A review of hydroclimate variability and changes in the Blue Nile Basin, Ethiopia.

Understanding the factors that influence hydroclimate variability is crucial for developing sustainable water management strategies in dynamic environments. The Blue Nile Basin, a significant freshwater resource in Africa, is facing challenges related to hydroclimate changes that impact sustainable development. Since the 1970s, the hydroclimate patterns of the region have undergone notable changes, prompting the need for a review of the literature on hydroclimate variability of the basin. Therefore, this study aims to offer a brief overview of the latest literature on hydroclimate variability and changes in the Blue Nile Basin. Based on the review of hydroclimate studies in the basin, it is evident that there have been significant advancements in our understanding of this complex system. However, the review also highlights that there are still areas of research that require further development to provide more comprehensive knowledge of the basin's hydroclimate. The projected intensification of hydroclimate change throughout the 21st century underscores the urgency for continued research efforts. The observed warming trend in the temperature of the basin and the discrepancies amongst research outputs on precipitation changes are important areas that require further investigation. Additionally, the inconsistency in reported changes in the watershed's hydrology and streamflow across the basin emphasizes the need for continued research to understand the factors behind these changes. Overall, this review provides valuable insights into the current state of hydroclimate studies in the basin and highlights the key areas for future research efforts to enhance our understanding of this vital system.

Spatio-Temporal Change of Land Use/Land Cover and Vegetation Using Multi-MODIS Satellite Data, Western Ethiopia.

Land use and land cover (LULC) change and variability are some of the challenges to present-day water resource management. The purpose of this study was to determine LULC and Normalized Difference Vegetation Index (NDVI) fluctuations in western Ethiopia during the last 20 years. The first part of the study used MODIS LULC data for the change analysis, change detection, and spatial and temporal coverage in the study region. In the second part, the study analyzes the NDVI change and its spatial and temporal coverage. In this study, The Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data were applied to determine LULC and NDVI changes over four different periods. Evergreen broadleaf forests, deciduous broadleaf forests, mixed forests, woody savannas, savannas, grasslands, permanent wetlands, croplands, urban and built-up lands, and water bodies are the LULC in the period of analysis. The overall classification accuracy for the classified image from 2001 to 2020 was 85.4% and the overall kappa statistic was 81.2%. The results indicate a substantial increase in woody savannas, deciduous broadleaf, grasslands, permanent wetlands, and mixed forest areas by 119.6%, 57.7% 45.2%, 37%, and 21.3%, respectively, followed by reductions in croplands, water bodies, savannas, and evergreen broadleaf forest by 90.1%, 19.8%, 13.2%, and 4.8%, respectively, for the catchment between 2001 and 2020. The result also showed that the area's vegetation cover increased by 64% from 2001 to 2022. This study could provide valuable information for water resource and environmental management as well as policy and decision-making.

Estimation of groundwater recharge variability using a GIS-based distributed water balance model in Makutupora basin, Tanzania.

Groundwater recharge estimation in the Makutupora basin (1500 km2) is vital considering the potentiality of the basin to Dodoma city. This study aims to apply the WetSpass model to estimate the long-term average seasonal (dry and wet) and annual groundwater recharge for the Makutupora basin. Data required for this study were biophysical data (topography, land use, soil, slope, and depth to the groundwater) and long-term hydro-meteorological data (2000-2020). Data were collected by using both field visits and disk transfer from respective institutions and websites. Hydro-meteorological data were prepared for dry and wet seasons. Raster maps were prepared in ArcMap 10.4 using the Inverse Distance Weighting (IDW) interpolation technique followed by resampling into a 200 × 200 m grid size. Resampled raster maps were converted from raster to ASCII format suitable to input in the WetSpass model. The findings indicated that more recharge was dominating in the wet season ranging between 0 and 120 mm/year with a mean value of 24.65 mm (99%) while less recharge occurs in the dry season ranging between 0 and 4.35 mm/year with a mean value of 0.24 mm/year (1%) and annually recharge ranges between 0 and 120.88 mm/year with a mean value of 24.88 mm. Mean annual precipitation computed from data for twenty (20) years was 694 mm/year out of which recharge accounted for 3.6%, surface runoff 33.9% and evapotranspiration 62.5%. The groundwater table receives total average volumetric recharge of 37.3 million m3 annually from precipitation for the entire basin area. The model was employed to realize the area's capacity for groundwater recharge to manage the water supply effectively, use it wisely, and plan for the future. Sustainable groundwater exploitation can be feasible only when there is knowledge of the rate at which groundwater is replenished annually. Therefore, the results of this study are useful in sustainable management plans, it may also be used as a benchmark for water supply authorities, policymakers and researchers to set proper protection measures and pumping policies.

