Radionuklidfreisetzung aus Kraftwerksschutt – Simulation von Elutionsversuchen unter transienten Randbedingungen

Für die umweltverträgliche Entsorgung des beim Rückbau kerntechnischer Anlagen anfallenden Betonbruchs werden Prognsemodelle des Radionuklidaustrags benötigt. Im Labor wurden Perkolationversuche mit radioaktiv kontaminiertem Bauschutt durchgeführt und Messreihen der wichtigsten bodenhydraulischen Größen sowie wichtige Stofftransportparameter ermittelt. Erstes Teilzeil des laufenden Forschungsvorhabens AuRa war die gekoppelte Modellierung der hydraulischen Prozesse und des nichtreaktiven Stofftransports. Die Auswertung der resultierenden Daten und die Überprüfung der Korrektheit der Prozesskenntnisse erfolgte mit der Methode der inversen Modellierung. Der Wasserfluss konnte mit der Richardsgleichung unter Verwendung des van Genuchten-Mualem-Modells der hydraulischen Funktionen zufriedenstellend beschrieben werden. Das Durchbruchsverhalten eines konservativen Tracers zeigte eindeutig auf, dass präferenzielle Flüsse auftraten. Diese konnten mit dem Mobil-Immobil-Modell zutreffend modelliert werden

Prediction of the absolute hydraulic conductivity function from soil water retention data

For modeling flow and transport processes in the soil–plant–atmosphere system, knowledge of the unsaturated hydraulic properties in functional form is mandatory. While much data are available for the water retention function, the hydraulic conductivity function often needs to be predicted. The classical approach is to predict the relative conductivity from the retention function and scale it with the measured saturated conductivity, Ks. In this paper we highlight the shortcomings of this approach, namely, that measured Ks values are often highly uncertain and biased, resulting in poor predictions of the unsaturated conductivity function. We propose to reformulate the unsaturated hydraulic conductivity function by replacing the soil-specific Ks as a scaling factor with a generally applicable effective saturated tortuosity parameter τs and predicting total conductivity using only the water retention curve. Using four different unimodal expressions for the water retention curve, a soil-independent general value for τs was derived by fitting the new formulation to 12 data sets containing the relevant information. τs was found to be approximately 0.1. Testing of the new prediction scheme with independent data showed a mean error between the fully predicted conductivity functions and measured data of less than half an order of magnitude. The new scheme can be used when insufficient or no conductivity data are available. The model also helps to predict the saturated conductivity of the soil matrix alone and thus to distinguish between the macropore conductivity and the soil matrix conductivity.</p

Impact of Temporal Macropore Dynamics on Infiltration : Field Experiments and Model Simulations

Macropores greatly affect water and solute transport in soils. Most macropores are of biogenic origin; however, the resulting seasonal dynamics are often neglected. Our study aimed to examine temporal changes in biopore networks and the resulting infiltration patterns. We performed infiltration experiments with Brilliant Blue on pastureland in the Luxembourgian Attert catchment (spring, summer, and autumn 2015). We developed an image-processing scheme to identify and quantify changes in biopores and infiltration patterns. Subsequently, we used image-derived biopore metrics to parameterize the ecohydrological model echoRD (ecohydrological particle model based on representative domains), which includes explicit macropore flow and interaction with the soil matrix. We used the model simulations to check whether biopore dynamics affect infiltration. The observed infiltration patterns revealed variations in both biopore numbers and biopore–matrix interaction. The field-observed biopore numbers varied over time, mainly in the topsoil, with the largest biopore numbers in spring and the smallest in summer. The number of hydrologically effective biopores in the topsoil seems to determine the number and thereby the fraction of effective biopores in the subsoil. In summer, a strong biopore–matrix interaction was observed. In spring, the dominant process was rapid drainage, whereas in summer and autumn, most of the irrigated water was stored in the examined profiles. The model successfully simulated infiltration patterns for spring, summer, and autumn using temporally different macropore setups. Using a static macropore parameterization the model output deviated from the observed infiltration patterns, which emphasizes the need to consider macropores and their temporal dynamics in soil hydrological modeling

Prediction of the absolute hydraulic conductivity function from soil water retention data

