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To model the water, solute and energy transport in porous media, it is essential to have accurate information about the soil hydraulic properties (SHP), i.e. the water retention curve (WRC) and the soil hydraulic conductivity curve (HCC). Having reliable data information to parameterize these models is important, but equally critical is the selection of appropriate SHP models. While various expressions for the WRC are commonly compared, the capillary conductivity model proposed by Mualem (1976a) is widely used but seldom compared to alternatives. The objective of this study was to compare four different capillary bundle models in terms of their ability to accurately predict the HCC without scaling the conductivity function by a measured conductivity value. These expressions include two simpler models proposed by Burdine (1953) and Alexander and Skaggs (1986), which assume a bundle of parallel capillaries with tortuous flow paths, and two more sophisticated models based on statistical cut-and-random-rejoin approaches, namely those proposed by Childs and Collis-George (1950) and the aforementioned model of Mualem (1976a). In order to check whether different parametrizations of the WRC interfere with the suitability of the conductivity models, we utilized four different capillary saturation models in combination with each of the conductivity prediction models, resulting in a total of 16 SHP model schemes. All schemes were calibrated using 12 carefully selected datasets that provided water retention and hydraulic conductivity data over a wide saturation range. Subsequently, the calibrated models were tested and rated by their ability to predict the hydraulic conductivity of 23 independent datasets of soils with varying textures. The statistical cut-and-random-rejoin models, particularly the Mualem (1976a) model, outperformed the simpler capillary bundle models in terms of predictive accuracy. This was independent of the specific WRC model used. Our findings suggest that the widespread use of the Mualem model is justified.

The integral suspension pressure method (ISP) is an automated method to evaluate sedimentation experiments for particle size analysis of soil materials. In contrast to the traditional pipette and hydrometer methods, it is based on the continuous measurement of the suspension pressure at one depth in the sedimentation cylinder. The particle size distribution is determined by inverse simulation (Durner et al., 2017). The ISP is promising because it is semi-automated, continuous, based on process simulation, and does not hinge on oversimplifying assumptions. Most importantly, disturbance of the settling particles is minimized whereas disturbance is unavoidable when applying the traditional methods. ISP has been made commercially available by the METER Group AG (Munich) with a device called PARIOTM. This implementation of ISP leads to a computerized system which yields quasi-continuous particle-size distribution curves.Practical experience with PARIO has shown that, despite cutting-edge pressure sensor technology with a resolution of 0.1 Pa, the accuracy of the particle-size analysis was less than expected from a theoretical analysis, and that the time required to determine the clay content exceeded theoretical expectations. In this contribution, we analyze the reasons for disturbances of the methodology in practical applications and show ways to improve accuracy by compensating different errors. Furthermore, we show how an extended version of ISP called ISP+, which considers a single additional measurement in the objective function (Durner and Iden, 2019), leads to stable estimates of the clay fraction while considerably reducing the measurement time.References:Durner, W., S.C. Iden, and G. von Unold (2017): The integral suspension pressure method (ISP) for precise particle-size analysis by gravitational sedimentation., Water Resources Research, 53, 33-48, doi:10.1002/2016WR019830.Durner, W., and S.C. Iden: ISP+: improving the Integral Suspension Pressure method by an additional measurement, Geophysical Research Abstracts Vol. 21, EGU2019-12761, 2019.

Studying the role of gravel, stones, or rock fragments on effective soil hydraulic properties (SHPs) is crucial for understanding and predicting soil water processes such as evaporation, redistribution, and water and solute transport through soils containing significant amounts of coarse inclusions. We conducted a laboratory study in which we investigated the effect of stones on the water retention and unsaturated hydraulic conductivity curves of soil–stone mixtures. Stony soils were created by packing predefined masses of soil particles (sand and sandy loam) with diameters