The ISBA surface scheme in a macroscale hydrological model applied to the Hapex-Mobilhy area: Part I: Model and database

As a first step towards coupling atmospheric and hydrological models, this article describes the implementation of the Interface Soil Biosphere Atmosphere (ISBA) surface scheme within a macroscale hydrological model at the regional scale. The introduction of the diurnal cycle in the hydrological model allows a coupling with the atmosphere through the energy balance and water budget computations. The coupled model is used in a forced mode, i.e. atmospheric forcing is imposed on the surface scheme without any retroaction of the surface exchanges on the atmosphere. The initial version of ISBA was modified to account for sub-grid runoff. Existing classifications of soil and vegetation at high spatial resolution (1 km) were used in conjunction with satellite information to monitor the monthly evolution of the vegetation. A dense surface network facilitated the interpolation of atmospheric fields with a time step of 3 h. The two years considered show large differences of annual precipitation and potential evaporation, which have an impact on the regime of the river flows. A one-dimensional study presents the sensitivity of the partitioning of precipitation into evaporation and runoff to the sub-grid runoff and drainage parameterizations, soil depth, and vegetation cover.