A review of two strongly contrasting geomorphological systems within the context of scale

Abstract Scale issues are often addressed in contemporary geo‐ecological studies and form one of the major challenges in the fields of physical geography, hydrology and ecology. In this paper the application of hierarchy theory and response units is proposed as an approach towards scale‐transcending environmental studies on degradation and geomorphological development. Goals of the research were to establish which processes are important at what spatio‐temporal scale, how hydro‐geomorphological response is influenced by biological processes and whether hierarchy theory and the response unit approach can be used as an up‐scaling methodology. Results from two climatologically and geomorphological different regions are discussed, one dominated by water shortage (SE Spain) and the other by water surplus (Luxembourg). In both cases detailed process research was carried out at scales ranging from the micro‐plot to the catchment. Process research was concentrated on understanding and quantifying sediment and water transfer through the geo‐ecosystems studied. Outcomes showed that in both cases the role of biological processes was important in the hydrological and degradation response of both areas. This was not only true for the finest scale levels but also had its impact on the emerging properties and response at the hillslope and catchment level. Connectivity of runoff‐generating and runoff‐absorbing areas was important on all scale levels. Connectivity is dominated by both the rainfall magnitude–frequency–duration characteristics and physically and biologically controlled thresholds, which range from initial soil moisture contents, vegetation patterns or soil biological activity, to the presence of water harvesting structures. The complex interrelationships of the processes involved showed that linear up‐scaling from fine to broad scale is impossible, as many thresholds and non‐linear processes are involved at specific scales. The identified response units are used to integrate these complex relationships in a relatively manageable way, and may provide a useful framework for up‐scaling, and for understanding catchment hydro‐geomorphological response and development. Copyright © 2002 John Wiley & Sons, Ltd.