Climate and hydrological variability: the catchment filtering role

Measuring the impact of climate change on flood frequency is a complex and controversial task. Identifying hydrological changes is difficult given the factors, other than climate variability, which lead to significant variations in runoff series. The catchment filtering role is often overlooked and in fact, this may hinder the correct identification of climate variability signatures on hydrological processes. Does climate variability necessarily imply hydrological variability? The research herein presented aims to analytically derive the flood frequency distribution basing on realistic hypotheses about the rainfall process and the rainfall–runoff transformation. The peak flow probability distribution is analytically derived to quantify the filtering effect operated by the rainfall–runoff process on climate change. A sensitivity analysis is performed according to typical semi-arid Mediterranean climatic and hydrological conditions, assuming a simple but common scheme for the rainfall–runoff transformation in small-size ungauged catchments, i.e. the CN-SCS model. Variability in peak flows and its statistical significance are analysed when changes in the climatic input are introduced. Results show that in regard to changes in the annual number of rainfall events, the catchment filtering role is particularly significant when the event rainfall volume distribution is not strongly skewed. Results largely depend on the return period: for large return periods, peak flow variability is significantly impacted by the climatic input, while for lower return periods, infiltration processes smooth out the effects of climate change.