‘As simple as possible but not simpler’: What is useful in a temperature-based snow-accounting routine? Part 1 – Comparison of six snow accounting routines on 380 catchments

This paper analyzes the behavior of hydrological snow accounting routines (SARs) used in combination with hydrological models to simulate streamflow at the catchment scale. To reach conclusions as general as possible, we compare the performance of six existing SARs combined with two different precipitation-runoff models. The SARs are temperature-based, have different levels of complexity (understood here as the number of optimized parameters and model functions), include various processes and also differ by the way they account for the spatial heterogeneity of snow cover. The SARs were tested on a set of 380 catchments significantly affected by snow and located in four countries (France, Switzerland, Sweden and Canada), showing different climatic conditions and altitude ranges. The value of each SAR is evaluated solely in terms of flow simulation quality at the catchment outlet. Several efficiency criteria are used, some of them specifically focusing on the time periods affected by snow accumulation and melt.As expected, the use of a snow accounting routine on snow-affected catchments significantly improves model efficiency, and this is true even for the simplest SARs. More interestingly, our results show that the most complex SAR does not yield the highest performance. Surprisingly, a lumped routine (i.e. without distribution in elevation bands) appears to be the most efficient on average on the whole catchment set. Results seem particularly sensitive to the spatial variability of processes in the snowpack and to the determination of the precipitation phase (solid or liquid). One critical point remains the identification of the solid precipitation correction factor necessary to compensate for snowfall measurement errors. In the companion article, we further investigate the sensitivity of model results to the description of snow processes in the SAR and try to identify the most important components of a parsimonious and general SAR.