Hydroclimatology of the Nile: Results from a regional climate model

A Regional Atmospheric Climate MOdel RACMO has been run over the Nile Basin (12°S, 36°N and 10°W to 54.4°E, with 50 km horizontal resolution). The initial and lateral boundary conditions are taken from the ECMWF ERA-40 reanalysis. The model has been adjusted to fit the complex hydrology of the Upper Nile wetlands. Modest changes were incorporated to the parameterization of orography (filtering), solar radiation (aerosols), runoff (drainage coefficient) and soil moisture (layer depth), which produced improved model results. Observational data sets were used to evaluate the model results including radiation, precipitation, runoff and evaporation data. The evaporation data were derived from satellite images in conjunction with a land surface energy balance model over a major part of the Upper Nile.

The paper presents the model evaluation results over the sub-basins: Blue Nile, White Nile, Atbara river and the Main Nile for the period 1995-2000. Limitations in both the observational data and the model are discussed. It is concluded that the model provides a sound representation of the regional water cycle over the Nile. The model is used to describe the regional water cycle in the Nile basin in terms of atmospheric fluxes, land surface fluxes and land surface climate feedbacks. The monthly moisture recycling ratio (i.e. locally generated/total precipitation) over the Nile varies between 8 and 14%, with an annual mean of 11%, which implies that 89% of the Nile water resources originates from outside the basin physical boundaries. The majority of this water resources originates from the Indian Ocean. The monthly precipitation efficiency varies between 12 and 53%, and the annual mean is 28%. The mean annual result of the Nile regional water cycle is compared to that of the Amazon and the Mississippi basins.
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