This session originates directly from research activities carried out under the PUB initiative [Predictions in Ungauged Basins, an IAHS initiative e.g., external link: http://pub.iwmi.org] and focus on the estimation streamflow-regime indicators (e.g., annual flow, flow-duration curves, floods and low-flows) through regionalization of hydrological information. In particular, the attention is focussed on objective and process-based criteria to be used for catchment classification, spatial interpolation of streamflow indices, identification of hydrologically homogeneous groups of basins. Original contributions are solicited that address the investigation of similarities among catchment characteristics and processes. Four different aspects are of particular importance:

- the role of catchment similarities in the explanation of regional hydrological patterns;
- pooling schemes in the context of hydrologically homogeneous groups of sites;
- analysis of possible common dominant processes as a basis for describing catchment similarity;
- assessment of uncertainty of regional streamflow prediction in ungauged sites.

The session format consists of a standard EGU oral&poster session and an open forum discussion in a splinter meeting room afterwards. The open forum will enable us to further discuss the topic in a friendly and informal atmosphere.
The publication of the session’s proceedings in a guest-edited Special Issue of a relevant International Journal is foreseen.