The search for regularities in hydrologic relationships is discussed against the background of the general types of predictive models used in science. The various approaches to the study of water are compared and contrasted. The ideas discussed are illustrated by examples from the development of techniques in flood hydrology and by personal conclusions on the sources for new hypotheses in flood hydrology and the possibility of their verification.

By the single assumption that the reservoir action in a catchment can be separated from translation, the general equation of the unit hydrograph is shown to be This is simplified by two further simple assumptions to give which can be conveniently calculated.

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This Horton Memorial Lecture reviews the approach of Robert Horton (1875-1945) to key problems in hydrology, meteorology, and geography and then speculates on how Horton might today approach the problem of analyzing the sensitivity of catchment runoff to climate change. It is suggested that the techniques used by Horton can give us an insight into the nature of the latter problem through a partial analysis of the phenomena involved.

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