Sensitivity analysis using mass flux and concentration

Abstract

Sensitivity analysis for hydrochemical models requires consideration of the multivariate nature of watershed response. A robust multiobjective generalized sensitivity analysis (MOGSA) procedure, recently developed at the University of Arizona, was used to fully investigate the single objective parameter sensitivity of the Alpine Hydrochemical Model (AHM). A total of 20000 simulations for a two-year period were conducted for the Emerald Lake watershed in Sequoia National Park, California. For each simulation 21 objective functions were evaluated: they were discharge and both concentration and mass flux for ten chemical species. The MOGSA procedure revealed that only 2000 simulations were necessary to establish the parameters sensitive to mass flux or concentration. We found significant differences in parameter sensitivity for concentration versus mass flux objective functions. For example, a snowpack elution parameter and a number of hydrologic parameters were sensitive for Cl- concentration, while only the snowpack elution parameter was 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 discharge events of the early summer. Our results indicate that using mass instead of concentration permits better identification of the model parameters 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.