A positive feedback for variability over large land regions may be found in
the mechanism of precipitation recycling. The precipitating water over large
land regions originates either from outside of the region 
. oceans or
surrounding seas) or it may have been derived from local evapotranspiration.
If land evapotranspiration partially feeds precipitating clouds, then the
land hydrology may be said to exert influence on its own forcing. When a
distributed dynamic system has boundary conditions that depend on its
states, then the problem is essentially nonlinear irrespective of the form
of the model.
Brubaker et al. [1993] use observation-based vapor transport climatology to estimate that on an annual basis up to 30% of the precipitation over large land areas is derived from local evaporation. On a seasonal basis, the fraction may be as high as 40%. Zangvil et al. [1993] focus on the United States Midwest region and conclude that local evaporation (generally highest at mid-day) is the main source of day-time precipitation. Large events, however, still require large advection of moisture in order to maintain the precipitation rate. Eltahir and Bras [1993a] estimate the precipitation recycling patterns over the Amazon basin. Depending on the length of the path over which airmasses traverse continental regions, the recycling fraction may grow and local evaporation may then constitute the major source of precipitation water. Chen and Pfaendtner [1993] use various sources of observations to analyze the transport of water vapor from source to sink regions and they illustrate the geographic patterns by combining water vapor transport and precipitation estimates.
Rodriguez-Iturbe et al. [1992] and Entekhabi et al. [1993] construct simple hydrologic balance models for large land regions that included recycling in order to illustrate the consequences of this form of land-atmosphere interaction. They showed that such nonlinear systems, when forced by simple random fluctuations, yield bimodal, intransitive climates where the system locks into a drought mode and persists there for a few years. After a random period the local hydrology escapes drought and locks into a wet-mode and persists there instead. Due to the land-atmosphere interaction, the probability distribution of the hydrologic state is bimodal with the unique characteristic of prolonged and persistent droughts and wet-periods. This is a simple illustration of the potentially significant role of land regions on the regional climatic variations. This influence is enhanced with increasing continentality.