This report presents the findings of a study to assess changes to flows into, and downstream of, the Usangu Wetlands, located in the headwaters of the Great Ruaha River, Tanzania. Hydrological data, in conjunction with remote sensing techniques, were used to provide insights into changes that have occurred to the Eastern Wetland. Results indicate that, between 1958 and 2004, inflows to the wetland declined by about 70 percent in the dry season months (July to November) as a consequence of increased human withdrawals, primarily for irrigation. This resulted in a decrease in the dry season area of the wetland of approximately 40 percent (i.e., from 160 km2 to 93 km2). In the last decade, outflows from the wetland have ceased for extended periods. An environmental flow model indicates that a minimum dry season outflow of approximately 0.6 m3s-1 is essential to sustain the basic ecological condition of the river. To maintain this outflow from the wetland, a minimum average dry season inflow of approximately 7 m3s-1 (i.e., approximately double current dry season flows) is required. To achieve this, dry season flows in the perennial rivers discharging into the wetland would have to be apportioned so that 20 percent is used for anthropogenic purposes and the remaining 80 percent discharges into the wetland. There is significant potential for improving water use efficiency. However, to ensure minimum downstream flow requirements, consideration should also be given to active water management within the wetland itself.
IWMI Research Report 104. Colombo, Sri Lanka: International Water Management Institute. 48p. ISBN 978-92-9090-645-2