Near Surface Soil Moisture Controls Beyond the Darcy Support Scale: A Remote Sensing Perspective

Abstract:

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 and the factors causing it. With the advent of a remote sensing era, it is essential to develop a comprehensive understanding of soil moisture variability and the factors creating 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 airborne remote sensor footprint scale (~800 m) to a satellite footprint scale (12800 m) across 3 hydro-climates- humid (Iowa), sub-humid (Oklahoma) and semi-arid (Arizona). We evaluated the effect of physical factors on soil moisture variability at coarse spatial support scales but fine (daily) temporal spacing scales which are typical of remotely sensed soil moisture data. The hierarchy or ranking scheme defined in the study is a function of the areal extent of controls of the different physical factors and the magnitude of their effect in creating spatial variability of soil moisture. We found that even though the areal influence of soil on soil moisture variability remained significant at all scales, it decreased as we went from airborne scale to coarser scales whereas the influence of topography and vegetation increased for all three hydro-climates. The magnitude of the effect of these factors, however, was dependent on antecedent soil moisture conditions and hydro-climate.