Emerging Measurement Techniques for Vadose Zone Characterization

Variables and parameters required to characterize soil water flow and solute transport are often measured at different spatial scales from those for which they are needed. This poses a problem since results from field and laboratory measurements at one spatial scale are not necessarily valid for application at another. Herein lies a challenge that vadose zone hydrologists are faced with. For example, vadose zone studies can include flow at the groundwater-unsaturated zone as well as at the soil surface-atmosphere interface at either one specific location or representing an entire field or landscape unit. Therefore, vadose zone measurements should include techniques that can monitor at large depths and that characterize landsurface processes. On the other end of the space spectrum, microscopic laboratory measurement techniques are needed to better understand fundamental flow and transport mechanisms through observations of pore-scale geometry and fluid flow. The Vadose Zone Hydrology (VZH) Conference made very clear that there is an immediate need for such microscopic information at fluid-fluid and solid-fluid interfaces, as well as for methodologies that yield information at the field/landscape scale. The need for improved instrumentation was discussed at the ASA-sponsored symposium on “Future Directions in Soil Physics” by Hendrickx (1994) and Hopmans (1994). Soil physicists participating in the 1994-1999 Western Regional Research Project W-188 (1994) focused on “improved characterization and quantification of flow and transport processes in soils,” and prioritized the need for development and evaluation of new instrumentation and methods of data anlysis to further improve characterization of water and solute transport. The regional project documents the critical need for quantification of water flow and solute transport in heterogeneous, spatially variable field soils, specifically to address preferential and accelerated contaminant transport. Cassel and Nielsen (1994) describe the contributions in computed tomography (CT) using x-rays or magnetic resonance imaging (MRI) as “an awakening,” and they envision these methodologies to become an integral part of vadose zone research programs. The difference in size between measurement and application scales poses a dilemma for the vadose zone hydrologist.