The science of hydrology is concerned with the processes of the hydrological cycle, their three-dimensional distribution across spatial scales ranging from regions, through catchments and river corridors, to river channels, and across temporal scales ranging from centuries to seconds. Within this wide range of time and space scales, hydrologists are concerned both with the average condition, seasonal variability and temporal trend in water resources and with the magnitude and frequency of hydrological events such as extreme rainstorms, ¯oods and droughts. Thus hydrology is and has always been a truly four-dimensional discipline, focusing upon the complexities of water storage and movement across the landscape in space and time. Furthermore, hydrologists have always been concerned with ecology to the extent that vegetation is an important control on hydrological processes. Interception, evapotranspiration and in®ltration processes are particularly heavily in¯uenced by the character and dynamics of the vegetation cover. In the last 50 years, however, hydrological processes have increasingly become a focus for ecologists and geomorphologists, creating a signi®cant interdisciplinary area of science.
The foundations of the evolution in understanding of the interface between hydrology and ecology can be illustrated by a series of important publications. The 1950s and 1960s saw major developments in research on the role of vegetation, which was mainly undertaken by hydrologists. Three examples of this research focus are H. L. Penman's (1963) book on Vegetation and Hydrology, the symposium edited by W. E. Sopper and H. W. Lull (1967) on Forest Hydrology, and the monograph by Hack and Goodlett (1960), which relates vegetation, geomorphology and hydrology at the catchment scale. During the 1960s and 1970s, there was an enormous expansion in the study of process geomorphology, with river systems providing a major theme. Texts such as Leopold et al. (1964), Gregory and Walling (1973), Schumm (1977), Kirkby (1978) and Dunne and Leopold (1978) re¯ect the explosion of research at the interface between hydrological processes, sediment transfer processes and ¯uvial geomorphology. Through the 1980s and 1990s, this led to an increasing appreciation by hydrologists and geomorphologists of the important multidirectional interactions between ecological, hydrological and geomorphological processes, which is re¯ected in a number of volumes including Petts (1984), Viles (1988), Thornes (1990) and Hupp et al. (1995). In parallel with the interdisciplinary research that was being driven from the physical sciences, the 1970s, 1980s and early 1990s saw the publication of many important interdisciplinary texts, which re¯ect research driven by ecologists. Some examples include Ward and Stanford (1979), Welcomme (1979), Malanson (1993), Mitsch and Gosselink (1993) and Maser and Sedell (1994).
The 1980s and 1990s have seen an acceleration in research at the interface of ecology and hydrology, with the publication of many substantive reviews and the proposal of a number of key concepts. Building on the ideas of hydrologists and geomorphologists, Vannote et al. (1980) published their very in¯uential paper on The river continuum concept'. This longitudinal' concept was developed in relation to regulated rivers by Ward and Stanford (1983) in their paper on the Serial discontinuity concept', and both concepts have been developed further along lateral ', vertical' and temporal' dimensions (Sedell et al., 1989;Ward, 1989;Stanford and Ward, 1993;Ward and Stanford, 1995) to provide a four-dimensional framework for ecohydrological study. A parallel awareness of dierences in ecological functioning between rivers of diering size has led to considerations of scale (e.g. Frissell et al., 1986) and connectivity (e.g. Bayley, 1995;