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The semi-arid savannah environment (SASE) of sub-Saharan Africa are characterized by low erratic rainfall which result to high risk of droughts, intra-seasonal dry spells and frequent food insecurity. The main occupation is subsistence small-scale rainfed agriculture and livestock production, which normally compete for the limited water resources. The main challenges to improving the livelihoods of the small-scale farmers are how to upgrade rainfed agriculture to improve rural livelihoods and conserve nature, and upgrade upstream landuse in balance with water needs for human and ecosystems downstream. There is an increased interest in opportunities of improving rainfed agriculture through adoption of rainwater harvesting (RWH) technologies. However, there is inadequate knowledge on hydrological impacts and limits of up-scaling rainwater harvesting at a river basin scale. Rainwater harvesting has a potential of addressing spatial and temporal water scarcity for domestic, crop production, livestock development, environmental management and overall water resources management is SASE. However, this potential has not been exploited despite the occurrence of persistent low agricultural production and food shortage in sub-Saharan Africa. The need to quantify this perceived potential and related hydrological impacts on a river basin led to the on-going research project titled "hydrological impacts of up-scaling RWH on upper Ewaso Ng'iro river basin water resources management". It is envisaged that the study will contribute to formulation of sustainable RWH up-scaling strategies to enhance food production and hydro-ecological balance in semi-arid savannahs of Africa. This paper presents the preliminary findings of the study mainly focusing on assessment of the potential of RWH technologies for improving food and water availability especially in semi-arid regions of eastern Africa. This was achieved by evaluating six RWH case studies selected from four countries (Ethiopia, Kenya, Tanzania and Uganda). Despite the success of a number of RWH systems, the rate of adoption is still low, hence making their impacts marginal. Nevertheless, there is a knowledge gap on the limits of up-scaling RWH in a river basin, which the other components of the study will address. The assessment of the hydrological impact of up-scaling RWH technologies is expected to provide answers to the question, what is the limit of up-scaling rainwater harvesting in a river basin?