Subgrid runoff parameterization

A subgrid parameterization of runoff based on the TOPMODEL hydrological framework is introduced within the ISBA surface scheme. Indeed, by the mean of a topographic index of hydrological similarity, the TOPMODEL framework suggests an explicit scaling equation linking local scale and macro scale water deficit. The idea is then to couple this scaling equation with ISBA to be able to associate the water content of each macro pixel (here 64km²) simulated by ISBA to the corresponding subgrid spatial distribution of the local pixels (here 75×75m²) of the digital elevation model (DEM). The main advantage is that this subgrid runoff parameterization depends only on the topography, without any calibration parameters. The ISBA-TOPMODEL model is tested in the Ardeche basin (France), for the period 1981–1995. Soil, vegetation and atmospheric forcing are taken form the GEWEX-Rhone database. The comparison between the simulation obtained with ISBA-TOPMODEL, the former version of ISBA including the Variable Infiltration Capacity subgrid runoff scheme, and the observations is presented.