Soil Moisture Controls Beyond Darcy Scale.

Variability observed in near-surface soil moisture is a function of spatial and temporal scale and an understanding of the same is required in numerous environmental and hydrological applications. Past literature has focused largely on the Darcy support scale of measurement for generating knowledge about soil moisture variability. With the advent of a remote sensing era, it is essential to develop a comprehensive understanding of soil moisture variability and the factors causing it at the remote sensing footprint scale. This understanding will facilitate knowledge transfer between scales which remains an area of active research. In this study, we have presented the hierarchy of controls that physical factors namely, soil, vegetation and topography exert on soil moisture distributions from field scale (~800 m) to a footprint scale (12800 m) using remotely sensed soil moisture data. The hierarchy or ranking scheme is a function of the spatial extent of controls of the different physical factors (spatial effect) and the magnitude of the short term mean ‘moisture gradient’ (temporal effect) in the areas controlled by each factor. The study has been conducted over 3 hydro-climates (humid (Iowa), sub-humid (Oklahoma) and semi-arid (Arizona)) to assess the transferability of these results across hydro-climates. It has been found that within a particular hydro-climate, there exists a specific hierarchy to the degree of influence that different physical factors have on soil moisture and this hierarchy changes with change in antecedent moisture conditions. In Arizona, where the soils are poor, we observed that the contribution of soil decreased with increase in scale whereas the contribution of topography and vegetation increased. However, across hydro-climates the results were not transferable. It indicates that the composition of heterogeneity of a particular hydro-climate (i.e. combination of topography, soil and vegetation) influences the effect that each physical factor has on the soil moisture distribution. However, the effect of different physical factors within a hydro-climate itself is dependent upon the spatial support scale of analysis.