Landscape unit delineation
Portion of a catchment with similar landscape morphological characteristics (topography/landform assemblage).
The aim is to delineate substantial areas of the catchment that are physiographically similar. The number of landscape units should not be large (typically up to four), but a higher number may be necessary if the catchment is particularly large and complex. These units are important for understanding the hydrological responsiveness of a catchment and also its sediment source / delivery characteristics, and so topography and rock type are the key characteristics underpinning unit delineation, although other factors (e.g. climate, vegetation cover and land use) may be considered to help confirm the appropriateness of divisions based on topographic and geological information.
Overall, topographic information underpins delineation of areas of internally consistent elevation range, relief and topographic dissection. Geology (lithology and tectonics) is also a fundamental control on topography as well as hydrological processes and the delivery of sediment to the fluvial system. Landscape units can be composed of many rock types, but broad groupings, as they affect landform and hydrological processes, are needed.
As a first step in delineation of landscape units, consideration is given to topography in terms of the broad elevation, relief and degree of dissection of the landscape. This enables the catchment to be subdivided into major landscape units such as: plains; undulating, lower elevation, hilly areas; and higher elevation, mountain areas. Appropriate threshold elevations or elevation ranges at which to separate plains from low (hills) and high (mountain) areas are likely to depend on the biogeographic region or subregion within which the catchment or its subcatchments are located. However, variations in rock type, land use and ‘natural’ vegetation cover may all be informative for delineation, since they often show a clear structure with increasing elevation. Furthermore, guidance from the Water Framework Directive (high: > 800 m; mid-altitude: 200-800 m; lowland: < 200 m) is a potentially useful starting point.
It may then be important to introduce subdivisions of these initial landscape units, into any clear, characteristic sub-types that are likely to be important for understanding hydromorphology (e.g. very steep mountain zones; intermontane plains, etc). Geology (lithology) can also be highly relevant when identifying subdividisions of the initial landscape units. For example, a subdivision of the initial units according to the hydrological (aquifers, aquicludes, aquifuges) or stability characteristics of the major groupings of rock type could be crucial for understanding hydromorphology. Thus, it might be appropriate to subdivide a single initial landscape unit such as a mountainous area, into a unit characterized by metamorphic rocks and a unit characterised by sedimentary rocks, because of differences in their detailed morphology (e.g. slope failures, landslide tracks, tallus slopes) that are indicative of their different resistance to erosion.