
Lake Tana is the biggest fresh water body in Ethiopia. Recently the Lake and its catchment water resources have been identified as one of the most important sources for hydropower, irrigation, and fishery development. However, there are huge uncertainties in estimating the water balance terms of both the Lake and its catchment. These will have an effect in developing the resources in a sustainable manner. The purpose of this article is to model and simulate the outflow series from the catchment measured at the Outlet of the Lake. This will give an opportunity to look the development impacts and their sustainability from the net supply perspective. A first order autoregressive model with model parameters Ø1 = 0.7845 and σa = 0.612 is identified as an adequate model to represent the deseasonalized series of the net supply of the catchment.