Soil-atmosphere Interactions Research Articles

Morphology characterization of unsaturated soils under drying-wetting cycles: crack opening and closure

The volumetric and hydrological responses of clayey soils subjected to drying-wetting (D-W) cycles are of paramount importance for the integrity of geoenvironmental infrastructures. The study aimed to investigate the cracking behavior of clayey soils under D-W cycles by using advanced 2D imaging and 3D scanning techniques to capture the initiation and propagation of desiccation cracks within a soil specimen. The temporal variation in the soil water content and the corresponding 2D digital photography and 3D morphology of cracks were simultaneously monitored, and the cracking characteristics were interpreted. It was found that the time-dependent evaporation process was independent of the D-W cycles. Both 2D and 3D characterization showed the cracking hysteresis phenomenon in the unsaturated soil, which indicates the dependency of the crack opening and closure on the degree of saturation. D-W cycles led to the formation of subcracks and the increase in the total crack length, reflecting the soil degradation. Additionally, it was demonstrated that the 3D characterization exhibited the advantage of capturing the volumetric change and the subtle change in the macroporosity of the cracked soil over the 2D visualization. The current study provides a perspective of combining 2D and 3D characterization for interpreting the volumetric change of cracked soils and enhancing the understanding of the hydromechanical responses and the soil-atmosphere interactions.

Hydro-mechanical analysis of slope stability using anisotropic constitutive model

This study introduces a novel and robust numerical approach to analyzing rainfall-induced landslide hazards, incorporating coupled hydro-mechanical behavior, anisotropic properties of unsaturated geomaterials, and soil–atmosphere interaction. The advanced hyperbolic Mohr-Coulomb model with anisotropy is adopted for mechanical responses, while generalized Darcy’s law is employed for hydraulic behaviors. This approach allows for anisotropic specification of hydromechanical properties like stiffness, strength, and intrinsic permeability. Atmospheric boundary conditions enable analysis of soil–atmosphere interactions, encompassing infiltration and runoff during extreme rainfall. Parametric analyses were conducted to evaluate slope safety factors under various anisotropic angles.

MATHEMATICAL MODELS OF METHANE CONSUMPTION BY SOILS: A REVIEW

MATHEMATICAL MODELS OF METHANE CONSUMPTION BY SOILS: A REVIEW

Improving soil resilience to drought climate by an eco-friendly approach

With the intensification of global warming, the increasing extreme drought climate has seriously threatened the stability of critical geotechnical infrastructure. This study proposes a novel, eco-friendly approach to improve soil resilience to drought climate. In order to verify the feasibility and durability of the proposed approach, a series of simulated climatic wetting-drying (WD) cycles were carried out using four sets of soil specimens treated with different W-OH (an eco-friendly polymer) concentrations. During this period, the water evaporation history, desiccation cracking process, WD cycle durability and microstructure evolution of soil specimens treated with different W-OH concentrations were investigated. The results reveal that the W-OH modification has a significant suppression effect on the whole process of soil evaporation, which can effectively prolong the soil evaporation time of constant rate stage and falling rate stage. The addition of W-OH not only significantly reduces crack ratio, total crack length, and average crack width, but also slows down the crack growth rate. Compared with untreated soil specimen, the treatment with low concentration of W-OH (2% and 4%) can effectively mitigate the formation of desiccation cracks, while the treatment of higher concentration of W-OH (6%) can directly prevent the occurrence of such cracks. The microstructure analysis results indicate that the W-OH can transform the soil structure into a denser state through two ways of bridging and encapsulation, which significantly increases the migration resistance of pore water and improves soil strength properties. This investigation is expected to improve fundamental understanding of soil-atmosphere interactions under eco-friendly polymer modification and provides insights into the potential application of eco-friendly polymer modification in enhancing soil resilience to extreme drought climate.