For modeling flow and transport processes in the soil-plant-atmosphere system, knowledge of the unsaturated hydraulic properties in functional form is mandatory. While much data are available for the water retention function, the hydraulic conductivity function often needs to be predicted. The classical approach is to predict the relative conductivity from the retention function and scale it with the measured saturated conductivity, Ks. In this paper we highlight the shortcomings of this approach, namely, that measured Ks values are often highly uncertain and biased, resulting in poor predictions of the unsaturated conductivity function. We propose to reformulate the unsaturated hydraulic conductivity function by replacing the soil-specific Ks as a scaling factor with a generally applicable effective saturated tortuosity parameter τs and predicting total conductivity using only the water retention curve. Using four different unimodal expressions for the water retention curve, a soil-independent general value for τs was derived by fitting the new formulation to 12 data sets containing the relevant information. τs was found to be approximately 0.1. Testing of the new prediction scheme with independent data showed a mean error between the fully predicted conductivity functions and measured data of less than half an order of magnitude. The new scheme can be used when insufficient or no conductivity data are available. The model also helps to predict the saturated conductivity of the soil matrix alone and thus to distinguish between the macropore conductivity and the soil matrix conductivity

Effects of improved water retention by increased soil organic matter on the water balance of arable soils: A numerical analysis

Abstract Climate change will lead to prolonged droughts in various regions of the world, which may significantly affect agricultural production. This is particularly problematic for soils with low water retention capacity, which cannot store sufficient water for crops. In this paper, we investigate how a change in the water‐holding capacity of the soil material, as could be achieved by increasing the soil organic carbon (SOC) amount, affects the components of the soil water balance (evaporation, transpiration, and groundwater recharge). Specifically, we state the hypothesis that an increased water‐holding capacity in a shallow soil layer, as it is achieved through SOC enrichment at the soil surface, will result in more water being stored near the soil surface and lost to unproductive evaporation, thereby reducing the amount of water available to plants and groundwater recharge. The hypothesis was tested by numerical simulations, employing the Hydrus‐1D program package to model the water balance in a soil–plant–atmosphere system for an arable crop in hydrologically contrasting years. The study considered soils with varying textures and different depths of a soil layer with increased SOC content. The soil hydraulic properties (SHP) of the soil material, including the effect of SOC on the SHP, were determined using a recently developed pedotransfer model based on data from over 500 samples. We showed that both the improved water retention by SOC and its vertical distribution affect the soil water balance in a complex manner. In sandy soils, increasing the water‐holding capacity in shallow layers up to 0.1 m led to enhanced evaporation and thus a decrease in water availability for crops. However, deeper incorporated SOC could ameliorate these negative effects. Our findings suggest that not only the amount but also the vertical SOC distribution should be considered if enrichment of SOC shall be applied to mitigate the effect of droughts

Modeling the Impact of Ditch Water Level Management on Stream–Aquifer Interactions

Decreasing groundwater levels in many parts of Germany and decreasing low flows in Central Europe have created a need for adaptation measures to stabilize the water balance and to increase low flows. The objective of our study was to estimate the impact of ditch water level management on stream-aquifer interactions in small lowland catchments of the mid-latitudes. The water balance of a ditch-irrigated area and fluxes between the subsurface and the adjacent stream were modeled for three runoff recession periods using the Hydrus-2D software package. The results showed that the subsurface flow to the stream was closely related to the difference between the water level in the ditch system and the stream. Evapotranspiration during the growing season additionally reduced base flow. It was crucial to stop irrigation during a recession period to decrease water withdrawal from the stream and enhance the base flow by draining the irrigated area. Mean fluxes to the stream were between 0.04 and 0.64 ls−1 for the first 20 days of the low-flow periods. This only slightly increased the flow in the stream, whose mean was 57 ls−1 during the period with the lowest flows. Larger areas would be necessary to effectively increase flows in mesoscale catchments

Preface: Linking landscape organisation and hydrological functioning: from hypotheses and observations to concepts, models and understanding

