Entekhabi [this issue] documents how atmospheric processes propogate through the complete regional hydrologic system and vice versa. Both Entekhabi and the NRC [1991] report, Opportunities in the Hydrologic Sciences, firmly establish that since the atmospheric forcing of surface hydrologic processes depends upon the land surface conditions, a two-way land-atmospheric interaction exists. Bales and Harrington [1994] also emphasize the importance of quantifying energy fluxes across the land-atmosphere because of their dominant role in snowpack evolution and meltwater generation. Since the land, biosphere, atmosphere and ocean systems are coupled across a wide range of spatial and temporal scales, each scientific discovery tends to lead to a deeper and larger scientific question [ Entekhabi, this issue].
Spatial variability of land surface processes affects the dynamics of the atmosphere (and vice versa) at different scales. Avissar [this issue] argues that an important hydrologic challenge is to develop a soil-vegetation-atmosphere-transfer scheme (SVATS) which can provide the characteristic length scale of the land-surface patchiness and the distribution of surface heat fluxes. To obtain the distribution of heat fluxes required, Avissar argues that such hydrologic SVATS schemes will likely require high-resolution topography, soil texture and vegetation type. At the global scale, development of such data sets remains an enormous challenge. Avissar [this issue] further argues that since such high-resolution macroscale field experiments are not likely to be available in the foreseeable future, we will have to rely on theoretical analyses, for the moment, to guide more focused field experiments. Nevertheless, Entekhabi argues that some of the new large-scale data sets (also see Kustas [this issue], and Engman [this issue]) combined with the scientific questions raised in his review and posed by the NRC [1991] should lead to advances in our understanding of the coupled land-atmosphere interaction.