This paper is a natural follow up of a recent and comprehensive trend detection study that have identified numerous regions in Brazil with a relatively large proportion of gauges with significant monotonic trends in mean annual flows. The aim of this paper is to provide a first quantitative evaluation of how much of the observed change in mean annual flow is due to climate and how much is due to other potential causes, such as human activities in the basin. We use the relatively simple decomposition method, based upon a Budyko-like equation, to accomplish the task for the São Francisco River basin. These preliminary results show that the relative contribution of climate and human activities in the basin varies over the region. In the Northwest part of the basin, changes are mainly due to climate, but this picture changes smoothly as we move southwards. In the Southern part of the basin, in about 50% of gauges, changes due to human activities on the surface are the predominant factor, while in the other 50%, both factors are equivalent.
