Assessment of source regions of the Zambezi River: implications for regional water security

Abstract. The Wilderness Project, in collaboration with the National Geographic Okavango Wilderness Project and the Wild Bird Trust, conducted the first known scientific on-river expeditions along the entire length of the Zambezi River, from its traditional source in Zambia and its most distal source in the Angolan Highlands Water Tower, all the way to the Indian Ocean. By combining ground measurement and high-resolution earth observation data, this study describes the overlooked contribution of Angolan Highlands Water Tower to the Zambezi River. Although preliminary, our results represent the first comprehensive assessment from integrated multiple lines of evidence. Our findings show: (1) The Zambezi River length, measured from the Lungwebungu River, is 342 km longer (total length of 3421 km to the Indian Ocean) than the traditional source, (2) Our estimates suggest that, during the late wet-season between March and May 2023, the Angolan portion of the Upper Zambezi and Lungwebungu sub-basins contributed approximately 73 % of the flow measured upstream of the Barotse Floodplain, (3) The Lungwebungu and Upper Zambezi sub-basins show evidence of reducing elevated conductivity, salinity, and TDS, likely introduced by mining activities in the Kabompo sub-basin, and (4) The Upper Zambezi sub-basin contains the highest proportion of wetland coverage, with 95 % (19 184 km2) located in Angola. Specifically, this coverage is concentrated over the Kameia and Luena wetlands, which are important for supporting overall river health. This study highlights the important role Angola plays in hosting the most distant source, and contributing a significant portion of the primary aquifer system of the Zambezi River; this hydrological contribution warrants greater recognition. Additionally, it underscores the need for improved monitoring and analysis of hydrological flows of the Zambezi Basin, particularly its Angolan tributaries. The Zambezi River Basin, the largest in southern Africa, faces ongoing challenges from climate change, development, and human water use. Collaborative efforts among basin countries are crucial to balance development with environmental needs, safeguarding ecosystem services and the natural dynamics that sustain the Upper Zambezi's ecological integrity.