Simulations of soil moisture and surface water balance using the simple biosphere model 2

Soil moisture and water balance for global and regional scales have been ca lculated using a land-surface process model (SiB2) forced by observed and m odel assimilated data. The simulated runoff for each grid cell has been pro vided as input to a global river routing model, in order to simulate river discharge rates. The simulated soil moisture and water balance have been co mpared with available observations for their annual mean and seasonal cycle s and for global, basin and grid point scales. The global distributions of the annual-mean soil moisture and wetness have been reasonably simulated. T here were large inter-annual variations of soil moisture in both the simula tions and observations at local stations. The simulated annual discharges f or major river basins agree reasonably well with observations, but with som e underestimates for large discharges and some overestimates for small disc harges. The seasonal cycle of river discharges has been well simulated for specific basins in the tropics, midlatitudes, and high latitudes, although for some basins the annual mean is underestimated. In the tropics, the seas onal cycles of soil moisture and the surface water balance are dominated by the precipitation cycle. In mid- and high latitudes, soil moisture and the water balance are affected by both the temperature and precipitation cycle s, and by the snow accumulation/melting cycle. The range of seasonal soil m oisture variations becomes smaller with increasing latitude. The seasonal c ycles of soil moisture for selected grid points have been compared with sel ected station observations. Even though there are differences in forcing an d in some specific surface boundary parameters at the stations, the simulat ed soil moisture agrees well with multiyear observations at a majority of t he stations. However, for almost all the selected grid cells, the seasonal variations are smaller, the snow melt and soil drying processes are late by about one month, and the soil is relatively wet in summer, compared with o bservations. These errors can be partly attributed to the unrealistically c ool temperatures provided to the model as forcing data, favoring less surfa ce evaporation and a later seasonal cycle, especially for mid- and high lat itudes.