The ISBA surface scheme in the MODCOU macroscale hydrological model

• The ISBA surface scheme in the MODCOU macroscale hydrological model applied to the Adour ( Hapex-Mobilhy area) and the Rhone river basins

Summary

    As a first step towards coupling atmospheric and hydrological models, Habets et al. (1999a, 1999b, 1999c) describe the implementation of the ISBA surface scheme within a macroscale hydrological model at the regional scale: the Modcou model (Ledoux et al. 1989). This study corresponds to the French contribution to the GEWEX/WCRP programme (Météo-France, ENSMP/CIG, CEMAGREF, CETP / CNRS / UVSQ) and was supported by the Programme National de Recherche en Hydrologie and by the Programme National d'Etude de la Dynamique du Climat. 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 ISBA-MODCOU model has two time steps (Figure 1): a fast time step to resolve the interactions between the surface and the atmosphere and a slow time step (one day) for the simulation of the surface and underground flows in the hydrological model. The gravitational drainage and the surface runoff computed by ISBA are transfered to MODCOU every day. The coupled model has been tested in two river basins on the long term (more than 10 years of integration): Firstly, the Adour basin (Hapex-Mobilhy area) was considered because a significant data base on the hydrology, the atmosphere and the soil and vegetation types was available (Habets et al. 1998, 1999). The second phase of the program was to transfer the whole system to the Rhone basin (Etchevers 1999, Golaz 1999): a considerably larger area with a higher topography (the Rhone basin encompasses a fraction of the Alps mountains) and a larger variability of climate. In the two basins, the ISBA-MODCOU model is used in a forced mode, i.e. the atmospheric forcing is imposed on the surface scheme without any retroaction of the surface exchanges on the atmosphere. The atmospheric forcing (precipitation, snowfall, incoming radiation fluxes, atmospheric quantities) has been regularly interpolated (resolution of 8 km), every 3 hours, on the long term: 10 years for the Adour basin (1986-1995) and 15 years in the Rhone basin (1981-1995).

    The initial version of ISBA has been modified to account for sub-grid runoff using the VIC approach (Wood et al. 1992). This sub-grid runoff scheme has been modified in order to reduce strongly runoff over very dry soils using a threshold set to the wilting point. Surface parameters were prescribed in the simulation domain from existing classifications of soil and vegetation at high spatial resolution (1km) (Corine Land Cover and INRA data bases) in conjunction with satellite information to monitor the monthly evolution of the vegetation.

    In the Adour basin, the ISBA-MODCOU model has been calibrated for the year 1986 using the Hapex-Mobilhy data set (Habets et al. 1999) and then intergrated during 10 years. The comparison with point observations of evaporation and soil water content shows that the surface scheme satisfactorily reproduces the annual water budget for very diverse land uses. The model is used as a tool for understanding how changes in meteorological conditions may affect the annual water balance and the ressource availability of a large basin. The impact of the interannual variability of precipitation on the water budget and on the river flows has been analyzed. The reduction of precipitation contributed to a comparable reduction of total runoff, and therefore a drastic decrease in the river flows as confirmed by the observations.

    In the Rhone basin, the simulation clearly shows the importance of topography and snow on the hydrological regime of the Rhone river and its tributaries. The simulated spatial variability of evaporation and total runoff are very large in the basin. Small annual evaporation and large runoff are found in the Alps because of the snow processes. On the other hand, the areas experiencing Mediterranean climate conditions (large annual global radiation, low precipitation) are characterized by negligeable annual runoff. The simulation has been used as a reference to test aggregation methods accounting for the sub-grid variability of surface processes within a large area (128 km by 128 km). Habets et al. (1999b) showed that the aggregated surface fluxes, drainage and runoff can be computed with an error lower than 5 % provided that the sub-grid variability of precipitation, runoff and vegetation is taken into account. If these sub-grid processes are not aggregated, the errors in the simulation of the various terms of the water balance may exceed the annual reference by 20 %.

    The next steps of the hydrological modelling programme are:

• (i) to analyze the simulation of the energy and water budgets on the long term (1981-1995) (P. Etchevers /Météo-France, C. Golaz ENSMP/CIG)
• (ii) to test the impact of interactive vegetation using the ISBA-Ags version in the Adour and Rhone basins (S. Voirin and A. Boone/ Météo-France)
• (iii) to improve the representation of sub-grid runoff (intruducing the Top Model concept) and drainage
• (iv) to implement the ISBA-MODCOU model within the Garonne basin

List of Figures