Relevance of the net natural inflow for investigating long-term hydrological changes in a managed lake catchment

The net natural inflow to a lake is an important quantity for developing lake management strategies and assessing hydrological change. The net natural inflow embeds the relevant hydrological processes governing the lake levels and it determines the lake levels and the emissary discharge under natural conditions, i.e. without withdrawals. The net natural inflow can be calculated by using an inverse procedure that reduces some of the sources of uncertainty that characterize the hydrological balance. In this paper we use the inverse procedure to analyze the net natural inflow of an Italian lake (Lake Bracciano) for which over a century of data are available. The behavior of the net natural inflow to the lake is analyzed, highlighting the high inter-annual variability and the consequent exposure of the lake to extreme events, in particular droughts. Evidence of hydrological change is discussed, as well as the importance of the availability of long time series for meaningful lake analysis and management. The analysis demonstrates the advantages of using the net natural inflow for lake hydrological analysis.