SENSITIVITY OF THE HYDROLOGICAL CYCLE TO THE PARAMETRIZATION OF SOIL HYDROLOGY IN A GCM

The sensitivity of the hydrological cycle to soil hydrology is investi gated with the LMD GCM. The reference simulation includes the land-sur face scheme SECHIBA, with a two-reservoir scheme for soil water storag e and runoff at saturation. We studied a non-linear drainage parametri zation, and a distributed surface runoff parametrization, accounting f or the subgrid scale variability (SSV) of soil moisture capacity, thro ugh a distribution where the shape parameter was b. GCM results show t hat the drainage parametrization induces significant reductions in soi l moisture and evaporation rate compared to the reference simulation. They are related to changes in moisture convergence in the tropics, an d to a precipitation decrease in the extratropics. When drainage is im plemented, the effect of the SSV parametrization (b = 0.2) is also to reduce soil moisture and evaporation rates compared to the simulation with drainage only. These changes are much smaller than the former, bu t the sensitivity of the hydrological cycle to the SSV parametrization is shown to be larger in dry periods, and to be enhanced by an increa se of the shape parameter b. The comparison of simulated total runoffs with observed data shows that the soil hydrological parametrizations does not reduce the GCM systematic errors in the annual water balance, but that they can improve the representation of the total runoffs ann ual cycle.