The link between landscape properties and hydrological functioning is the very foundation of hydrological sciences. The fundamental perception that landscape organisation and its hydrological and biogeochemical processes co-develop is often discussed. However, different landscape characteristics and hydrological processes interact in complex ways. Hence, the causal links between both are usually not directly deducible from our observations. So far no common concepts have been established to connect observations, properties and functions at and between different scales.This special issue hosts a broad set of original studies indicating the current state and progress in our understanding of different facets of dynamic hydrological systems across various scales. It is organised as a joint special issue in HESS and ESSD, with the purpose of providing the scientific insights in combination with the underlying data sets and study design. While the individual studies contribute to distinct aspects of the link between landscape characteristics and hydrological functioning, it remained difficult to compile their specific findings to more general conclusions. In this preface, we summarise the contributions.In the search for ways to synthesise these individual studies to the overall topic of linking landscape organisation and hydrological functioning, we suggest four major points how this process could be facilitated in the future: (i) formulating clear and testable research hypotheses, (ii) establishing appropriate sampling designs to test these hypotheses, (iii) fully providing the data and code, and (iv) clarifying and communicating scales of observations and concepts as well as scale transfers

Variability of earthworm-induced biopores and their hydrological effectiveness in space and time

Earthworms create biopores and thereby increase the susceptibility of soils to preferential flow which, on the one hand, reduces surface runoff and soil erosion, but, on the other hand, enhances vertical water and solute transport. Spatial and temporal variability in earthworm abundances might lead to spatial and temporal variability in biopore densities and even in the hydrological effectiveness of these pores. In this paper, we present a reproducible sampling design for simultaneous earthworm-biopore observations and analyze the temporal variability in earthworm abundances, biopore densities and hydrological effectiveness of biopores and its differences between grassland and arable land. During one year we performed six field campaigns where we sampled earthworms and performed dye tracer experiments in the small Luxembourgian Wollefsbach catchment (4.4 km²) on three arable land sites and three grasslands. We quantified how abundances of ecological groups of earthworms affect biopore densities and hydrological effectiveness. Finally, we applied piecewise structural equation modeling (piecewise SEM) to find the most probable structure of the ecohydrological system of soil moisture, clay content, earthworms, biopore densities and their hydrological effectiveness. Our results show that earthworm abundance and biopore density as well as their hydrological effectiveness vary strongly between seasons and between grassland and arable land (0–300 earthworms m−2, 0–500 biopores m−2, 0–100% hydrologically effective biopores). Piecewise SEMs indicate that the temporal variability in earthworm abundances, biopore densities and hydrological effectiveness is significantly affected by two-month averaged soil moisture variability. We also found a significant effect of earthworm abundances on biopore densities and their hydrological effectiveness in 3 and 10 cm depth. Based on these results we recommend the consideration of spatial and temporal variability in biopore densities and their hydrological effectiveness for the reliable representation of macropore flow and related solute transport in soil hydrological models

Soil water retention and hydraulic conductivity measured in a wide saturation range

Soil hydraulic properties (SHPs), particularly soil water retention capacity and hydraulic conductivity of unsaturated soils, are among the key properties that determine the hydrological functioning of terrestrial systems. Some large collections of SHPs, such as the UNSODA and HYPRES databases, have already existed for more than 2 decades. They have provided an essential basis for many studies related to the critical zone. Today, sample-based SHPs can be determined in a wider saturation range and with higher resolution by combining some recently developed laboratory methods. We provide 572 high-quality SHP data sets from undisturbed, mostly central European samples covering a wide range of soil texture, bulk density and organic carbon content. A consistent and rigorous quality filtering ensures that only trustworthy data sets are included. The data collection contains (i) SHP data, which consist of soil water retention and hydraulic conductivity data, determined by the evaporation method and supplemented by retention data obtained by the dewpoint method and saturated conductivity measurements; (ii) basic soil data, which consist of particle size distribution determined by sedimentation analysis and wet sieving, bulk density and organic carbon content; and (iii) metadata, which include the coordinates of the sampling locations. In addition, for each data set, we provide soil hydraulic parameters for the widely used van Genuchten–Mualem model and for the more advanced Peters–Durner–Iden model. The data were originally collected to develop and test SHP models and associated pedotransfer functions. However, we expect that they will be very valuable for various other purposes such as simulation studies or correlation analyses of different soil properties to study their causal relationships. The data are available at https://doi.org/10.5880/fidgeo.2023.012 (Hohenbrink et al., 2023)