The land fraction of the Earth is small (30%) but its distribution into large contiguous areas and its distinctive hydrothermal inertia cause significant variations in regional climatic systems. Milly and Dunne [1994] use a GCM to investigate the influence of the land moisture inertia on the model climate. They analyze shifts in the extent and intensity of general circulation and the atmospheric hydrologic cycle in response to the wide spectrum of values for soil water storage capacity. Using the same GCM, Delworth and Manabe [1993] show that the presence of an interactive soil moisture reservoir acts to increase the variance and add memory to near surface atmospheric variables such as humidity. These modeling studies demonstrate that moisture inertia at the land surface boundary (introduced by adding soil reservoir) acts as a low-pass filter which shifts the variability from higher to lower frequencies. Those fluctuations in the atmosphere that are affected by the surface moisture source are in turn characterized by greater memory and larger range of anomalies. The soil moisture storage and transfer processes in GCMs are grossly simplified. Redistribution due to topography and numerous other important processes that may significantly contribute to land-atmosphere interaction are neglected.
The influence of these processes and feedback mechanisms on the near surface
atmosphere are largely unknown. An illustrative example of mechanisms
through which land-atmosphere interaction can modulate climatic fluctuations
may be posed using a conceptual figure (below) from McNab and Karl
[1989]. The top curve in Figure 1 represents typical fluctuations in the
precipitation anomaly time-series. Over land regions, the surface runoff
(cumulative hydrographs) responds to these anomalies with some delay. The
next hydrologic reservoir, soil moisture, responds to the runoff anomalies
with still more delay. The baseflow contribution to streamflow also follows
the anomalies of soil moisture with some lag as well. Finally the
groundwater system fluctuations lag those of the streamflow and soil
moisture. As the precipitation anomaly makes its way through components of
the land hydrologic cycle (storm runoff 
soil moisture
streamflow 
groundwater) its phase-lag
with respect to the precipitation anomaly is increased and, due to the
storage associated with each component, the fluctuations are dampened
significantly. Again, the soil hydrology is acting essentially as a low-pass
filter, but in both space and time [ Entekhabi and Rodriguez-Iturbe,
1993]. The net result is an extension of meteorological drought
(precipitation deficit) to delayed and prolonged hydrologic drought
(available soil water deficit).
Figure 1 demonstrates how an atmospheric forcing anomaly propagates down through the regional hydrologic system. An important effect that may be added to Figure 1 is land-atmosphere interaction. What would be the over-all effect if the regional soil moisture could affect the atmospheric forcing? That would correspond to the possibility of anomalies moving ``up'' as well as down the figure. If soil moisture positively affected precipitation, then a soil moisture anomaly due to an earlier precipitation anomaly would cause precipitation deficit currently and in the future due to the lagging effect. Such feedback mechanism would then lead to persistence and essentially locking of anomalous events until a large enough external fluctuation forces the system to regain its normal state. The subsurface branch of the hydrologic cycle can essentially transform, by extending the duration, the meteorologic drought into a hydrologic drought. Smith and Richman [1993] find fluctuations with 20 to 40 years time scale in Palmer Drought Index constructed based on precipitation and temperature observations. This index is essentially a surrogate for surface soil moisture. Smith and Richman [1993] also report significant streamflow increase and up to one meter rise in the groundwater table following wet climate anomalies.