Techniques to Answer Hydrologic Questions of the Unsaturated Zone: Borrow, Adapt, Steal, and Dream

Advances in critical-zone hydrology require information from the unsaturated zone, especially for problems related to groundwater contamination, sustainability of water supplies, and ecological availability of water. Highly desirable capabilities include:

· Detection of preferential flow paths, sometimes on an individual basis.

· Identification of easily measured surrogates for property-transfer models (perhaps from fields such as soil mechanics, tomographic imaging, aggregate analysis, and colorimetry), and ways to soundly connect them to the properties of most importance and difficulty.

· Practical means for instrument calibrations that depend on the medium, when the medium is heterogeneous.

· Nonintrusive methods that work at great, as well as shallow, depths.

· Reduction of departures from natural conditions (e.g. properties and processes of undisturbed soil) and improved isolation of measured quantities (e.g. water content independent of salinity and temperature).

Unsaturated zone processes are notoriously difficult to quantify. Soil and rock materials are opaque to visible light, spatially variable in the extreme at all scales, and easily disrupted by intrusive instrumentation. Relevant conditions and properties are rich in nonlinearities, discontinuities, hysteresis, and coupled interactions.

One way forward is to exploit improvements in technology we are already using (e.g. faster microprocessors). Another approach is to borrow from other fields including ones that have previously been fruitful in unsaturated-zone research (e.g. medical imaging, electrical engineering, nuclear physics), and where possible have the tools go bigger, smaller, deeper, and faster, with higher resolution in space and time. There also may be great potential in relatively unexploited evolving fields such as astrogeology, high-energy physics, archaeology, and acoustics. One inviting route is mimicry of biological systems: for example, just as a snake calibrates its infrared vision for rabbits, so might we calibrate electrical resistivity for water content, profiting further from following nature itself.