An important parameter in the assessment of the impact that conditions of limited soil water availability have on vegetation is the average length of the intervals during the growing season in which soil moisture is below some critical levels. These levels are related to the plant tolerance to water stress. The interannual rainfall variability induces important fluctuations on the average duration and frequency of the periods of water stress with important effects on the spatial and temporal structure of plant ecosystems. It is shown that the nonlinearities embedded in the dynamics controlling the soil water balance may drastically enhance the effects of the fluctuations present in the climatic forcing. Interannual climate variability leads to stronger year-to-year changes on the mean duration and frequency of periods of soil water deficit as well as to the emergence of preferential states in the probability distributions of these two variables. The sensitivity of these statistical properties is studied with respect to the characteristics of climate, soil, and vegetation.