Hydrologic Concepts of Variability and Scale

All hydrological phenomena have significant variations in time and space. Typically, these variations are driven by variations in physiographic factors such as climate, soils, vegetation, topography, geology, as well as by human and animal activities. These externally driven variations then propagate through hydrological systems, leading to an extremely rich variety of hydrological variability apparent at different temporal and spatial scales, in different physical settings. Virtually any quantitative approach to this problem requires the selection of a limited set of spatial and temporal scales within a particular physiographic setting. Any particular choice of time and space scales has a major influence on which aspects of this hydrological variability are perceived. This article surveys hydrological variability in both time and space, across a range of scales.