The interannual variability of summer mean soil moisture conditions in Switzerland during the 20th century was studied using a stochastic soil moisture model. The model was validated against high-quality observations of the water balance at a representative location. The summer mean degree of saturation was found to fluctuate around the point of stomatal closure at low elevations to the north of the Alps, clearly above this threshold at higher elevations and to the south of the Alps, and systematically below this limit in the inner-Alpine domain. In addition, the results indicated that the largest interannual variability occurs at low elevations. A linearization of the model revealed that interannual variations of summer mean soil moisture are driven in order of importance by variations in the rate of storm arrivals, joint variations in the rate of storm arrivals and maximum evapotranspiration, and variations in mean rainfall depth. This was explained in terms of the sensitivities of seasonal mean soil moisture with respect to the driving parameters.