The long-standing dilemma of European summer temperatures at the mid-Holocene and other considerations on learning from the past for the future using a regional climate model

Abstract. The past as an analogue for the future is one of the main motivations to use climate models for paleoclimate applications. Assessing possible model limitations in simulating past climate changes can lead to an improved understanding and representation of the response of the climate system to changes in the forcing, setting the basis for more reliable information for the future. In this study, the regional climate model (RCM) COSMO-CLM is used for the investigation of the mid-Holocene (MH, 6000 years ago) European climate, aiming to contribute to the solution of the long-standing debate on the reconstruction of MH summer temperatures for the region, and gaining more insights into the development of appropriate methods for the production of future climate projections. Two physically perturbed ensembles (PPEs) are first built by perturbing model physics and parameter values, consistently over two periods characterized by different forcing (i.e., the MH and pre-industrial, PI). The goal is to uncover possible processes associated with the considered changes that could deliver a response in MH summer temperatures closer to evidence from continental-scale pollen-based reconstructions. None of the investigated changes in model configuration produces remarkable differences with respect to the mean model behavior. This indicates a limited sensitivity of the model to changes in the climate forcing, in terms of its structural uncertainty. Additional sensitivity tests are further conducted for the MH, by perturbing the model initial soil moisture conditions at the beginning of spring. A strong spatial dependency of summer near-surface temperatures on the soil moisture available in spring is evinced from these experiments, with particularly remarkable differences evident over the Balkans and the areas north of the Black Sea. This emphasizes the role of soil–atmosphere interactions as one of the possible drivers of the differences in proxy-based summer temperatures evident between northern and southern Europe. A well-known deficiency of the considered land scheme of COSMO-CLM in properly retaining spring soil moisture, confirmed by the performed tests, suggests that more attention should be paid to the performance of the soil component of climate models applied to this case study. The consideration of more complex soil schemes may be required to help bridging the gap between models and proxy reconstructions. Finally, the distribution of the PPEs with changes in model configuration is analyzed for different variables. In almost all of the considered cases the results show that what is optimal for one period, in terms of a model configuration, is not the best for another characterized by different radiative forcing. These results raise concerns about the usefulness of automatic and objective calibration methods for RCMs, suggesting that a preferable approach is the production of small PPEs that target a set of model configurations, properly representing climate phenomena characteristic of the target region and that will be likely to contain the best model answer under different forcing.

Assessment of the importance of increasing temperature and decreasing soil moisture on global ecosystem productivity using solar-induced chlorophyll fluorescence.

The accurate assessment of the global gross primary productivity (GPP) of vegetation is the key to estimating the global carbon cycle. Temperature (Ts) and soil moisture (SM) are essential for vegetation growth. It is acknowledged that the global Ts has shown an increasing trend, yet SM has shown a decreasing trend. However, the importance of SM and Ts changes on the productivity of global ecosystems remains unclear, as SM and Ts are strongly coupled through soil-atmosphere interactions. Using solar-induced chlorophyll fluorescence (SIF) as a proxy for GPP and by decoupling SM and Ts changes, our investigation shows Ts plays a more important role in SIF in 60% of the vegetation areas. Overall, increased Ts promotes SIF by mitigating the resistance from SM's reduction. However, the importance of SM and Ts varies, given different vegetation types. The results show that in the humid zone, the variation of Ts plays a more important role in SIF, but in the arid and semi-arid zones, the variation of SM plays a more important role; in the semi-humid zone, the disparity in the importance of SM and Ts is difficult to unravel. In addition, our results suggest that SIF is very sensitive to aridity gradients in arid and semi-arid ecosystems. By decoupling the intertwined SM-Ts impact on SIF, our study provides essential evidence that benefits future investigation on the factors the influence ecosystem productivity at regional or global scales.

Evolution of Unsaturated Shear Strength and Microstructure of a Compacted Silty Clay on Wetting Paths

Abstract Silty clay is a common embankment material whose moisture increases over time due to soil–atmosphere interactions and climate changes. The objective of this study is to investigate the uns...

