Sensitivity analysis using mass flux and concentration

Sensitivity analysis for hydrochemical models requires consideration of the multivariate nature of watershed response. A robust multiobjective general ized sensitivity analysis (MOGSA) procedure, recently developed at the Univ ersity of Arizona, was used to fully investigate the single objective param eter sensitivity of the Alpine Hydrochemical Model (AHM). A total of 20000 simulations for a two-year period were conducted for the Emerald Lake water shed in Sequoia National Park, California. For each simulation 21 objective functions were evaluated: they were discharge and both concentration and m ass flux for ten chemical species. The MOGSA procedure revealed that only 2 000 simulations were necessary to establish the parameters sensitive to mas s flux or concentration. We found significant differences in parameter sens itivity for concentration versus mass flux objective functions. For example , a snowpack elution parameter and a number of hydrologic parameters were s ensitive for Cl- concentration, while only the snowpack elution parameter w as sensitive for Cl- mass flux. By using mass flux instead of concentration fewer mineral weathering parameters and more soil exchange parameters were sensitive. Mass flux calculations emphasize the spring snowmelt and peak d ischarge events of the early summer. Our results indicate that using mass i nstead of concentration permits better identification of the model paramete rs that most affect stream conditions during peak springtime flows and that some combination of mass flux and concentration objectives should be used in evaluating model performance. Copyright (C) 1999 John Wiley & Sons, Ltd.