By analyzing the data from the Second Global Soil Wetness Project (GSWP-2) sensitivity experiments, this study quantifies the sensitivities of soil wetness simulation to uncertainties in precipitation and radiation forcings and estimates the actual influence of these uncertainties. The sensitivity of soil moisture to uncertainties in radiation has a large seasonal cycle because of its temperature dependence. Sensitivity to uncertainties in precipitation is less temporally variable and is large in semiarid regions. Precipitation sensitivity is generally dominant over radiation sensitivity in cold climate, while in warm climate radiation sensitivity is dominant or both of them are high. However, actual uncertainties in precipitation are typically much larger than for net radiation in warm climate, so the practical impact is dominated by our lack of accurate precipitation estimates. Precipitation uncertainties account for about 2/3 of the total soil wetness uncertainties from all forcing data, while radiation uncertainties only account for about 1/6.