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