Sascha C. Iden

Extended multistep outflow method for the accurate determination of soil hydraulic properties near water saturation

Multistep outflow experiments are a well-established method to determine soil hydraulic properties. In the medium pressure head range where the specific water capacity is sufficiently high, the method yields reliable results. However, in the pressure head range corresponding to conditions close to and at water saturation, the method suffers from considerable uncertainties, in particular with respect to the determination of the hydraulic conductivity function. This is caused by the insensitivity of the experimental design with respect to the saturated hydraulic conductivity Ks. It is therefore generally recommended to perform an additional percolation experiment in order to determine Ks. The disadvantages resulting from this are an increased experimental cost and the necessity to combine information from two different experiments. In the case treated in this study the latter bears the risk of data inconsistency. We present a new experimental design which combines a water saturated percolation with an unsaturated multistep outflow experiment in a consecutive manner. The saturated percolation resembles a falling-head experiment with an initial ponding of 2-4 cm of water at the soil surface. The onset of unsaturated flow can be identified unambiguously by pressure head measurements inside the sample. We test this extended multistep outflow method (XMSO) by inversions using synthetic and real laboratory data. The soil hydraulic properties are parameterized with free-form functions in order to avoid model errors in the constitutive relationships and to quantify robustly the uncertainties of determination close to water saturation. Our results confirm that the XMSO method allows to identify correctly the soil hydraulic properties in the pressure head range near and at water saturation. In particular, the saturated hydraulic conductivity can be determined without bias and with great accuracy. Because of the improved information content and since the experiment is quick to perform and evaluated easily by inverse modelling, it can replace the MSO design in future applications.

Biofilm effect on soil hydraulic properties: Experimental investigation using soil‐grown real biofilm

Understanding the influence of attached microbial biomass on water flow in variably saturated soils is crucial for many engineered flow systems. So far, the investigation of the effects of microbial biomass has been mainly limited to water-saturated systems. We have assessed the influence of biofilms on the soil hydraulic properties under variably-saturated conditions. A sandy soil was incubated with Pseudomonas Putida and the hydraulic properties of the incubated soil were determined by a combination of methods. Our results show a stronger soil water retention in the inoculated soil as compared to the control. The increase in volumetric water content reaches approximately 0.015 cm3 cm−3 but is only moderately correlated with the carbon deficit, a proxy for biofilm quantity, and less with the cell viable counts. The presence of biofilm reduced the saturated hydraulic conductivity of the soil by up to one order of magnitude. Under unsaturated conditions, the hydraulic conductivity was only reduced by a factor of four. This means that relative water conductance in biofilm-affected soils is higher compared to the clean soil at low water contents, and that the unsaturated hydraulic conductivity curve of biofilm-affected soil cannot be predicted by simply scaling the saturated hydraulic conductivity. A flexible parameterization of the soil hydraulic functions accounting for capillary and non-capillary flow was needed to adequately describe the observed properties over the entire wetness range. More research is needed to address the exact flow mechanisms in biofilm-affected, unsaturated soil and how they are related to effective system properties. This article is protected by copyright. All rights reserved.

Unsaturated hydraulic properties of Sphagnum moss and peat reveal trimodal pore‐size distributions

In ombrotrophic peatlands, the moisture content of the vadose zone (acrotelm) controls oxygen diffusion rates, redox state, and the turnover of organic matter. Whether peatlands act as sinks or sources of atmospheric carbon thus relies on variably saturated flow processes. The Richards equation is the standard model for water flow in soils, but it is not clear whether it can be applied to simulate water flow in live Sphagnum moss. Transient laboratory evaporation experiments were conducted to observe evaporative water fluxes in the acrotelm, containing living Sphagnum moss, and a deeper layer containing decomposed moss peat. The experimental data were evaluated by inverse modeling using the Richards equation as process model for variably-saturated flow. It was tested whether water fluxes and time series of measured pressure heads during evaporation could be simulated. The results showed that the measurements could be matched very well providing the hydraulic properties are represented by a suitable model. For this, a trimodal parametrization of the underlying pore-size distribution was necessary which reflects three distinct pore systems of the Sphagnum constituted by inter-, intra-, and inner-plant water. While the traditional van Genuchten-Mualem model led to great discrepancies, the physically more comprehensive Peters-Durner-Iden model which accounts for capillary and noncapillary flow, led to a more consistent description of the observations. We conclude that the Richards equation is a valid process description for variably saturated moisture fluxes over a wide pressure range in peatlands supporting the conceptualization of the live moss as part of the vadose zone.

Hydraulic Properties and Non-equilibrium Water Flow in Soils

Accurate knowledge of hydraulic properties for unsaturated soils is critical in the estimation of soil water fluxes by simulation models that are based on the Richards equation. The purpose of this chapter is to review the characterization of unsaturated soil hydraulic properties for their applicability in models simulating unsaturated water transport. We start with a short review of the fundamentals that lead to the definition of the hydraulic functions in the framework of continuum hydromechanics. Next, we address problems of common parameterizations of hydraulic functions in the critical regions near and at saturation, towards dryness, and on hysteresis. We find that traditional approaches have deficiencies, but recent progress has been significant in particular with respect to including film and corner flow components in the hydraulic conductivity function. The chapter closes with a discussion of the phenomenon of dynamic non-equilibrium in soil water flow, which shows to our opinion toward the need for a paradigm change in the modeling of soil water transport.

Robust Inverse Modeling of Growing Season Net Ecosystem Exchange in a Mountainous Peatland: Influence of Distributional Assumptions on Estimated Parameters and Total Carbon Fluxes

While boreal lowland bogs have been extensively studied using the eddy‐covariance (EC) technique, less knowledge exists on mountainous peatlands. Hence, half‐hourly CO2 fluxes of an ombrotrophic peat bog in the Harz Mountains, Germany, were measured with the EC technique during a growing season with exceptionally dry weather spells. A common biophysical process model for net ecosystem exchange was used to describe measured CO2 fluxes and to fill data gaps. Model parameters and uncertainties were estimated by robust inverse modelling in a Bayesian framework using a population‐based Markov Chain Monte Carlo sampler. The focus of this study was on the correct statistical description of error, i.e. the differences between the measured and simulated carbon fluxes, and the influence of distributional assumptions on parameter estimates, cumulative carbon fluxes, and uncertainties. We tested the Gaussian, Laplace, and Students t distribution as error models. The t‐distribution was identified as best error model by the deviance information criterion. Its use led to markedly different parameter estimates, a reduction of parameter uncertainty by about 40%, and, most importantly, to a 5% higher estimated cumulative CO2 uptake as compared to the commonly assumed Gaussian error distribution. As open‐path measurement systems have larger measurement error at high humidity, the standard deviation of the error was modeled as a function of measured vapor pressure deficit. Overall, this paper demonstrates the importance of critically assessing the influence of distributional assumptions on estimated model parameters and cumulative carbon fluxes between the land surface and the atmosphere.