In arid and semi–arid regions, water resources are scarce and need climate change impact assessment for water conservation after each storm rainfall as rainfall harvesting in urban areas and runoff harvesting in rural areas. It is the main purpose of this paper to present necessary Water Harvesting (WH) preparation principles by considering simple climate change models for the Arabian Peninsula in general, and for the Kingdom of Saudi Arabia in particular. The proposed model is a mixture of the Global Circulation (Climate) Model (GCM)'s output scenarios with historical local rainfall records, and hence, prediction of monthly rainfall amounts up to 2100.

Keywords: arid regions, downscaling, rainfall harvesting, scenario, Saudi Arabia, water harvesting, water resources, impact assessment, water conservation, climate change models