TOWARDS DEVELOPING A NEW SHORT-TERM MODEL FOR THE BIRKENES CATCHMENT - LESSONS LEARNED

New hydrochemical information is available on soil waters for the Birkenes catchment in southern Norway. This information is in discord with the existing Birkenes model: the two reservoirs of the model cannot easily be identified with particular areas or soil horizons in the catchment. Here, a new model is presented allowing such an identification while still representing a simple, lumped structure. The simulated flow patterns in the model are evaluated through a three-step procedure involving: (1) reproduction of the hydrograph, (2) reproduction of the chemically inert 18O isotope in runoff, and (3) simulation of H+, Ali (inorganic monomeric Al), and Ca in runoff by mixing of measured soil water 'end-member' compositions attributed to each model reservoir. The first two tests are met reasonably well within the proposed structure, whereas the third requirement, if indeed the measured end-member compositions are representative for the catchment, implies structural changes in the way the Birkenes model describes water routing. More field work is required on soil water chemistry, for instance in the deeper layers in the valley bottom, before a further iteration in model development can be undertaken. © 1990.