Abstract
Abstract. 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 thus may hinder the correct identification of climate variability signatures on hydrological processes. Does climate variability necessarily imply hydrological variability? This research aims to analytically derive the flood frequency distribution based on realistic hypotheses about the rainfall process and the rainfall–runoff transformation. The annual maximum peak flow probability distribution is analytically derived to quantify the filtering effect of 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 annual maximum peak flows and its statistical significance are analysed when changes in the climatic input are introduced. Results show that depending on changes in the annual number of rainfall events, the catchment filtering role is particularly significant, especially 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 affected by the climatic input, while for lower return periods, infiltration processes smooth out the impact of climate change.
Highlights
Many of the concerns about climate change are related to its effects on the hydrological cycle (Kundzewicz et al, 2007, 2008; Koutsoyiannis et al, 2009; Bloeschl and Montanari, 2010), and its impact on freshwater availability and flood frequency (Milly et al, 2002; Kay et al, 2006; Allamano et al, 2009)
Does climate variability necessarily imply hydrological variability? This research aims to analytically derive the flood frequency distribution based on realistic hypotheses about the rainfall process and the rainfall–runoff transformation
Results largely depend on the return period: for large return periods, peak flow variability is significantly affected by the climatic input, while for lower return periods, infiltration processes smooth out the impact of climate change
Summary
Many of the concerns about climate change are related to its effects on the hydrological cycle (Kundzewicz et al, 2007, 2008; Koutsoyiannis et al, 2009; Bloeschl and Montanari, 2010), and its impact on freshwater availability and flood frequency (Milly et al, 2002; Kay et al, 2006; Allamano et al, 2009). In the present study, modelling efforts are basically centred on the role of climatic variability and its effects on catchment hydrological response, with rainfall statistical properties and their future trends representing the major factors controlling flood frequency distribution. The frequency of occurrence of peak flows is estimated by means of a derived distribution approach, which is useful to obtain probability distributions of peak flows in ungauged or poorly observed basins In such cases design floods are calculated from a hydrological model, which is driven by historical or synthetic rainfall data (Haberlandt and Radtke, 2014). A simplified representation of hydrological processes is considered without including detailed effects Under such assumptions, the aim of this research is to quantify the actual extent to which the rainfall–runoff process filters the impact of rainfall variability on runoff annual maximum peak flow series. The rainfall–runoff model proposed assumes a simple but common scheme for small, fast-responding, ungauged catchments, subjected to erratic hydrological regimes (Ferrer Polo, 1993; Soulis and Valiantzas, 2012)
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