Strategies to overcome near surface disturbances while inverting time-lapse surface ERT data

An important basis for a reliable groundwater management is the detailed knowledge of the aquifer. We attempt here to determine groundwater flow velocity and direction of an hard rock aquifer in a groundwater protection area (Hastenrather Graben, Germany). In a common experimental set-up, we injected low salinity water in the aquifer and monitored the resistivity changes in the aquifer using time-lapse Electrical Resistivity Tomography (ERT). However, we witnessed strong influences from near surface resistivity fluctuations which were in the range of expected changes within the aquifer. We investigated these disturbances in more detail and determined two main sources of disturbance: a surface ditch, used for drainage with varying water fillings and, therefore, varying resistivities over time; secondly, changes of soil-atmosphere interactions, i.e., water accumulation in slight surface depressions after rain events. In addition to these main sources, the measured data is furthermore affected by random noise.In this paper, we compare three approaches to reduce the influence of these effects. After identifying disturbed areas, we exclude data near disturbed areas from the inversion as first approach; secondly, we refined the mesh at disturbed areas; and thirdly, we apply a region-based regularization to the time-lapse inversion.In a synthetic case study, all three approaches improve the inversion result. However, the region-based regularization recovers the input features best. The geometry of changes at depth are here less masked and the simulated injection body is better resolved. Nevertheless, the values are less accurately fitted than with a global regularization factor. Therefore, applying the approach on field data requires careful adjustment of the individual regularization factors to improve the time-lapse inversion results.We apply the combined first and third approach on our field data. As expected from the synthetic study, we see less masking and compensatory effects. The injection plume is visible in the resistivity distribution. The groundwater flow seems to be negligibly small, which is in accordance with an existing groundwater flow model of the aquifer.Our results confirm the fact that it is important to consider potential near-surface influences when analysing time-lapse ERT data. Based on a detailed synthetic case and an application to a aquifer injection study, we suggest to exclude data near heavily disturbed areas, in addition to applying a region-based regularization scheme.

Observations on soil-atmosphere interactions after long-term monitoring at two sample sites subjected to shallow landslides

Soil-atmosphere interaction has implications in different scientific research contexts and is increasingly investigated through field measurements. This paper reports a detailed description of interaction between shallow soil and atmosphere at two test sites in Oltrepò Pavese area (Northern Italy). The two test sites are in the same climatic area but are characterised by different geological features. In fact, the first objective is to compare the behaviour of two different soils, namely a clayey-sandy silt (CL) and a silty clay (CH), under similar meteorological events. Soil-atmosphere interaction is studied on the basis of long-term (about 87 and 42 months for the two test sites, respectively) monitoring data of both volumetric water content and soil water potential, recorded at different depths along two vertical soil profiles in the first two metres from ground level. Field measurements, together with meteorological data such as precipitation and air temperature, allow for clear identification of the seasonal fluctuations of unsaturated soil hydraulic properties. To infer detailed information, the recorded data were processed and relationships between soil water potential and water content were investigated. Different time spans, from several months to a few days, even including single rainy events, are considered to show the hydraulic soil behaviour. The hysteretic cycles of water content with respect to soil water potential and non-equilibrium flow are highlighted. In particular, the measured soil water potential is in the range of 0–800 kPa and of 0–1500 kPa for the CL and CH soil, respectively. At both sites, the observed hysteretic cycles are more frequent in the hot season (summer) than in the cold season (winter) and tend to reduce with depth. The experimental results are compared with the soil water characteristic curves (SWCCs) to assess whether and to what extent the SWCCs are reliable in modelling the hydraulic behaviour of partially saturated soils, under atmospheric forcing, at least in the considered climatic contexts.

Influence of soil-atmosphere interactions and unsaturated soil properties on slope stability

Results of coupled slope stability analysis performed on two urban slopes in Salvador-BA, considering suction variations due to soil-atmosphere interactions over the years of 2016 and 2018 are presented. Boundary conditions, unsaturated soil shear strength and hydraulic parameters were determined by surveying the local topography, analysing climatic and hydrological data, and gathering undisturbed soil samples from the slopes. Soil characterization, saturated and suction controlled triaxial tests and permeability, evaporation and soil water retention curve tests were performed. The simulation of the slope stability over time was performed using SEEP/W and SLOPE/W. A “soil-climate interaction function” and a modifier function were used as two distinct approaches to model the interactions between soil and the environment. The Vanapalli et al. (1996) [1] and Fredlund et al. (1978) [2] models were used to fit the cohesionversussuction curves. The most severe stability conditions were obtained in modelling considering the modifier function for soil-atmosphere interaction and Vanapalli et al. (1996) [1] model for slope 2 and the model of Fredlund et al. (1978) [2] for slope 1.

