Use of objective criteria for the assessment of biogeochemical ecosystem models

Ecosystem modeling is confronted with complex biological systems and changing environmental conditions. A model which describes ecosystem behavior under all conditions has not been found yet and there does not exist one 'true' model for a specific ecosystem. Often ecosystem models describe the measured data more or less well, but most judging criteria for model performance are rather subjective. Furthermore. from a mathematical view point the calibrations of ecosystem models are hardly ever unique. The aim of this stud; was to develop and use criteria which permit an objective comparison of different models to the observed field data and to each other. A given model which describes a specific system significantly better will be declared the 'valid' model while the other will be rejected. The term 'valid' is used here in a sense that any model that could not be proven invalid would be a valid model for the system. We used the biogeochemical soil models MAGIC (Cosby, B.J., Hornberger, G.M., Wright, R.F., 1985. Modelling the effects of acid deposition: assessment of a lumped-parameter model of soil water and stream water chemistry. Water Resour. Res. 21, 51-63) and the SO-Model (derived from the Batch Equilibrium Model (BEM; Prenzel, J., 1991. Introduction to BEM (Batch Equilibrium Model), vol 28. Berichte des Forschungszentrums Waldokosysteme/Waldsterben, Gottingen, 51 pp.). The data set used was the soil solution chemistry in a forest ecosystem of the Selling area (North-West Germany). To test the performance of the models four criteria were used: the efficiency (Martinec, J., Range, A., 1989. Merits of statistical criteria for the performance of hydrologic models. Water Resour. Bull. 25 (2), 421-432; Hinzman, L.D., Kane, D.L., 1991. Snow hydrology of a headwater artic basin; 2. Conceptual analysis and computer modelling. Water Resour. Res. 27, 1111-1121), the Normalized Mean Absolute Error (NMAE, given by Janssen, P.H.M., Heuberger, P.S.C., 1995. Calibration