Eco-hydrology of the Pangani River downstream of Nyumba ya Mungu reservoir, Tanzania

Flow duration curves (FDC) and hydrographs were developed by using the collected historical daily flow data and used for assessing the flow variation before and after Nyumba ya Mungu (NYM) dam construction. The results showed that there is no seasonality and that for most part of the year the flow is within 20-40 m3/s. From the comparison it was found that there is 42.7% decrease in Q5, 2.1% decrease in Q95, 38.9% increase in Q75, 23.4% increase in Q50 and decrease of percentage exceedence of 50 m3/s by 61.5%. Physical habitat simulation model (PHABSIM) was used for hydraulic simulation incorporating geometric and flow data collected in the field. From the simulations minimum flow for habitat protection is about 15 m3/s and for flood formation flows should be greater than 50 m3/s. Thus the required reservoir outflow for sustainable water resources management at the NYM should follow the above specifications.