Evaluation of antecedent rainfall effects in the analysis of the probability of transient failure in unsaturated slopes

The occurrence of extreme events of precipitation is commonly responsible for inducing surface mass movements in slopes. Landslides are due to the soil-atmosphere interactions. The main inducer of failure in unsaturated slopes is the infiltration, which triggers the reduction of suction, favoring the decrease of stability. Thus, this paper presents the implementation of analytical models to evaluate the probability of transient failure of slopes under unsaturated conditions for a local scale when subjected to antecedent precipitation. In this sense, the model proposed involves an analytical solution to Richards equation for unsaturated flow under transient conditions, capable of analyzing the soil behavior in the infiltration process. Therefore, these analytical models and the geotechnical parameters taken from the case study of a landslide that occurred in Nova Friburgo (Brazil) allowed the simulation of the infiltration process using the precipitation data corresponding to the day of the landslide, as well as four months previous to the collapse. The calculation routine is capable of generating the decreasing performance of the transient factor of safety and the increasing probability of transient failure through a probabilistic approach, allowing easy predicting failure and management of threat of landslides.

Improvement of the soil-atmosphere interactions and subsequent heavy precipitation modelling by enhanced initialization using remotely sensed 1 km soil moisture information

This study assesses the impact of an improved soil moisture (SM) initialization using direct insertion methodology for convection-resolving modelling of heavy precipitation events (HPEs). State-of-the-art 1 km SM data from the Soil Moisture and Ocean Salinity (SMOS) mission, SMOS-BEC L4 version 3 are used for this purpose. A strategy is developed to prepare the SMOS-L4 surface soil moisture (SSM) product for the COnsortium for Small-scale MOdelling (COSMO) model initialization by applying a cumulative density function (CDF)-matching bias-correction and the exponential filter method to calculate corresponding SM profiles (L4-Expo). The processed satellite-derived product is validated with 38 observing sites from three in-situ SM networks REMEDHUS (19), SMOSMANIA (11) and VAS (8). All networks measure at a soil depth of 5 cm, only at the SMOSMANIA network additional measurements at 10, 20 and 30 cm are available. Four HPEs are selected to evaluate the impact of the high-resolution realistic initialization. The results show a high agreement index (AI = 0.91) and a low root-mean-square deviation (0.03 m3/m3) of the high-resolution, bias-corrected SMOS-L4 SSM product compared to in-situ observations. Moreover, the derived L4-Expo SM profile field agrees with ground-based observations and successfully removes the wet bias of the original COSMO simulated SM profile. Enhanced SM initialization with the SMOS-L4 SM derived profiles improves precipitation modelling in all selected HPEs as a consequence of improved near-surface 2 m-temperature and induced changes in the pressure field (−0.5 hPa), atmospheric humidity distribution (dqs = 5–15%) as well as wind circulations (dw700hPa = 25%), thus convergence/divergence fields. The sensitivity study, applying the same methodology for SM initialization for a SMOS-L3 (~25 km), shows a weaker improvement of the precipitation forecast of an analysed HPE than the 1 km SMOS-L4 product. Our results highlight the benefit of high-resolution SSM remote sensing satellite data for scientific disciplines like meteorology in overcoming present limitations such as the uncertainty associated to SM initialization in models.

Soil–atmosphere interaction in the overburden of a short-lived low and intermediate level nuclear waste (LLW/ILW) disposal facility

