Can multi-objective calibration of streamflow guarantee better hydrological model accuracy?

Hydrological models often require calibration. Multi-objective calibration has been more widely used than single-objective calibration. However, it has not been fully ascertained that multi-objective calibration will necessarily guarantee better model accuracy. To test whether multi-calibration was effective in comparison to single-calibration in terms of model accuracy, two strategies were tested out. For these strategies, the objective functions used included the Nash-Sutcliffe efficiency and its logarithmic form, which highlight high flow and low flow, respectively. These two indexes were first used for multi-objective calibration, and then they were separately employed for single-objective calibration. To assess the calibration strategies' accuracy, the simulated streamflow was compared with observed streamflow, particularly high flow and low flow. This study was conducted in the upper stream of the Heihe River basin in northwest China using the FLEX-Topo model and MOSCEM-UA algorithm. The results show that the simulation based on the Nash-Sutcliffe efficiency performed best both in modelling the dynamics and simulating the high flow of the observed streamflow. Thus, it seems that multi-objective calibration does not necessarily lead to better model accuracy. This conclusion might provide useful information for hydrologists in calibrating their models, making their simulations more reliable.