Hydrology and forests in the Blue Nile basin : what can be learned from half a century of observations and community perception for water management?

Abstract: Forest cover change is considered a major cause of water scarcity during the dry season in the Blue Nile River Basin of Ethiopia, which is part of the Nile River system. However, this is an over-simplification of the complex reality of forest and flow relationships, especially in the tropics. The objectives of this study were to explore the spatial relationship of land use and flow regime, detect temporal changes in the hydrological regime, determine changes in the forest cover, and summarize the results to define the relationship between forest cover and hydrological regime. Two broad approaches were used to address these aims: observational data analysis and community perception. Thirty-two watersheds were covered in the spatial study, and 45 years of data for a dozen of these watersheds were analyzed in the temporal study. Statistical methods were used to explore the spatial relationship of land use and flow, and both statistical and modeling methods were used to detect hydrological change over time. Remote sensing analysis was used to document forest cover changes. Natural grassland and woodland had a positive, while grazing land and open bush land had a negative correlation with low flow regimes at the spatial scale. There were no major temporal changes in the flow regime, or clear results to attribute land degradation or land use change to hydrological changes and specific changes within each watershed. The change related to forest cover were watershed specific, although there were general differences between southern and northern watersheds regarding the time of deforestation. The community perception indicated the relationship of forest cover change and flow regime was more complex than just deforestation causing loss of dry season flow. According to the elders, forest and flow relationships were watershed specific, even sub-watershed specific. The lack of a clear relationship between forest cover change and flow regime in the temporal dimension could be attributed to the scale of watersheds, uncertainty about the measurement of flow extremes, and the impact of variability in rainfall within the region. The watershed-specific nature of the relationship between forest and flow within the Basin, confirmed by the community perception, indicates forest hydrology studies should be tailored to watershed scale, or even sub-watershed scale i. e. hill-slope scale, and address the relevance of water availability at farm and river scale.