Soil Water Flow at Different Spatial Scales

A major challenge in hydrological sciences is the modeling of flow and transport processes and their measurement across a range of spatial or temporal scales. Such needs arise, for example, when watershed processes must be determined from soil hydrological data collected from a limited number of in situ field measurements or analysis of small soil cores in the laboratory. The scaling problem cannot be solved by simple consideration of the differences in space or timescale, for several reasons. First, spatial and temporal variability in soil hydrological properties create uncertainties when changing between scales. Second, flow and transport processes in vadose zone hydrology are highly nonlinear. As a result, vadose zone properties are nonunique and scale-dependent, resulting in effective properties that vary across spatial scales and merely serve as calibration parameters in hydrologic models. Therefore, their estimation for heterogeneous materials can only be accomplished using scale-appropriate measurements and models. We present examples of soil water flow at the pore, local, and regional scales. The inherent complexity of flow in heterogeneous soils, and the need to integrate theory with experiment, requires innovative and multidisciplinary research efforts to overcome limitations imposed by current understanding of scale-dependent soil water flow and transport processes. Keywords: vadose zone; unsaturated water flow; flow modeling; scaling; calibration

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