Climate change modeling for water resources management : Tana Sub-Basin, Ethiopia

Abstract

This study, conducted in the Tana Sub-basin, Ethiopia, aimed to model the impact of climate change on water resources management. The Soil and Water Assessment Tool (SWAT), SPI generator, and RStudio were employed to conduct a comprehensive analysis of climate variability, hydro-climatic extremes, and the impact of land use land cover change on water resources within the region. The findings highlight the significant impacts of both climate variability and land use land cover change on water resources management in the Tana Sub-basin. Changing climate patterns and hydro-climatic extremes were identified as key factors posing increasing challenges to water availability and sustainable management within the region. In analyzing the variability and trends of climate parameters in the Tana Sub-basin, various statistical methods and indices were employed to assess precipitation and temperature patterns. The findings indicated a statistically non-significant increasing trend in rainfall across the Sub basin, with values ranging from 1.64 to 5.37 mm/year. With regard to temperature, an increase was observed, but it was also not statistically significant. The seasonality index ranged between 0.87 and 1.03. In 36.69% of the Sub-basin, rainfall occurs in marked seasonal patterns with a long dry season and the remaining 63.31% is concentrated in three or fewer months, indicating a different rainfall distribution pattern. The assessment of precipitation concentration found that 57.5% of the rainfall data exhibited strong irregular concentration; 41.5% showed irregular concentration, and 1% exhibited moderate concentration. This decreasing trend in projected mean annual precipitation and increasing trend in temperature under the RCP4.5 and RCP8.5 scenarios from 2020 to 2100 indicated significant changes in climate conditions in the Tana Sub-basin. In conclusion, the study underscores the presence of climate change, variability, and trends in the Tana Sub-basin, highlighting the need to align agricultural and water resource management practices with the observed climate variability. Hydro-climatic extremes in the Tana Sub-basin, including drought events, were investigated using several statistical measures and tests including coefficient of variation, seasonality index, precipitation concentration index, Mann-Kendall trend test, and Sen’s slope estimator. Three categories of drought were identified: meteorological drought, agricultural drought, and hydrological drought. The frequency of meteorological drought ranged from 1.05% to 10.04%, agricultural drought from 1.26% to 12.21%, and hydrological drought from 0.21% to 14.5% in the study area. The variability in drought occurrence indicates that certain areas and seasons in the Tana Sub-basin are more susceptible to drought conditions than others. The observation of a potential trend in drought and wet extent occurrences in the study area until 2100 suggests that it may experience significant shifts in hydro-climatic patterns due to climate change. This highlights the importance of considering both spatial and seasonal patterns when assessing drought risks and implementing appropriate measures for water resource management and agricultural practices. In conclusion, the study emphasizes the significance of understanding spatial and seasonal variations in drought occurrence and aligning agricultural practices and water resource management accordingly. This study also assessed the impacts of land use land cover change on water resources using the Soil and Water Assessment Tool. Three land use land cover maps (1986, 2000, and 2014) were analyzed to assess and quantify the changes and their impact on water resources. The findings indicated that changes in land use land cover have a significant impact on various components of the water balance in the study area. Compared to the baseline year of 1986, the average annual water yield increased by 14.88% and 12.6% in 2000 and 2014, respectively. Baseflow exhibited an increase of 18.4% in 2000 but decreased by 7.16% in 2014. Surface runoff increased by 12% and 16.16% in 2000 and 2014, respectively. Evapotranspiration decreased by 18.39% and 13.49% in 2000 and 2014, respectively. The expansion of cultivated land and the decline of forestland and grassland have implications for water resources and hydrological processes. The study thus demonstrates that land use land cover changes in the Tana Sub-basin have significant implications for surface runoff, water yield, evapotranspiration, and baseflow. The analysis of water consumption by different sectors in the Sub-basin in 2020 indicated total annual water consumption for irrigation and horticulture of approximately 555.76 and 46.52 MCM, respectively. Livestock consumes about 31.78 MCM annually. Urban and rural domestic iii water consumption is estimated to be around 22.03 and 33.43 MCM/year, respectively. The highest water-consuming sectors in the Sub-basin are rainfed agriculture and hydropower, accounting for more than 3,700 MCM annually. The projected increase in water demand, as indicated by the estimated water requirements of 6079.01 MCM in 2025, 6423.99 MCM in 2030, and 7519.93 MCM in 2035, emphasizes the urgency of sustainable water resource management. The study proposes adaptive strategy options for integrated water resource management as a crucial step in addressing the scarcity of water and ensuring sustainable use of water resources in the area. Assessing and updating water demand and available water resources are fundamental for informed decision-making and effective management of water resources in the Tana Sub-basin. Implementation of the adaptive strategy options outlined is essential to mitigate the challenges posed by increasing water demand and consumption, ultimately contributing to sustainable management of water resources in the region