USE OF OBJECTIVE CRITERIA FOR THE ASSESSMENT OF BIOGEOCHEMICAL ECOSYSTEM MODELS

Ecosystem modeling is confronted with complex biological systems and c hanging environmental conditions. A model which describes ecosystem be havior under all conditions has not been found yet and there does not exist one 'true' model for a specific ecosystem. Often ecosystem model s describe the measured data more or less well, but most judging crite ria for model performance are rather subjective. Furthermore. from a m athematical view point the calibrations of ecosystem models are hardly ever unique. The aim of this stud; was to develop and use criteria wh ich permit an objective comparison of different models to the observed field data and to each other. A given model which describes a specifi c 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 a cid deposition: assessment of a lumped-parameter model of soil water a nd stream water chemistry. Water Resour. Res. 21, 51-63) and the SO-Mo del (derived from the Batch Equilibrium Model (BEM; Prenzel, J., 1991. Introduction to BEM (Batch Equilibrium Model), vol 28. Berichte des F orschungszentrums 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 hydrol ogic 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), th e Normalized Mean Absolute Error (NMAE, given by Janssen, P.H.M., Heub erger, P.S.C., 1995. Calibration of process orientated models. In: van Grinvsen. J.J.M. (Ed.) Modelling Water; Carbon and Nutrient Cycles in Forests: Application of 16 Simulation Models to a Spruce Stand at Sel ling, Germany. Ecological Modelling, vol. 83, pp. 55-66), the confiden ce interval test (CIT, developed in this study) and the model rejectio n criteria (Sun, N.Z., 1994. Inverse Problems in Groundwater Modelling . Dordrecht, 337 pp.). Whereas the efficiency and NMAE are related to the averaged data, the CIT and the model rejection criteria include th e spatial heterogeneity at every time step. When evaluated visually, b oth model results might be accepted. From the application of the model performance criteria we selected the MAGIC model as the 'valid' model for our system. (C) 1998 Elsevier Science B.V. All rights reserved.