Reproducibility of the Wet Part of the Soil Water Retention Curve: A European Interlaboratory Comparison

Abstract

Abstract. The soil water retention curve (SWRC) is a key soil property required for predicting basic hydrological processes. SWRC is often obtained in laboratory with non-harmonized methods. Moreover, procedures associated to each method are not standardized. This can induce a lack of reproducibility between laboratories using different methods and procedures or using the same methods with different procedures. The goal of this study was to estimate the inter/intralaboratory variability of the measurement of the wet part (from 10 to 300 hPa) of the SWRC. An interlaboratory comparison was conducted between 14 laboratories, using artificially constructed, porous and structured samples as references. The bulk densities of samples were different at the very beginning of the experiment. This resulted in a variability of retention properties between the samples, which was estimated by a linear mixed model with a "sample" random effect. Our estimate of inter/intralaboratory variability was therefore not affected by intrinsic differences between samples. The greatest portion of the differences in the measurement of SWRCs was due to interlaboratory variability. The intralaboratory variability was highly variable depending on the laboratory. Some laboratories successfully reproduced the same SWRC on the same sample, while others did not. The mean intralaboratory variability over all laboratories was smaller than the mean interlaboratory variability. A possible explanation for these results is that all laboratories used slightly different methods and procedures. We believe that this result may be of great importance regarding the quality of SWRC databases built by pooling SWRCs obtained in different laboratories. The quality of pedotransfer functions or maps that might be derived is probably hampered by this inter-/intralaboratory variability. The way forward is that measurement procedures of the SWRC need to be harmonized and standardized.

Hydro-pedotransfer functions: A roadmap for future development

“…Furthermore, the temperature and relative humidity in the laboratory impact the measurements by altering the surface tension of the water and the vapor fluxes in the sample during equilibration (Hopmans and Dane, 1986). In a recent study on the reproducibility of the wet part of the soil water retention curve Guillaume et al (2023) conducted an inter-and intra-laboratory method comparison and found that both inter-and intralaboratory variability can be a substantial source of scatter and error in the data, even when methods have been harmonised.…”

“…The effect on PTFs, however, remains largely unknown. While inter-laboratory comparisons exist for textural analysis, the same is very rare for hydro-physical properties such as retention curve or hydraulic conductivity (Guillaume et al, 2023). This type of exercise requires reference samples, which drain over predefined pressure head ranges, sufficiently enough so that inter-and intra-laboratory measurement uncertainty may be disentangled.…”

Stochastic analysis of plant available water estimates and soil water balance components simulated by a hydrological model

“…In the example of Figure 2, but also in the other examined layers, differences between methods and variability due to replication can be observed. Variability of this order of magnitude may be caused by hysteresis, equipment calibration, or other sources (Gubiani et al, 2013;Guillaume et al, 2023;Schelle et al, 2013). It will determine the standard deviation of fitted parameters and the uncertainty of modeling results.…”