Africa Center of Excellence for Water Management

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Browsing Africa Center of Excellence for Water Management by Subject "Analytical Hierarchical Process"

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    Modelling Spatial Patterns of Runoff, Soil Erosion, and Best Management Practices in the Dirima Watershed, Tana Basin, Upper Blue Nile, Ethiopia
    (Addis Ababa University, 2025-03-24) Simir Birihan; Tenalem Ayenew (Professor)
    Soil erosion is a pressing issue in worldwide watersheds, particularly in the Ethiopian highlands which leads to land degradation, deterioration of water resources and reduced agricultural productivity. This problem is not evaluated and researched well particularly in the study area of the Dirima watershed, which spans 162 km² within the upper Tana basin. This study addressed these challenges by modelling runoff-generating areas (RGAs), soil erosion dynamics, and evaluating Best Management Practices (BMPs) using hydrological modelling. To do this, the study was focused on four key objectives: assessing hydrological responses from three soil layers using the XAJ model, identifying and quantifying spatial runoff generation and soil erosion hotspots through SWAT+ and AHP, developing rating curves to estimate streamflow and sediment yields, and evaluate best management practices (BMPs) using the SWAT+ model. The hydro-climate datasets were used for model’s parameter calibration for 1996-2005 and validation for 2006-2009. This study successfully calibrated the XAJ hydrological model for the Dirima watershed in Ethiopia using the DEoptim algorithm, achieving baseline performance measures of RMSE = 12, NSE = 0.76, PBIAS = 10.5%, and R2 = 0.78, and validation performance of RMSE = 3.65, NSE = 0.85, PBIAS = 9.9%, and R2 = 0.85. Projections using CMIP6 climate scenarios (SSP2-4.5, SSP3-7.0, SSP5-8.5) indicate future average Tmax increases of 1.59°C, 1.93°C, and 2.48°C, and Tmin increases of 1.83°C, 2.33°C, and 2.85°C, respectively. Changes in the upper soil layer are projected as follows: evaporation (1.72, 1.71, and 1.79mm), soil moisture (0.63, 0.62, and 0.64mm), and runoff (1.07, 0.97, and 1.18mm) in SSP2-4.5, SSP3-7.0, and SSP5-8.5, respectively. Critically, projected precipitation and streamflow values are consistently lower than ET and temperature across all SSP scenarios, suggesting potential water scarcity challenges requiring proactive water resource management strategies. Analysis of the Dirima River, known for its floods and sedimentation affecting Kolladiba town, involves establishing stage-discharge and sediment concentration-discharge relationships. This study uses a power function for stage discharge, divided into two distinct stage regimes (0.3-3 m and 3-6 m), reflecting varying flow conditions. Sediment transport is modelled linearly across three discharge regimes (0-42, 42-190, 190-560 m³/sec) to account for sediment transport processes and behaviours. These curves are crucial for estimating discharge, flood forecasting, and sediment management in the Dirima watershed, enhancing understanding of watershed responses to environmental changes. Field data updates are essential for model accuracy. The Dirima watershed's water balance analysis reveals significant losses, with acceptable model performance of SWAT+ (Calibration: R²=0.71, PBIAS=-0.9, NSE=0.7; Validation: R²=0.84, PBIAS=-28, NSE=0.76). Prediction uncertainty is characterized by p-factors (53%/58%) and r-factors (61%/67%). Surface processes (53%), soil water (20.9%), and groundwater (20.2%) drive the water balance. High rainfall loss (78-80%) via runoff and evapotranspiration indicates limited watershed storage. In the dry season, evapotranspiration, lateral flow, and return flow increase by 59%, 0.9%, and 39%, respectively. Declining aquifer storage threatens water supplies for Gondar City, Kolladiba town, and industries. Customized water management strategies are crucial to address these challenges. The SWAT+ model and AHPs approaches were used to categorize the Dirima watershed into five RGAs ranging from 1 very low (VL) to 5 very high (VH). This categorization resulted in the VL category accounting for 8%,566 mm of runoff and a sediment yield of 33 t/ha/yr. The "L" category, represents 17% with 606mm of runoff and 53.5 t/ha/yr of sediment yield. The M, category 32% of the catchment, generates 662mm of runoff. The H and VH categories, covering 27% and 16%, produce substantial runoff of 682 and 690.2mm, respectively. Sediment yield reached 186.5 t/ha/year in the VH category. The findings emphasized the need for targeted erosion prevention strategies to mitigate these risks. To mitigate loss, BMPs including terraces (ter), vegetative filter strips (VFS) and grass waterways (GWWY) were evaluated both individually and in combination. The ter reduces surface runoff and sediment yield by 25.46% and 57.32% respectively, while VFS is effective in reducing runoff by 24.58% and sediment by 31.92%. The mean runoff is reduced by 16.97%, and sediment is reduced by 38.21% because of the GWWY. And also, that combining BMPs is synergistically better than individual BMPs alone Comb-1 can reduce runoff by 27.84% and sediment by 62.67%; Comb-2 can reduce runoff by 28.85% and sediment by 64.96%; for the full combination of Comb-3, it has the best performance that can achieve a 33.95% reduction in runoff and a 76.42% reduction in sediment yield. These interventions highlight potential cost savings by preserving soil fertility and improving agricultural sustainability. This study offers insights into sustainable watershed management and supports broader environmental conservation efforts. Also, this study shows the applicability of hydrological models and how much the Dirima watershed contributes to Lake Tana eutrophication and sedimentation. By integrating SWAT+, field data, and BMPs under different climate scenarios, the research provides a framework for effective land and water management strategies in the Lake Tana Basin and similar regions.