Impact of Climate Change on Flood Frequency Using Different Climate Models and Downscaling Approaches S. Camici 1 ; L. Brocca 2 ; F. Melone 3 ; and T. Moramarco, M.ASCE 4 Abstract: The analysis of the climate change impact on flood frequency represents an important issue for water resources management and flood risk mitigation. However, for small/medium catchments (<1,000 km 2 ), the spatial-temporal resolution of global circulation models (GCMs) output is not adequate (>40,000 km 2 ) and downscaling procedures are required. In this paper, two different GCMs selected among the Coupled Model Intercomparison Project 3 models, the Hadley Center Coupled Model, the Parallel Climate Model, and two statistical downscaling approaches, (1) delta change, and (2) quantile mapping, are compared. For the generation of long hourly time series of rainfall, temperature, and discharge, stochastic weather generators coupled with a continuous rainfall-runoff model are employed. Therefore, the frequency of annual maxima rainfall and discharge is projected for the future period 2070–2099 over three small subcatchments in the Upper Tiber River Basin, central Italy. Results reveal that both the GCMs and downscaling methods play a significant role in the deter- mination of the climate change impact for future scenarios, mainly in terms of annual maxima values. By comparing the future (2070–2099) with the baseline period (1961–1990), all GCMs project a decrease of mean annual rainfall (∼30%) and an increase of mean annual temper- ature (∼40%). However, in terms of annual maxima (of rainfall and discharge) the results are found to be dependent on the selected GCM and downscaling method. On one hand, through the application of the delta change method, both GCMs project a decrease in the flood frequency curves. On the other hand, if the quantile mapping downscaling method is considered, the Hadley Center Coupled Model 3 projects a decrease in the frequency of annual maxima discharge; the opposite occurs for the Parallel Climate Model. The hydrological characteristics of the study catchments play an important role in the assessment of the climate change impacts. For that, the need to use ensemble GCM results and multiple downscaling methods is underlined. DOI: 10.1061/(ASCE)HE.1943-5584.0000959. © 2014 American Society of Civil Engineers. Author keywords: Climate changing; Flood frequency; Downscaling method; Global circulation model (GCM); Weather generator. Introduction Understanding the climate change impact on the hydrologic cycle evolution is one of the major challenges in the context of water resources management. The Intergovernmental Panel on Climate Change (IPCC 2007) stated that it is evident by now that recent climate changes have had serious impacts on the intensity and fre- quency of floods in many regions of the world. In this context, the European Directive 2007/60/CE, which is the reference framework for the assessment and management of flood risks in the European countries, advises to take account the effects of climate changes on the occurrence and the adverse impacts of flood events. Therefore a robust procedure for the flood frequency analysis in a context of climate change is required. The methodology usually followed to assess the hydrological consequences of climate change consists on a chain formed by Xu et al. (2005), as follows: (1) global circulation models (GCMs), (2) downscaling techniques, and (3) hydrological models. Global circulation models provide a global scale the changes in atmos- pheric variables under the climate change scenarios defined by the IPCC. These climate projections are defined at a coarse grid (approximately 150–300 km) and hence cannot be used as input of hydrological models for climate change impact assessments. Thus statistical or dynamical downscaling procedures (Fowler et al. 2007; Prudhomme et al. 2002), allowing researchers to obtain finer spatial/temporal resolution of GCMs outputs, are strictly needed. In this context, many studies have been conducted to analyze the uncertainty in climate change impact on runoff by using different GCMs, downscaling techniques, and hydrological models (Dibike and Coulibaly 2005; Kay et al. 2009; Prudhomme and Davies 2009; Chiew et al. 2010; Segui et al. 2010; Chen et al. 2012; Teutschbein and Seibert 2012; Samadi et al. 2012). In accordance with previous studies (Wilby and Harris 2006; Déqué et al. 2007; Minville et al. 2008), Kay et al. (2009) found that the GCM struc- ture is by far the largest source of uncertainty on flood frequency estimation and for that the use of at least two different GCMs was recommended in climate change impact assessment (Prudhomme and Davies 2009; Veijalainen et al. 2010). On the other hand, also the choice of the downscaling method is an important source of uncertainty (Schmidli et al. 2007), which has to be accounted especially for small basins (Segui et al. 2010). Horton et al. (2006), by analysing the climate change effects on runoff regimes for several catchments in the Swiss Alps, showed that the use of different dynamical downscaling methods [regional climate models (RCMs)] forced with the same GCM provided a 1 Ph.D. Student, Research Institute for Geo-Hydrological Protection, National Research Council, 06128 Perugia, Italy (corresponding author). E-mail: s.camici@irpi.cnr.it 2 Researcher, Research Institute for Geo-Hydrological Protection, National Research Council, 06128 Perugia, Italy. E-mail: l.brocca@irpi .cnr.it 3 Senior Researcher, Research Institute for Geo-Hydrological Protec- tion, National Research Council, 06128 Perugia, Italy. E-mail: f.melone@ irpi.cnr.it 4 Senior Researcher, Research Institute for Geo-Hydrological Protection, National Research Council, 06128 Perugia, Italy. E-mail: t.moramarco@irpi.cnr.it Note. This manuscript was submitted on October 19, 2012; approved on December 16, 2013; published online on December 18, 2013. Discussion period open until October 20, 2014; separate discussions must be submitted for individual papers. This paper is part of the Journal of Hydrologic En- gineering, © ASCE, ISSN 1084-0699/04014002(15)/$25.00. © ASCE 04014002-1 J. Hydrol. Eng. J. Hydrol. Eng.