Regionalization of precipitation and air temperature over high-altitude catchments - learning from outliers

Closing the water balance of mountainous catchments may sometimes become tricky, even before applying a hydrological model. In this paper, we focus on mountainous, snow-affected catchments and try to understand the reasons for the unrealistic hydrological behaviours that they sometimes show: when annual runoff is greater than the annual areal precipitation estimate, something is obviously wrong, but finding the appropriate means to adjust the water balance is far from a trivial matter. This paper aims to improve our knowledge of the water balance of mountainous and snow-affected catchments in two different countries: Sweden and France. We use a simple non-dimensional framework to detect outliers and then propose an regionalization of precipitation and air temperature in order to better estimate inputs over high-altitude catchments. Since we are interested in catchment water balance, we evaluate our regionalized input estimates by comparing them to streamflow measurements. The results are mixed: in Sweden, our approach is successful because it can make most outlier catchments become regular ones; but in France it is disappointing because it does not solve most of the water balance problems identified. However, for both countries, the regionalization approach significantly improves the performance of a rainfall-runoff model at a daily time step.