Soil water characteristic determination from concurrent water content measurements in reference porous media

We introduce and verify the use of calibrated reference soils or other porous media having known and reproducible water retention characteristics as a means to determine the unknown water retention properties of soils in situ. Pockets of the reference soil may be buried at experimental soil locations, with embedded time domain reflectometry (TDR) probes in both the target and adjacent reference media. Monitoring changes in water content by TDR allows inference of the soil water characteristic (SWC) properties of surrounding soil in hydraulic equilibrium via the known reference media SWC relationship. An expression relating the van Genuchten retention coefficients of any two paired porous media is used to estimate the unknown coefficients. Advantages include in situ measurements across the entire soil wetness range, an ability to closely match pore-size distributions (hence hydraulic continuity) with a wide range of porous media, and measurement efficiency based on using the same instrumentation for all measurements. Seven different soils were used in experiments conducted in a laboratory pressure plate apparatus, a greenhouse, and the field. Soil water characteristic relationships obtained using the reference soils approach were generally similar to those measured using pressure plate apparatus. Potential operational concerns include realizing consistent bulk densities for the buried reference soil pockets, and ensuring that water content (theta) measurements are obtained near effective saturation in field applications. Practical utility of the proposed method may be enhanced by identification or manufacture of a range of reference porous media having reproducible SWCs, e.g., that are invariant under different packing conditions.