Delineation of groundwater potential zones of the transboundary aquifers within the semiarid Bulal catchment, Southern Ethiopia.

In the semiarid Bulal transboundary catchment of southern Ethiopia, groundwater is the only reliable drought-resilient water source. The central and southern parts of the catchment are dominantly overlain by the transboundary aquifers of the Bulal basalts, while the basement rocks outcrop in the eastern part. This study uses an integrated geographic information system (GIS), remote sensing (RS), and analytical hierarchal process (AHP) to identify and delineate the groundwater potential zones of the semiarid Bulal catchment within the Ethiopian territory. Based on their relative importance to groundwater occurrence and movement, ten input parameters were chosen. According to Saaty's AHP approach, the input themes and each of their distinct features were given normalized weights. A composite groundwater potential zone index (GWPZI) map was generated by integrating all the input layers employing the GIS-overlay analysis technique. The map was validated using the yield of wells from the catchment. The GWPZI map depicts four groundwater potential zones: high (representing 27% of the total area), moderate (20%), low (28%), and very low (25%). The geological feature has the greatest influence on the distribution of groundwater potential. Areas with high potential are mainly overlain by the Bulal basaltic flow, while low groundwater potential zones are in the regolith over the basement rocks. Unlike conventional methods, our novel approach is effective in identifying relatively shallow GWPZs throughout the catchment, and it can be applied in similar semiarid regions. The GWPZI map serves as a quick guide for effectively planning, managing, and developing the catchment's groundwater resources.

Impacts of climate and land use/cover changes on streamflow at Kibungo sub-catchment, Tanzania.

The impacts of changing climate and land use/cover on streamflow in the Kibungo sub-catchment were evaluated using the Soil and Water Assessment Tool (SWAT). Rainfall, minimum, and maximum temperature data for six stations from the ensemble mean of the RCMs (RCA4, RACMO22T, CCLM4-8-17) were used under RCP 4.5 and RCP 8.5. The homogeneity and trend test were used to detect the change point and identify the pattern in the annual time series, respectively. Land Change Modeler (LCM) was used to predict land use maps of 2040 and 2070 from historical maps. Streamflow was simulated for 2021-2040 and 2041-2070 based on the homogeneity test results. The model calibration (2009-2016) and validation (2009-2016) for streamflow were successful. The homogeneity test detects a change point in 2040. A significant decrease in annual rainfall by 22.9 mm/yr (RCP 4.5) and 57 mm/yr (RCP 8.5) for 2021-2040 and an insignificant decrease was obtained during 2041-2070 under both emission scenarios. The annual temperature increased insignificantly by 0.004 °C/yr under RCP 4.5 while a significant increase of 0.21 °C/yr under RCP 8.5 was observed for 2021-2040. For 2041-2070, a significant increase of 0.016 °C/yr (RCP 4.5) and 0.045 °C/yr (RCP 8.5) was observed. The change in land use/cover resulted in increasing the build-up area (84%), agricultural fields (55.6%), and a decrease in the forest area (10.5%) during 2021-2040. During 2041-2070, the build-up area increased by 32.1%, the agricultural field by 36%, and the forest area decreased by 11%. Streamflow decreased significantly by 65.4 m3/yr (RCP 4.5) and 195.9 m3/yr (RCP 8.5) from 2021 to 2040. An insignificant decrease of 13.7 m3/s (RCP 4.5) and 7.63 m3/s (RCP 8.5) was observed during 2041-2070. This study provides an insight to the managers, planners, and policymakers on environmental protection/conservation for sustainable utilization of water resources at the Kibungo sub-catchment.

