exhibited normal to moderate variability in annual and June to September rainfall, respectively, whereas 96.43% of rainfall stations showed high variability from March to May. The MK test showed that most of the significant trends in annual rainfall were decreasing except in two stations.</p> <p>The second research chapter analysed the long-term dynamics of land-use/land-cover changes and population growth in the ARB using remote sensing and Geographic Information System (GIS). Landsat images for 1988, 2002, and 2018 were processed, classified, and analysed. The accuracy assessment showed that the classification was relatively acceptable and effective in detecting the long-term land-use changes in ARB. The results of this research showed that cropland increased by 12% between 1988 and 2002, and by 2018 it had increased by 15%. Similarly, the built-up area expanded by 52 km² (184%) between 1988 and 2002, and by 2018 it had reached 225%. The analysis showed that the cropland and built-up area expanded at the expense of forest and shrubland, with shrubland and forest being reduced by 4% and 25% respectively over the 30 year study period. Higher levels of deforestation, combined with population growth, urbanization, and cropland expansion, have impacted the available water resources and runoff in the area. The findings from this study can help in the design of sustainable environmental management strategies and practices to ensure the sustainability of the ecosystem and natural resources. The results can also be used to address food security issues in the ARB since there is a notable increasing trend in population growth with a commensurate decrease in agricultural land, thereby increasing food security concerns.</p> <p>The third research chapter aimed at projecting and analysing climate change in the ARB using bias-corrected Global and Regional Climate Model simulations. The analysis included a baseline period from 1986 to 2005 and two future scenarios (the 2050s and 2070s) under two representative concentration pathways (RCP4.5 and RCP8.5). Following the evaluation of bias correction methods, the Distribution Mapping (DM) and Power Transformation (PT) were used for temperature and precipitation projection, respectively. According to this research, the 2050s and 2070s RCP4.5 simulations showed an increase in precipitation during half of the months with 32 and 10%, respectively. Moreover, the 2050s and 2070s RCP8.5 simulation indicated a decrease in precipitation with 18 and 26%, respectively. The 2050s and 2070s RCP8.5 simulation indicated a significant decrease in precipitation in four of the months (February/March to May) with the highest decreasing rate of 34.7%. The 2050s and 2070s RCP4.5 simulation showed an increase of 0.48-2.6 °C in maximum temperature. In the case of RCP8.5, the simulated maximum temperature increase reached 3.4 °C and 4.1 °C in the 2050s and 2070s, respectively. The future precipitation and temperature change projected in ARB might worsen the water stress and incidence of dry spells in the basin, and hence mitigation strategies and management options to reduce this negative impact should be developed.</p> <p>The fourth research chapter identified the variability of potential evapotranspiration (PET) and water availability in the ARB under different climate change scenarios and relates it with the long-term drought occurrences in the area. The PET and water availability of the ARB were estimated during the period of 1995–2009 and two future scenarios (the 2050s and 2070s). The representative concentration pathways (RCP4.5 and RCP8.5) simulations showed an increase in the monthly mean PET from March to August in the 2050s, and all the months in the 2070s. The study also identified a shortage of net water availability in the majority of the months investigated and the occurrence of mild to extreme drought in about 40%–50% of the analysed years at the three study locations namely; Holetta, Koka Dam, and Metehara. The decrease in water availability and an increase in PET, combined with population growth, will aggravate the drought occurrence and food insecurity in the ARB. Therefore, integrated watershed management systems and rehabilitation of forests, as well as water bodies, should be addressed in the ARB to mitigate climate change and water shortage in the area.</p> <p>The fifth research chapter investigated the perceptions of farmers towards climate and environmental change and the impacts of those changes on their farming systems, water resources, and irrigation. The study used a semi-structured questionnaire where a total of 201 rural households from nine different villages in ARB of Ethiopia were interviewed. The data collected were analysed using descriptive statistics, quantitative data analysis, and multinominal logistic regression by applying the Statistical Package for Social Scientists (SPSS) software. The finding of this research showed that about 80, 97 and 98% of the respondents perceived the occurrence of climate variability in the Upper, Middle, and Lower ARBs, respectively. Between 68-80% of the respondents observed the variability of rainfall through an increase or decrease of rainfall at different seasons of the year. More than 70% of the respondents also confirmed that there was a temperature change in their areas. The present study identified that most of the farmers in the ARB perceived the occurrence of climate change, including its impact on their farming systems, river water availability, and irrigation practice. The results also showed that almost all of the interviewed farmers in the ARB lack adequate training and knowledge of adaptation strategies towards climate and environmental change. The high awareness of climate change and environmental variability by farmers in the ARB may help decision and policymakers in establishing participatory climate adaptation and mitigation strategies.</p> <p>In general, the findings of this research provide valuable information on the characteristics, variability, and trend of rainfall and streamflow necessary for the design of sustainable water management strategies that could be applied to reduce the impact of droughts and floods in the ARB. Moreover, the outcomes of this study will provide statistical data on the current and future climatic and non-climatic changes in ARB. This will help the government, policymakers, private investors, and the community to design and implement sustainable participatory environmental management strategies and techniques with a view of reducing the risk of climate change in the ARB.</p>