The impact of climate fluctuations on the dynamics of soil moisture is studied through a stochastic model of soil water balance. The analysis focuses on the changes of soil water content induced by the interannual variability of rainfall observed at the decade-to-century timescale. Extensive data analyses have been performed to characterize the statistical properties of such a variability. Particular attention is paid to the year-to-year variability of the average value of soil moisture during the growing season because of its relevance to the mechanisms affecting the physiology of plants and the dynamics of ecosystems. It is found that the probability distribution of the average seasonal soil moisture may be either unimodal or bimodal depending on the different combinations of climate, soil, and vegetation parameters. The possible occurrence of a double mode has both hydrologic and ecologic implications that are analyzed here. ¿ 2000 American Geophysical Union