Apart from the notable exception of the analysis of the transient dynamics caused by the initial condition of soil moisture at the start of the growing season (Sections 3.3 and 7.1), until now only statistically steady conditions during the growing season have been considered. In the context of this book, however, two other sources of temporal dynamics may also be important: the seasonality of both the rainfall regime and the evapotranspiration demand, and the interannual rainfall fluctuations. The former may be especially relevant in Mediterranean climates or at mid latitudes where the rainfall regime may have multiple maxima (e.g., spring and fall) and the passage between the growing season and the dormant season is more gradual. In some cases, however, when the seasonality is marked but the regimes of the two seasons are distinct, the steady-state analysis of the growing season often proves to be satisfactory, provided that the initial soil moisture conditions are properly accounted for (see Chapters 5 and 7). The second form of variability, i.e., the interannual one, is in many aspects even more important, and tends to be more intense in arid and semi-arid regions. Its origin stems from the fact that each growing season is characterized by a rainfall regime in part controlled by general circulation and global climate characteristics, which are affected by year-to-year variability. This brings about interannual fluctuations in the growing season rainfall regime that are more intense than what would simply result from different realizations of precipitation obtained using the same stochastic process with constant coefficients (e.g., α and λ).