Parameter estimation for a global model of terrestrial biogeochemical cycling by an iterative method

We have implemented a geographically distributed ecosystem model TERRA for the carbon, nitrogen, and water dynamics of the global terrestrial biospher e. The ecosystem model in each grid cell has state variables of soil water; vegetation carbon, soil carbon, vegetation nitrogen, soil organic nitrogen , soil inorganic nitrogen, and a variable for allocation. Eight parameters associated with eight carbon or nitrogen fluxes are determined during model calibration at specific sites for each of 17 vegetation types that cover t he globe. Calibration is performed by an iterative method that brings calcu lated fluxes into agreement with observed fluxes over successive iterations . For the 17 vegetation types, calibration required a geometric mean number of 123 iterations for convergence with a geometric S.D. of 2.25. The minim um and maximum iterations required was 52 and 1060 for xeromorphic woodland and tropical evergreen forest, respectively. The parameter controlling gro ss primary productivity C-max was found to be correlated with maximum proje cted leaf area index (plus cover of nonvascular plants) with a correlation coefficient of 0.90 (r(2) = 0.81). Correlation of parameters with the input fluxes and average standing crops corresponding to the iteration equation was high in those cases in which temperature and precipitation were not exp licit factors in the iteration equation. The parameters K-fall, L-nc, and N -loss had correlation coefficients of 1.0 each with the appropriate ratio o f the matching observed transfer flux and standing crop. In iteration equat ions with explicit dependence on temperature, soil moisture, and other fact ors, the correlation coefficients were less than 1.0 with these ratios; the parameters C-max, K-r, K-d, N-max, and N-up had correlation coefficients o f 0.83, 0.66, 0.60, 0.61, and 0.87, respectively. These correlation coeffic ients indicate the importance of environmental factors such as temperature and soil moisture in the calibration process and the robustness of the iter ation method in determining parameters in systems with time-varying coeffic ients. (C) 2001 Elsevier Science B.V. All rights reserved.