Natural Hazards and Earth System Science www.nat-hazards-earth-syst-sci.net 1561-8633 1684-9981 10 7 2010 10.5194/nhess-10-1605-2010 http://www.nat-hazards-earth-syst-sci.net/10/1605/2010/ http://www.nat-hazards-earth-syst-sci.net/10/1605/2010/nhess-10-1605-2010.html http://www.nat-hazards-earth-syst-sci.net/10/1605/2010/nhess-10-1605-2010.pdf 1605 1615 2010-07-23 Physical controls on the scale-dependence of ensemble streamflow forecast dispersion G. Mascaro gmascaro@unica.it E. R. Vivoni R. Deidda Dipartimento di Ingegneria del Territorio, Università di Cagliari, Cagliari, Italy School of Earth and Space Exploration & School of Sustainable Engineering and the Built Environment, Arizona State University, Tempe, Arizona, USA The accuracy of ensemble streamflow forecasts (ESFs) is impacted by the propagation of uncertainty associated with quantitative precipitation forecasts (QPFs) through the physical processes occurring in the basin. In this study, we consider consistent ESFs (i.e., observations and ensemble members are equally likely) and we study the effect of basin area (A) and antecedent rainfall (AR) on the ESF dispersion, a metric of flood forecast skill. Results from a set of numerical experiments indicate that: (i) for small basins (≲180 km2), ESF dispersion is mainly dominated by the runoff generation process and does not depend on the basin size A; (ii) for larger areas, ESF dispersion decreases with A according to a log-linear relation due to the decreasing variability of ensemble QPFs and, possibly, to the channel routing process. In addition, we found that, regardless the basin size, the ESF dispersion decreases as AR increases, and that the influence of AR is larger for basins with fast response times. Physical controls (land cover, soil texture and morphometric features) on the analyzed basin response confirm these interpretations. Anderson, J. 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