Influence of runoff parameterization on continental hydrology: Comparison between the Noah and the ISBA land surface models

[ 1] A comprehensive set of hydrological parameterizations without any basin-scale calibration was recently introduced into the global ISBA land surface model in order to improve the simulation of the hydrological impacts of both seasonal climate anomalies and global warming. In this study, the same approach is introduced into the Noah land surface model, in order to improve the representation of surface runoff. The Dunne runoff is computed using topographic information via a TOPMODEL approach and the Horton runoff using an explicit distinction between frozen and unfrozen soils, while land surface heterogeneities are introduced via a tile approach. The validation is conducted at a 1 degrees by 1 degrees horizontal resolution using global off line simulations that were driven by Global Soil Wetness Project atmospheric forcing. The simulated runoff is converted into discharges using the TRIP river routing model and compared to observations from a dense network of gauging stations distributed over the world's largest river basins. Results confirm that the relationships between the Dunne runoff and the topography, as well as between the Horton runoff and the frozen soils, are among the main hydrological processes to simulate realistic global river discharges. The comparison between the old and the new versions of Noah and ISBA reveal that the model disparity, in term of water flux production, is globally reduced due to the use of similar surface hydrology that lead to comparable improvements of discharge scores. This confirms the relevance of these surface hydrological processes for regional and global applications.