Global validation of the ISBA sub-grid hydrology |
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| Authors: | B Decharme H Douville |
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| Institution: | (1) CETP-IPSL-CNRS, 10 avenue de l’Europe, 78140 Velizy-Villacoublay, France;(2) Météo-France, CNRM/GMGEC/UDC, 42 Avenue G. Coriolis, 31057 Toulouse, France |
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| Abstract: | Over recent years, many numerical studies have suggested that the land surface hydrology contributes to atmospheric variability
and predictability on a wide range of scales. Conversely, land surface models (LSMs) have been also used to study the hydrological
impacts of seasonal climate anomalies and of global warming. Validating these models at the global scale is therefore a crucial
task, which requires off-line simulations driven by realistic atmospheric fluxes to avoid the systematic biases commonly found
in the atmospheric models. The present study is aimed at validating a new land surface hydrology within the ISBA LSM. Global
simulations are conducted at a 1° by 1° horizontal resolution using 3-hourly atmospheric forcings provided by the Global Soil
Wetness Project. Compared to the original scheme, the new hydrology includes a comprehensive and consistent set of sub-grid
parametrizations in order to account for spatial heterogeneities of topography, vegetation, and precipitation within each
grid cell. The simulated runoff is converted into river discharge using the total runoff integrating pathways (TRIP) river
routing model (RRM), and compared with available monthly observations at 80 gauging stations distributed over the world’s
largest river basins. The simulated discharges are also compared with parallel global simulations from five alternative LSMs.
Globally, the new sub-grid hydrology performs better than the original ISBA scheme. Nevertheless, the improvement is not so
clear in the high-latitude river basins (i.e. Ob, MacKenzie), which can be explained by a too late snow melt in the ISBA model.
Over specific basins (i.e. Parana, Niger), the quality of the simulated discharge is also limited by the TRIP RRM, which does
not account for the occurrence of seasonal floodplains and for their significant impact on the basin-scale water budget. |
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