A catchment is an area of land that is drained by a river and its tributaries.
Delineation is based entirely on topographic and river network information. The catchment boundary to any required (usually gauged) point on the river network is defined by applying GIS tools to an appropriate digital elevation model. In theory, this process should be relatively easy using existing digital elevation models (e.g. SRTM, ASTER GDEM, EU-DEM) and widely available GIS algorithms. In practice the process is often quite difficult. In particular, delineation of headwater streams can be problematic if valley width is less than DEM resolution, while vertical accuracy of DEMs often causes problems in flat, plain regions. Further complications in terms of subsequent interpretation of hydrology can arise due to water transfer infrastructure and changes in underlying geology, which may lead to the effective watershed not coinciding with the topographic watershed.
Simple GIS tools are available to create an inferred river network and to delineate the catchment boundary for any location on the network based on a DEM. Accurate digital mapping products could also be utilised to improve the accuracy of river network used in the process (e.g. OS Mastermap dataset for the UK). At pan-European scale, the CCM2 River and Catchment Database v2.1 (Table 3.2) is a purpose-designed product. The CCM2 database was originally defined using the SRTM 90 m DEM but it has been refined continually to remove errors in river line positions. While exact channel planform boundaries are not defined in CCM2, the database can be utilised to accurately define catchment boundaries and quantify the upstream contributing areas to any point on a river network.
For all analyses at finer spatial scales, the river course is best delineated using a digital representation of the actual network rather than any inferred network derived from DEM analysis.