Journal of Hydrometeorology | Essou et al. 
Precipitation forcing is critical for hydrological modelling as it has a strong impact on the accuracy of simulated river flows. In general, precipitation data used in hydrological modelling are provided by weather stations. However, in regions with sparse weather station coverage, the spatial interpolation of the individual weather stations provides a rough approximation of the real precipitation fields. In such regions, precipitation from interpolated weather stations is generally considered unreliable for hydrological modelling. Precipitation estimates from reanalyses could represent an interesting alternative in regions where the weather station density is low. This article compares the performances of river flows simulated by a watershed model using precipitation and temperature estimates from reanalyses and gridded observations. The comparison was carried out based on the density of surface weather stations for 316 Canadian watersheds located in three climatic regions. Three state-of-the-art atmospheric reanalyses – ERA-Interim, CFSR and MERRA – and one gridded observations database over Canada – NRCan – were used. Results showed that the Nash-Sutcliffe values of simulated river flows using precipitation and temperature data from CFSR and NRCan were generally equivalent regardless of the weather station density. ERA-Interim and MERRA performed significantly better than NRCan for watersheds with weather station densities of less than 1 station per 1000km2 in the Mountainous region. Overall, these results indicate that for hydrological modelling in regions with high spatial variability of precipitation such as Mountainous regions, reanalyses perform better than gridded observations when the weather station density is low.
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