Regional hydrochemical modelling by delineation of chemical hydrological response units (CHRUs) within a GIS: An approach of observing man-made impacts in the Broel River catchment (Germany)

Characterizing and modelling hydrochemical catchment dynamics implies dealing with the challenges of upscaling point measurements to a regional, spatial distribution and with the regional spatial dynamics of nonpoint sources and their relation to catchment management. These two challenges can be met by applying the concept of ‘chemical response units (CHRUs)’ and using GIS-analysis to delineate their spatial distribution. CHRUs can be defined as areas having similar hydrochemical dynamics dependent on their landuse management. This concept was applied to the mesoscale catchment of the Broel River (A = 216km2) in the middle-mountain range of the Rheinische Schiefergebirge in Germany. The catchment is underlayn by impermeable Devonian shale where losses of water due to deep percolation are negligible. The climate is oceanic and mean annual rainfall ranges between 950 and 1100 mm. The landuse pattern of the catchment was mapped in the field and digitized with the aid of a GIS-system. About 34% is covered by coniferous and deciduous forest, 11% — impervious, 52% — range land and the remaining 3% — agriculture. The catchment was instrumented to capture atmospheric dry and wet deposition as bulk samples, and two river gauging stations were equipped with instruments for continuous logging of electrical conductivity EC (mS/m) and water temperature. Additionally, water samples were taken in two-week intervals for complete anorganic analysis. An intensive questionary of farmers regarding their fertilizing practices was carried out and trade figures of fertilizers, sold to the farming community, were supplied by local traders to estimate chemical-fertilizer input. Data from neighbouring forested test catchments were used to evaluate the salt contribution from forested areas. The database also included other physiographic factors like soils and topography and GIS-overlay analysis was used to delineate specific CHRUs. Their hydrochemical dynamics is strongly characterized by landuse and atmospheric deposition. The annual chemical balances of the Broel catchment were calculated using these CHRUs and the results compared with the measured output at the two gauging stations. First, results from these balance calculations showed deviations associated with undetected point sources but also good fits during periods with unique hydrological conditions. In summary, the concept of CHRUs proved to be a practical method for the evaluation of the chemical dynamics of the heterogeneous mesoscale Broel River catchment. Future research is directed at combining the CHRU-concept with a hydrological model like MMS/PRMS, WASNOD&STOMOD or APEX, and thereby interfacing the hydrochemical and the hydrological dynamics, on a basin-scale.