The central monitoring site of the Eifel/Lower Rhine Valley Observatory is the catchment area of the River Rur. It covers a total area of 2354 km² and exhibits a distinct land use gradient: The lowland region in the northern part is characterised by urbanisation and intensive agriculture whereas the low mountain range in the southern part is sparsely populated and includes several drinking water reservoirs. Furthermore, the Eifel National Park is situated in the southern part of the Rur catchment serving as a reference site. Intensive test sites are placed along a transect across the Rur catchments in representative land cover, soil, and geologic settings: In order to obtain spatially distributed information about river discharge rates, the Rur catchment is partitioned into a nested set of sub-catchments that will span distinct assemblages of hydrologic features and several orders of magnitude in drainage area. More detailed measurements and characterisation of smaller, focal catchments are embedded within progressively larger catchments, allowing the critical evaluation and development of scaling strategies. Additionally, for the analysis of the groundwater flow system and groundwater exchange rates on a regional scale, the monitoring of natural tracers of ground and surface waters will be carried out.