Geospatial application on mapping groundwater recharge zones in Makutupora basin, Tanzania.

Management of groundwater systems is indispensable to countries that depend on groundwater as the primary source of community water supply (e.g. Dodoma, Tanzania). Urbanization and industrialization lead to groundwater over-pumping and reduced recharge zones in the basin. This study used Remote Sensing and geospatial datasets to determine the groundwater recharge zones (GWRZ) followed by sensitivity analysis to identify the influence of geologic and hydrologic factors on the variation of the GWRZ in the case of the Makutupora basin, Tanzania. The implementation of weighted overlay analysis aimed to determine the GWRZ using different thematic maps created from land use land cover (LULC), drainage density, lithology, lineament density, rainfall, slope and soil datasets. The analytical hierarchy process (AHP) and multi-influencing factor (MIF) are the multi-criteria decision analysis (MCDA) implemented to assign weights to the selected influencing factors. Either, the map removal method was implemented for the sensitivity analysis. Pumping wells were overlaid to validate the GWRZ map determined. The overlay of seven thematic maps resulted in the GWRZ map being categorized as good (35.79% for AHP and 21.68% for MIF), moderate (40.98% for AHP and 58.39% for MIF) and poor (23.22% for AHP and 19.95% for MIF). Good recharge potential areas lie in an area characterized by thick forest, high lineament and water bodies around the northwestern and central-eastern side of the basin. Validation of GWRZ indicated that 33.33% for AHP and 30% for MIF are in good GWRZ, 41.6% for AHP and 28% for MIF are in moderate GWRZ and 25% for AHP and 42% for MIF are in poor GWRZ. The sensitivity analysis revealed the high effect of GWRZ on the removal of the LULC, lithology and lineament thematic layer in both AHP and MIF-generated GWRZ maps. This implies that the expansion of settlements is not considering recharge zone protection. Lineaments are also a very important factor governing groundwater recharge which needs to be protected. The result displays that urbanization dramatically reduced the potential area for groundwater recharge. Protecting the potential recharge zone from any activity that reduces the recharge is vital for the sustainability of groundwater.

Identification of hydrogeochemical processes in groundwater of Dawa River basin, southern Ethiopia.

Dawa River basin in southern Ethiopia is covered by volcanic, basement, and sedimentary rocks. Locating good quality groundwater is a challenge in most parts of the basin. Statistical analysis and graphical plots of 94 hydrochemical data of groundwater were used as a main tool to acquire an insight into the major processes that control groundwater chemistry. In the volcanic terrain groundwater is dilute (mean total dissolved solids (TDS): 152 mg/l), while salinity is the highest in the sedimentary terrain (mean TDS: 1750 mg/l). NO3 (-) varies from below the detection limit to 433 mg/l NO3 (-). In 26 % of the water samples, nitrate concentration is above the human-affected value, 5 mg/l NO3 (-). In 6 % of the samples, NO3 (-) concentration is above the limit recommended in drinking water, 50 mg/l NO3 (-), by WHO. Concentration range of the other major ions is also high and hydrochemical water types are diverse, suggesting the effect of various hydrogeochemical processes on the water chemistry. Chemical data analysis revealed that in the volcanic and most parts of the basement terrains silicate hydrolysis is the dominant process. Gypsum dissolution is the main process in the sedimentary terrain. Dissolution of gypsum is also important at few locations along dry riverbeds in the semiarid area where the effect of evaporation on the water chemistry is considerable. Loading of factors with K(+) and SO4 (2-), K(+) and NO3 (-), and NO3 (-) and correlation of SO4 (2-) with Cl(-), along with the observed high nitrate concentration, indicate the effect of surface contamination sources on the water quality.