Why do we have so many different hydrological models? A review based on the case of Switzerland

Hydrology plays a central role in applied as well as fundamental environmental sciences, but it is well known to suffer from an overwhelming diversity of models, in particular to simulate streamflow. Based on Switzerland's example, we discuss here in detail how such diversity did arise even at the scale of such a small country. The case study's relevance stems from the fact that Switzerland shows a relatively high density of academic and research institutes active in the field of hydrology, which led to an evolution of hydrological models that stands exemplarily for the diversification that arose at a larger scale. Our analysis summarizes the main driving forces behind this evolution, discusses drawbacks and advantages of model diversity and depicts possible future evolutions. Although convenience seems to be the main driver so far, we see potential change in the future with the advent of facilitated collaboration through open sourcing and code sharing platforms. We anticipate that this review, in particular, helps researchers from other fields to understand better why hydrologists have so many different models.