The ISBA surface scheme in a macroscale hydrological model applied to the Hapex-Mobilhy area - Part II: Simulation of streamflows and annual water budget

A macroscale hydrological model including an improved representation of the surface processes is validated at a regional scale using the data set coll ected during the Hapex-Mobilhy experiment (1986), Using observed atmospheri c forcing with a resolution of 5 km and realistic fields of vegetation and soil types in a large area, the hydrological model correctly simulates dail y streamflows for two contrasting climatological years in 1986 and 1987, pr ovided that a sub-grid scale parameterization of surface runoff is included in the surface scheme. The comparison with point observations of evaporati on and soil water content shows that the surface scheme satisfactorily repr oduces the annual water budget for very diverse land uses, It is shown that , although the annual rainfall was significantly lower in 1987, the annual evaporation was higher than in 1986 because precipitation fell mainly in su mmer. The reduction of precipitation contributed to a comparable reduction of total runoff, and therefore a drastic decrease in the river flows as con firmed by the observations. The second part of the study addresses the issu e of modelling the surface water budget at a large grid scale. Once validat ed, the macroscale hydrological model is used as a reference to test aggreg ation methods. Aggregated fluxes of evaporation, drainage and surface runof f are found to be close to the reference fluxes provided that sub-grid scal e surface runoff and interception losses are taken into account in the aggr egation procedure. (C) 1999 Elsevier Science B.V, All rights reserved.