This paper presents a conceptual framework for assessing the impacts of land use changes in the upper Ewaso Ng"iro river basin in Kenya. It is based on a people-water-ecosystem nexus, and it presents the key issues, their interactions and how they can be addressed. The paper presents hydrological assessment of up-scaling rainwater harvesting conceptual framework, which assesses the impacts of land use changes on hydrological regime in a river basin. The results will enhance the formulation of sustainable land and water resources management policies and strategies for water-scarce river basins.