Performance of the theta probe ML2 in the presence of nonuniform soil water profiles

An attempt is made to investigate the soil permittivity regime which is predicted by ML2 theta probe in cases where a nonuniform soil moisture profile prevails. Our investigation was performed in laboratory columns, under temperature-controlled conditions for a number of different porous media. Five distinct cases, where the moisture regime could be established in a predesigned manner were tested. These are: (a) Air over a water or ethanol layer of changing thickness. (b) Air over a saturated porous material of changing thickness and the same in the reverse order. (c) Dry porous material over the same but saturated porous material of changing thickness and vice versa. (d) Gradual wetting from the top surface of initially dry soils. (e) Initially saturated vertical soil profiles, left to gradually dry by evaporation from both open surfaces (top and bottom).From our experimental investigations it is shown that the bulk dielectric constant predicted by the ML2 probe in nonuniformly wet profiles irrespective of their layer thickness is closely described by the arithmetic averaging scheme. This fact makes the water content predictions from the manufacturer calibration equations for these cases, seriously overestimated. A comparison of the actual water contents with those predicted with the calibration equations of Robinson et al. [Robinson, D.A., Jones, S.B., Blonquist, J.M., Friedman, S.P., 2005. A physical derived water content/permittivity calibration model for coarse–textured layered soils. Soil Sci. Soc. Am. J. 69 (5), 1372–1377] gives very satisfactory results.