Hydrological Behavior of the Mountainous Region of Rio de Janeiro: Piabanha Watershed

The mountainous region of Rio de Janeiro has a history of natural disasters. The aim of this study was to understand the hydrology of this region. A dataset (up to 70 years) from four rainfall gauges and three stream gauges in Piabanha watershed (2065 km2) was used for this purpose. Mean annual rainfall ranged from 2900 mm (at the headwaters) to 1250 mm in the central area of the watershed. The results showed good correlation between rainfall and stream gauges data. There is a seasonal pattern of dry winters (average month rainfall ranging from 15 to 90 mm) and 20 m3/s mean monthly discharge at the watershed outlet and a rainy summer (average month rainfall ranging from 150 to 450 mm) and 80 m3/s mean monthly discharge at the watershed outlet. The historical rainfall and discharge dataset showed a cyclicity of about 15 years bounded by the driest years. The most catastrophic events that occurred in the mountainous region of Rio de Janeiro were during the years around the top of the rainfall and discharge cycles. According to the historical trend the last cycle began at 1999 and the present moment (2015-2016) matches the end of a drier period. However during this last cycle rainfall and discharge trends are not very clear. Together with an increase of great magnitude natural disasters frequency in recent years this may be a sign of an anthropogenic effect.