Abstract The long-term safety of shallow disposal facility for short-lived low and intermediate level nuclear waste (LLW/ILW) relies on the performance of engineered barriers (caps and liners). Soil-atmosphere interaction may affect the capping material at shallow depths, due to changes of its coupled hydro-thermal-mechanical behavior under the effect of atmospheric conditions. This study aims to investigate the soil-atmosphere interaction at different time scales in the overburden of an LLW/ILW disposal facility, located in middle France. Actual meteorological data from Valencia El Saler station in Spain, including solar radiation, air temperature, latent heat, rainfall, and actual evaporation, at four different time scales (30 min, daily, weekly and monthly), were applied as the future climate conditions of the studied area based on climate analogues. A numerical approach combining a coupled hydro-thermal model and a soil-atmosphere interaction model was employed. Results show that the employment of meteorological data at a short time scale (30 min) can increase the simulation accuracy by capturing the rainfall events and accelerating the soil-atmosphere interactions. Furthermore, the effect of future climate conditions in the long-term (7 years) on soil hydro-thermal behavior was examined. This study allows a further detailed inspection of the climate’s role in the LLW/ILW storage system.

Analysis of water vapor adsorption in soils by means of a lysimeter and numerical modeling

AbstractDaily temperature oscillations can cause adsorption (and desorption) of atmospheric water vapor by soils. The resulting daily fluctuations in the amount of liquid water in the soil can be measured by high‐precision weighing lysimeters. We analyzed the data of a lysimeter in a sandy dune sediment in southern Spain using Codebright, a thermohydraulic numerical model for unsaturated flow that takes into account water, vapor, and heat transport in the soil, as well as soil–atmosphere interactions such as precipitation, evaporation, and solar radiation. The analysis shows that daily temperature oscillations, psychrometrics, and soil water retention can explain the fluctuations of the amount of liquid water in the soil. The retention curve, especially its driest part, is essential for the existence of these fluctuations. The fluctuations could not be reproduced by a model using the van Genuchten retention curve with a constant residual saturation. On the other hand, satisfactory results could be obtained by models using retention curves that at their driest part still show a change of saturation with suction. Moreover, the models suggest within the top few decimeters of the soil a pattern of alternating bands of condensation and evaporation, which follows the daily temperature oscillations that fade out deeper in the soil.

Soil–atmosphere interactions for analysing slopes in tropical soils in Singapore

The modelling of soil–atmosphere interactions for slopes requires a proper understanding of the relevant soil parameters (soil water retention curve and the permeability function). This paper reports on numerical investigations of an instrumented residual soil slope in Singapore. The objective of the paper is to provide practical suggestions for choices of hydraulic parameters that can provide realistic predictions in analysing practical problems. The analyses use an uncoupled unsaturated flow model and a hydromechanical coupled model with time-dependent boundary conditions. The results obtained using measured weather data for the slope were validated against field measurements of pore-water pressure changes. The models were able to capture the trend of changes in the pore-water pressure due to rainfall events provided that account was taken of a more permeable surficial layer due to the presence of desiccation cracking and root passages. A comparative study of stability shows that the mobilised shear strength drops quickly during a rainfall event but recovers much more slowly during drying. This shows how a series of regular rainstorms with short periods of drying in between can cause a ratcheting effect, with rapid loss of strength during each period of rain that is not recovered during the intermediate drying periods.

Soil moisture–precipitation coupling over Central Europe: Interactions between surface anomalies at different scales and the dynamical implication

The sign of soil moisture–precipitation coupling and its scale dependence are controversial issues when examining soil–atmosphere interactions. Using the operational convection‐permitting COSMO model with 2.8 km horizontal grid spacing, the interplay of soil‐moisture heterogeneities on different scales and the role of the midtropospheric background wind are addressed by performing a series of real‐case simulations for 11 synoptically weakly and moderately forced summer precipitation events across Central Europe. The cumulative effect of soil‐moisture bias and heterogeneities on different scales is investigated, with consideration of the prevailing synoptic forcing, by explicitly perturbing soil‐moisture conditions. A soil‐moisture bias of ±25% is combined with different soil‐moisture heterogeneity length‐scales ranging from 30 to 140 km introduced by chessboard patterns.For synoptically moderately forced cases, our experiments only show a small impact of soil‐moisture perturbations on convective afternoon precipitation. Averaged over all seven synoptically weakly forced cases, however, we find (a) a positive coupling between the overall soil‐moisture bias and the domain‐averaged precipitation and (b) a negative local soil moisture–precipitation coupling. Increased precipitation over drier soils is related to an interaction between thermally induced vertical circulations and the background wind, causing a persistent updraft region at the downstream flank of the dry patches. Convective precipitation is triggered preferentially near these soil‐moisture gradients. The circulation cells are most dominant for soil‐moisture perturbations at scales between 40 and 80 km. The spatial locking of convection at soil‐moisture boundaries at these scales results in an earlier triggering of convection and an overall reduction of the day‐to‐day variability of area‐averaged precipitation.

