John B. Thornes

Abstract

This paper discusses the concept of magnitude and frequency introdu-ced by Wolman and Miller in the middle of the 20th century. The concept is outlined and exemplified from recent examples and reference is made to the need for revision in the light of (a) the interaction between extreme events and human activity; (b) developments in hillslope hydrology, and (c) emergence of our understanding of non-linear behaviour.The extremeness of extreme events is identified through work-done plots and thro-ugh conventional statistical probability density functions. It is shown to be controlled (for runoff events) in the short term by vegetation cover, surface crusting and channel network evolution. For the longer term the paper addresses the impact of climatic changes through the vegetation cover by investigating the lagged nature of the response and the amplifica-tion or damping of the response through non-linear behaviour.

Keywords: extreme events, logistic behaviour, non-linearity, magnitude and fre-quency, stability and instability, vegetation

John B. Thornes, Research Chair in Physical Geography School of Social Sciences and Public Policy King's College London London WC2R 2LS, UK