Tesfaye et al. (2026) Vegetation-driven evapotranspiration enhancements modulate the climate in the Nile River basin

Identification

• Journal: Journal of Hydrology Regional Studies
• Year: 2026
• Date: 2026-01-22
• Authors: Samuale Tesfaye, Gebeyehu Taye, Dirk Hölscher
• DOI: 10.1016/j.ejrh.2026.103152

Research Groups

• Tropical Silviculture and Forest Ecology, University of Göttingen, Germany
• Land Resources Management and Environmental Protection Department, Mekelle University, Ethiopia
• African Center of Excellence for Water Management, Addis Ababa University, Ethiopia
• Centre of Biodiversity and Sustainable Land Use, University of Göttingen, Göttingen, Germany

Short Summary

This study investigates how vegetation dynamics influence regional climate in the Nile River basin (NRB) from 1982 to 2020, finding that increased vegetation density primarily causes a cooling effect through enhanced evapotranspiration, offsetting approximately 9.5% of the basin's warming, with weak and spatially inconsistent impacts on precipitation.

Objective

• To examine how air temperature and precipitation, at annual and seasonal scales, respond to changes in vegetation dynamics across different sub-basins, climatic conditions, and vegetation types in the Nile River basin.
• To quantify the sensitivity of air temperature and precipitation to variations in vegetation dynamics.
• To identify the contributions of different biogeophysical mechanisms (e.g., evapotranspiration, albedo) in driving interannual and seasonal vegetation dynamics that influence air temperature and precipitation.

Study Configuration

• Spatial Scale: Nile River basin (NRB), Northeastern Africa, spanning 4°0’0” S–35°0’0” N and 23°0’0” E–40°0’0” E, with a total drainage area of 3.2 million square kilometers.
• Temporal Scale: 1982–2020 (39 years), analyzed at annual and seasonal (June–September (JJAS), October–January (ONDJ), February–May (FMAM)) scales.

Methodology and Data

• Models used:
  • Community Earth System Model version 2 (CESM2)
  • Community Atmosphere Model version 6 (CAM6)
  • Community Land Model version 5 (CLM5) with satellite phenology mode (CLM5SP)
  • Multiple linear regression model for sensitivity analysis
  • Non-parametric Mann-Kendall Tau-b test with Sen’s slope estimates for trend analysis
• Data sources:
  • Leaf Area Index (LAI): Global Inventory Monitoring and Modeling Studies (GIMMS LAI4g), Global Land Surface Satellite (GLASS v.3), Long-term Global Mapping (GLOBMAP)
  • Air temperature (T), Precipitation (P), Surface downward solar radiation (SR): ERA5-Land monthly averaged reanalysis data (0.1° spatial resolution)
  • Evapotranspiration (ET): Global Land Evaporation Amsterdam Model (GLEAM v.3.2a)
  • Albedo (α): GLASS (Liu Q et al., 2013)
  • CO2 concentrations: Climate Model Intercomparison Project (CMIP)
  • Land cover type product: MODIS collection 6 land cover product (MCD12C1)
  • Sea surface temperature (SST), Sea ice fraction (SIC): Atmospheric Model Intercomparison Project (AMIP)
  • Digital Elevation Model (DEM): Shuttle Radar Topographic Mission (SRTM)

Main Results

• Approximately 54% of the Nile River basin's vegetated areas experienced significant increases in Leaf Area Index (LAI) from 1982 to 2020, with an average interannual growth rate of 0.04 m² m⁻² per decade.
• Enhanced vegetation density primarily exerts a cooling effect through increased evapotranspiration, reducing temperature across 43% of vegetated land, particularly in forest- and shrub-dominated areas.
• In high-elevation regions of Ethiopia, Kenya, and Uganda, vegetation induced surface warming by reducing albedo and enhancing solar energy absorption.
• Overall, vegetation change contributes to a net basin cooling of 0.02 ± 0.006 °C per decade, offsetting 9.5 ± 2.9% of the observed NRB warming over 39 years.
• Impacts on precipitation are weak and spatially inconsistent, with only semi-arid regions showing slight positive feedback.
• Seasonal variability is pronounced: evapotranspiration-driven cooling and positive precipitation responses dominate during June–September (JJAS) and October–January (ONDJ), while radiative warming and negative precipitation responses prevail from February–May (FMAM).
• Non-radiative processes, primarily evapotranspiration, are the dominant biogeophysical drivers of climate responses to vegetation change, influencing 83% of the vegetated area.

Contributions

• Provides the first comprehensive investigation into the contribution of vegetation dynamics to altering climatic processes in the Nile River basin, a highly vulnerable region.
• Quantifies the spatially heterogeneous and seasonally variable impacts of vegetation dynamics on temperature and precipitation, and disentangles the underlying biogeophysical mechanisms (evapotranspiration vs. albedo).
• Highlights the potential of vegetation-based land management strategies (e.g., afforestation, reforestation, land restoration) to partially mitigate regional warming, emphasizing the importance of geographically targeted interventions for maximum climate benefit.
• Utilizes a state-of-the-art coupled land-atmosphere climate model (CESM2) combined with long-term remote sensing data and a regression-based attribution method to isolate and quantify vegetation–climate feedbacks.

Funding

• Alexander von Humboldt Foundation

Citation

@article{Tesfaye2026Vegetationdriven,
  author = {Tesfaye, Samuale and Taye, Gebeyehu and Hölscher, Dirk},
  title = {Vegetation-driven evapotranspiration enhancements modulate the climate in the Nile River basin},
  journal = {Journal of Hydrology Regional Studies},
  year = {2026},
  doi = {10.1016/j.ejrh.2026.103152},
  url = {https://doi.org/10.1016/j.ejrh.2026.103152}
}