ALBERT

Description: Inverse modelling of in situ soil water dynamics is a powerful tool to test process understanding and determine soil hydraulic properties at the scale of interest. The observations of soil water state variables are typically evaluated using the ordinary least squares approach. However, the underlying assumptions of this classical statistical approach of independent, homoscedastic, and Gaussian distributed residuals are rarely tested in practice. In this case study, we estimated the soil hydraulic properties of a homogeneous, bare soil profile from field observations of soil water contents. We used a formal Bayesian approach to estimate the posterior distribution of the parameters in the van Genuchten–Mualem (VGM) model of the soil hydraulic properties. Three likelihood models that differ with respect to assumptions about the statistical features of the time series of residuals were used. Our results show that the assumptions of the ordinary least squares did not hold, because the residuals were strongly autocorrelated, heteroscedastic and non-Gaussian distributed. From a statistical point of view, the parameter estimates obtained with this classical statistical approach are therefore invalid. Since a test of the classic first-order autoregressive (AR(1)) model led to strongly biased model predictions, we introduced an modified AR(1) model which eliminates this critical deficit of the classic AR(1) scheme. The resulting improved likelihood model, which additionally accounts for heteroscedasticity and nonnormality, lead to a correct statistical characterization of the residuals and thus outperformed the other two likelihood models. We consider the corresponding parameter estimates as statistically correct and showed that they differ systematically from those obtained under ordinary least squares assumptions. Moreover, the uncertainty in the parameter estimates was increased by accounting for autocorrelation in the observations. Our results suggest that formal Bayesian inference using a likelihood model that correctly formalizes the statistical properties of the residuals may also prove useful in other inverse modelling applications in soil hydrology.

Description: In ombrotrophic peatlands, the moisture content of the acrotelm (vadoze zone) controls oxygen diffusion rates, redox state, and the turnover of organic matter. Thus, variably saturated flow processes determine whether peatlands act as sinks or sources of atmospheric carbon, and modelling these processes is crucial to assess effects of changed environmental conditions on the future development of these ecosystems. We show that the Richards equation can be used to accurately describe the moisture dynamics under evaporative conditions in variably saturated peat soil, encompassing the transition from the topmost living moss layer to the decomposed peat as part of the vadose zone. Soil hydraulic properties (SHP) were identified by inverse simulation of evaporation experiments on samples from the entire acrotelm. To obtain consistent descriptions of the observations, the traditional van Genuchten–Mualem model was extended to account for non-capillary water storage and flow. We found that the SHP of the uppermost moss layer reflect a pore-size distribution (PSD) that combines three distinct pore systems of the Sphagnum moss. For deeper samples, acrotelm pedogenesis changes the shape of the SHP due to the collapse of inter-plant pores and an infill with smaller particles. This leads to gradually more homogeneous and bi-modal PSDs with increasing depth, which in turn can serve as a proxy for increasing state of pedogenesis in peatlands. From this, we derive a nomenclature and size classification for the pore spaces of Sphagnum mosses and define inter-, intra-, and inner-plant pore spaces, with effective pore diameters of 〉  300, 300–30, and 30–10 µm, respectively.

Description: In ombrotrophic peatlands, the moisture content of the acrotelm (vadoze zone) controls oxygen diffusion rates, redox state, and the turnover of organic matter. Thus, variably saturated flow processes determine whether peatlands act as sinks or sources of atmospheric carbon, and modelling these processes is crucial to assess effects of changed environmental conditions on the future development of these ecosystems. We show that the Richards equation can be used to accurately describe the moisture dynamics under evaporative conditions in variably saturated peat soil, encompassing the transition from the topmost living moss layer to the decomposed peat as part of the vadose zone. Soil hydraulic properties (SHP) were identified by inverse simulation of evaporation experiments on samples from the entire acrotelm. To obtain consistent descriptions of the observations, the traditional van Genuchten–Mualem model was extended to account for non-capillary water storage and flow. We found that the SHP of the uppermost moss layer reflect a pore-size distribution (PSD) that combines three distinct pore systems of the Sphagnum moss. For deeper samples, acrotelm pedogenesis changes the shape of the SHPs due to the collapse of inter-plant pores and an infill with smaller particles. This leads to gradually more homogeneous and bi-modal PSDs with increasing depth, which in turn can serve as a proxy for increasing state of pedogenesis in peatlands. From this, we derive a nomenclature and size classification for the pore spaces of Sphagnum mosses and define inter-, intra-, and inner-plant pore spaces, with effective pore diameters of 〉 300 μm, 300–30 μm and 30–10 μm, respectively.