1 Trends in Rainfall and Runoff in the Blue Nile Basin: 1964-2003

Most Nile water originates in Ethiopia but there is no agreement on how land degradation or climate change affects the future flow in downstream countries. The objective of this paper is to improve understanding of future conditions by analyzing historical trends. During the period 1963 to 2003, average monthly basin wide precipitation and monthly discharge data were collected and analyzed statistically for two stations in the upper 30 % of Blue Nile Basin and one station at the Sudan-Ethiopia border. A rainfall runoff model examined the causes for observed trends. The results show that while there was no significant trend in the seasonal and annual basin-wide average rainfall, significant increases in discharge during the long rainy season (June to September) at all three stations were observed. In the upper Blue Nile the short rainy season flow (March to May), increased while the dry season flow (October to February) stayed the same. At the Sudan border the dry season flow decreased significantly with no change in the short rainy season flow. The difference in response was likely due to weir construction in the nineties at the Lake Tana outlet that affected significantly the upper Blue Nile discharge but only affected less than 10 % of the discharge at the Sudan border. The rainfall runoff model reproduced the observed trends, assuming that an additional ten percent of the hillsides were eroded in the 40 year time span and generated overland flow instead of interflow and base flow. Models concerning future trends in the Nile cannot assume that the landscape runoff processes will remain static