With a natural catchment area of nearly 1500 km², Vosso is the most extensive river network in Hordaland. The catchment area was increased artificially with the transfer of the upper parts of the Steinsland- and Ekso water systems. Most of the transfer occurs via the Evanger power station. As a consequence of the power plant development, Vosso's catchment area was increased by 1600 km².
The water flowing out of Lake Vangsvatnet is recorded at the measuring station at Bulken. This is one of the oldest measuring stations in the country in an essentially unregulated water system - the station lies above the water transfer from the power development, and thus is not influenced by it. The station has carried out uninterrupted recording since 1891. The data from here have been used in many situations, for example, as a type water system for calculating variations in water supply as a consequence of eventual climatic changes. Also the water temperature in Vosso has been recorded at several places.
During the period from 1961-1990, the average water supply increased at Bulken by 13 % compared with the calculations from the previous 30 years. The increase has been greatest during the autumn months. This compares closely with the increase in measured precipitation, which was also greatest during the autumn. This increase in water supply continued during the first part of the 1990s, but 1996 was the driest year since 1960. The water supply in 1996 was a bit under 1/3 of what had been measured during the wettest year (1990).