Scale dependency of spatial variability of soil moisture in gravel-mulched field

Knowledge of the scale dependency of the spatial variability of soil moisture is of paramount importance in the study of soil–atmosphere interactions and hydrological processes. We present a case study of the effects of sampling area and spacing on the spatial variability of soil moisture in a 32 × 32 m gravel-mulched field in an arid area of northwestern China. The coefficient of variation, correlation length and Moran’s index, which are commonly used in spatial analysis, were applied to each scenario to characterise the degree of spatial variability of soil moisture. The spatial variability was weak at all sampling scales. All indices increased to various degrees with an increase in sampling area. The correlation length decreased with increasing sampling spacing, and neither the coefficient of variation nor Moran’s index were significantly correlated with sampling spacing, indicating that sampling spacing had little effect on soil-moisture variability. The spatial distributions of soil moisture were irregular, with peaks and valleys at different sampling spacings but tending to gradually stabilise as sampling spacing increased. A sampling spacing of 8 m was reasonable, because it best-characterised the spatial distribution of soil moisture. This study also provides a theoretical reference for the establishment of optimal sampling schemes, which allow a considerable savings in both time and cost, and provides theoretical support for ecological agricultural production in gravel-mulched fields.

Stem–root flow effect on soil–atmosphere interactions and uncertainty assessments

Abstract. Rainfall that reaches the soil surface can rapidly move into deeper layers in the form of bulk flow through the stem–root flow mechanism. This study developed the stem–root flow parameterization scheme and coupled this scheme with the Simplified Simple Biosphere model (SSiB) to analyze its effects on land–atmospheric interactions. The SSiB model was tested in a single-column mode using the Lien Hua Chih (LHC) measurements conducted in Taiwan and HAPEX–Mobilhy (HAPEX) measurements in France. The results show that stem–root flow generally caused a decrease in soil moisture in the top soil layer and moistened the deeper soil layers. Such soil moisture redistribution results in substantial changes in heat flux exchange between land and atmosphere. In the humid environment at LHC, the stem–root flow effect on transpiration was minimal, and the main influence on energy flux was through reduced soil evaporation that led to higher soil temperature and greater sensible heat flux. In the Mediterranean environment of HAPEX, the stem–root flow substantially affected plant transpiration and soil evaporation, as well as associated changes in canopy and soil temperatures. However, the effect on transpiration could be either positive or negative depending on the relative changes in the soil moisture of the top soil vs. deeper soil layers due to stem–root flow and soil moisture diffusion processes.

Mathematical and physical modelling of rainfall in centrifuge

Rainfall simulation in centrifuge models is important for modelling soil–atmosphere interactions. However, the presence of Coriolis force, drag forces, evaporation and wind within the centrifuge may affect the distribution of rainfall over the model. As a result, development of appropriate centrifuge rain simulators requires a demanding process of experimental trial and error. This paper highlights the key factors involved in controlling rainfall in centrifuge simulations, develops a mathematical model to simulate rainfall within a centrifuge and shows how to calibrate it experimentally. Finally, the paper summarises some important factors that should be considered in the development of rainfall simulators for centrifuges.

Mathematical and physical modelling of rainfall in centrifuge

Rainfall simulation in centrifuge models is important for modelling soil–atmosphere interactions. However, the presence of Coriolis force, drag forces, evaporation and wind within the centrifuge may affect the distribution of rainfall over the model. As a result, development of appropriate centrifuge rain simulators requires a demanding process of experimental trial and error. This paper highlights the key factors involved in controlling rainfall in centrifuge simulations, develops a mathematical model to simulate rainfall within a centrifuge and shows how to calibrate it experimentally. Finally, the paper summarises some important factors that should be considered in the development of rainfall simulators for centrifuges.