Streamflow Sensitivity in Ethiopia’s Upper Blue Nile River Basin: Human Land Use Changes Outweigh Climate Impacts

Illustrative Image: Streamflow Sensitivity in Ethiopia’s Upper Blue Nile River Basin: Human Land Use Changes Outweigh Climate Impacts
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A study by Abebe et al. (2024) titled “Attribution and Streamflow Sensitivity Analysis in the Upper Blue Nile River Basin Using a Top-down Modeling Framework” published in Abyssinia Journal of Engineering and Computing, reveals that streamflow increased in 68% of the sub-watersheds, with land surface changes—such as deforestation, soil degradation, and urbanization—exerting a stronger influence on runoff than climate variability.

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Land use changes—more than climate variability—are the primary drivers of streamflow increases across 68% of Upper Blue Nile sub-watersheds.
– Abebe et al. 2024

This study explores how streamflow in Ethiopia’s Upper Blue Nile River Basin (UBNRB) responds to both climatic variations and changes in watershed characteristics. Utilizing a top-down modelling approach that integrates the Budyko framework with elasticity analysis, the research quantifies streamflow sensitivity and attributes changes to either climatic or land surface factors. The UBNRB, located in northwestern Ethiopia, is a crucial water source, contributing over 60% of the total Nile River flow. It plays a vital role in regional agriculture, water supply, and hydroelectric power. The basin is predominantly humid, with an aridity index ranging from 0.6 to 1.1.

To assess the drivers of streamflow changes, the study uses the Budyko framework—which examines the balance between water availability (precipitation) and energy availability (potential evapotranspiration)—alongside elasticity analysis to measure how responsive runoff is to variations in precipitation, evapotranspiration, and watershed conditions. The WEP hydrological model is employed to simulate streamflow and evapotranspiration, while Pettitt’s test identifies statistically significant shifts in streamflow between a base period (1983–1998) and a change period (1999–2018). Findings reveal that streamflow increased in 68% of the sub-watersheds, with land surface changes—such as deforestation, soil degradation, and urbanization—exerting a stronger influence on runoff than climate variability. While precipitation exhibited a slight upward trend, changes in potential evapotranspiration were minimal. Streamflow elasticity to precipitation was high (ranging from 0.6 to 0.9), indicating that even small changes in rainfall could lead to substantial changes in runoff.

The eastern, southern, and northwestern parts of the basin experienced the most significant hydrological changes, highlighting the urgent need for localized water resource management. The study concludes that human-induced alterations in watershed characteristics can impact streamflow more profoundly than climate change alone, underscoring the importance of sustainable land-use planning to safeguard water resources in the UBNRB.

How the Study was Conducted

The study investigated streamflow variations in Ethiopia’s Upper Blue Nile River Basin (UBNRB), a region in northwestern Ethiopia that contributes over 60% of the Nile River’s annual flow. Researchers employed a top-down modeling approach to assess hydrological changes across 95 sub-basins under predominantly humid conditions (aridity index: 0.6–1.1).

To disentangle the impacts of climate and watershed characteristics on runoff, the Budyko framework was applied, linking precipitation and potential evapotranspiration to estimate water balance. This was complemented by elasticity analysis, which quantified streamflow sensitivity to changes in precipitation, evapotranspiration, land use, soil type, and vegetation.

Streamflow simulation and validation were conducted using the distributed Water and Energy Processes (WEP) model. Calibration and validation spanned 1992–2004 and 2005–2014, respectively, using data from the Kessie and El-Diem gauging stations. Model performance was evaluated using Kling-Gupta Efficiency (KGE), Nash-Sutcliffe Efficiency (NSE), and Percent Bias (PBIAS).

Change-point detection in streamflow patterns was carried out using Pettitt’s test, which identified two periods: a base period (1983–1998) and a change period (1999–2018). Supporting datasets included the HYDRO1k Digital Elevation Model (USGS), ENACTS climate data (National Meteorology Agency of Ethiopia), NASA/USGS land cover data, FAO soil information, and streamflow records from Ethiopia’s Ministry of Water and Energy.

What the Authors Found

The authors found that changes in watershed characteristics—such as land use, vegetation cover, and human activities—had a greater impact on streamflow dynamics in the Upper Blue Nile River Basin than climatic factors like precipitation or temperature.

Why is this important

Securing Water for Millions: The Upper Blue Nile River Basin contributes over 60% of the Nile’s flow, making it critical for water access in Egypt, Sudan, and Ethiopia. Understanding streamflow shifts supports better management of agriculture, irrigation, and flood risks.

Distinguishing Climate vs. Human Impact: By separating the effects of climate variability from human-induced watershed changes, the study highlights where targeted land use reforms or conservation strategies are more effective than relying solely on climate-based solutions.

Localized Watershed Insights: Analyzing 95 individual sub-watersheds reveals region-specific hydrological patterns, allowing for tailored water management strategies and smarter infrastructure investment in high-priority zones.

Robust Planning Tools: Using the Budyko framework and elasticity analysis, the study provides practical tools for predicting water availability and guiding evidence-based policy and engineering interventions.

Future-Ready Policy Guidance: Vulnerable regions like the Eastern and Southern UBNRB require urgent focus. The findings help governments and stakeholders plan for climate resilience and ecosystem protection.

What the Authors Recommended

• Focus on controlling land use changes—such as deforestation and poor agricultural practices—through afforestation, soil conservation, and sustainable land management, as these factors have a stronger impact on streamflow than climate variability.
• Prioritize water monitoring and strategic interventions in the Eastern and Southern headwaters and Northwestern lowlands, where streamflow changes were most pronounced.
• Develop region-specific management plans based on the unique hydrological behavior of each watershed instead of applying uniform policies across the entire basin.
• Incorporate models like the Budyko framework and elasticity analysis into national water policy to improve forecasting, decision-making, and resource allocation.
• Anticipate the effects of urbanization, agriculture, and infrastructure on water systems, and enforce land-use regulations to maintain basin stability.

In conclusion, the study underscores the dominant role of human-induced land surface changes over climate variability in shaping streamflow patterns in the Upper Blue Nile River Basin. By integrating robust analytical tools like the Budyko framework and elasticity analysis, the research offers critical insights for policymakers and water managers to implement targeted, region-specific strategies that promote sustainable land use and safeguard water security across this vital transboundary watershed.