Investigating hydro-climates of the Upper Blue Nile River Basin.

The Upper Blue Nile Basin (UBNB) is the main flow contributor to the downstream river. Given the competing demands, we characterize the water balance by statistical analysis of past observations and data assimilation models. UBNB runoff shows minimal trend (1% of variance) and a large annual cycle (69%) governed by the equatorial trough and Hadley circulation. The diurnal cycle linked to local heat fluxes contrasts with modulation by the slower Pacific El Niño Southern Oscillation and faster Indian Dipole. Using daily measurements over ~25 years, we found that the inflow to Lake Tana (average 48 m3/s) obtains a linear regression fit of r2 = 76% with the discharge at Khartoum (average 1380 m3/s) at a lag of +6 days, implying that the small catchment conveys the hydro-climate signal to the entire Nile Basin. The inflow-induced turbidity in Lake Tana was studied using changes in colour detected by satellite. Land-use changes in the UBNB were quantified by satellite infra-red temperature and aerosol density. Trends in vegetation temperatures showed a + 0.2°C/year rise in agricultural zones. Ensemble model-projected trends in runoff are upward and may keep pace with the growing demand for Nile water. [ABSTRACT FROM AUTHOR]