The objective of this study was to characterize a very coarse alluvial vadose zone of the Ljubljana basin (Slovenia), with a gravel content >75%. A combination of the simplified evaporation experiment (HYPROP) and the dew point method (WP4 PotentiaMeter) was used to obtain the information across a wide moisture range. We obtained water retention data on fine (diameter <2 mm) and coarse samples (diameter <8 mm) and compared them with the mass-based gravel correction. Measurements on coarse samples enabled us to determine (i) the influence of coarser particles on the shape of the water retention curve (WRC), (ii) if the retention curves are consistent with the decrease in the water content from fine to coarse to bulk samples, (iii) what known parametric models would be most suitable in representing hydraulic properties of coarse material, and (iv) if the mass-based gravel correction is performing well for a coarse set of samples. The results of the study revealed that the vadose zone is represented by four major sedimentary textures, with the most dominant being well graded gravel followed by poorly graded gravel. The parameterization of the obtained data showed that both fine and coarse sets of data were best described with the bimodal van Genuchten model of Durner with the constrained conductivity model of Peters and Durner (VGbi-PDI) and the bimodal van Genuchten model of Durner with the unconstrained conductivity model of Peters and Durner (VGbi-PDII) across the whole moisture range. The measurements on the coarse set of samples led us to the conclusion that the mass-based gravel corrections do not adequately describe the WRC of samples containing large amounts of gravel.

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