Flood frequency hydrology:: 2.: Combining data evidence

In a companion paper (Merz and Bloschl, 2008) we argue that it is very useful to expand the information beyond the flood sample at a site of interest to better represent the diversity of flood processes in estimating flood frequencies. In this paper we present a framework of how to combine different sources of information by hydrological reasoning to obtain more informed estimates of flood frequencies. These sources of information include the local flood peak sample and temporal, spatial, and causal expansion of information. As most of this information is independent, one would expect that the final estimate is more reliable than each of the individual sources, including the flood peak sample alone. To illustrate the proposed framework, four examples from Austria are given. In all four examples the statistical analyses of the flood records do not fully represent the site-specific flood behavior in the light of the more complete information. The strengths of the proposed framework are its flexibility, in that more weight can be given to sources that are known with better confidence than others, and the ability to account for local particularities of catchments in terms of hydrological processes and data availability.