Stewart and Abou Najm (2018) developed a comprehensive model (SA model) for single ring infiltration that consists of a couple of two-terms explicit infiltration equations similar, in form, to the approximate expansions proposed by Haverkamp et al. (1994) (HV model). Application of SA model requires the transition time, τ_crit, from transient to steady state to be known a-priori or establishing a constraint among the four constants that figure in the infiltration equations. Estimation of soil saturated hydraulic conductivity, K_s, and capillary length, λ, from single ring infiltration measurements also needs a scaling parameter referred to “a” to be known. SA model assumes this scaling parameter as a constant and fixes its value at a = 0.45. However, there is evidence that a cannot be considered a constant independent of soil type and initial water content.

This study investigates some open issues related to the use of the SA model for single ring infiltration: 1) how comparable is τ_crit with the maximum time, t_max, that separates transient from steady state condition in HV model; 2) how the scaling parameter a depends on different experimental conditions and how it can be related to HV parameters.

Preliminary theoretical considerations showed that the two characteristic times (τ_crit and t_max) are related and, for relatively dry initial conditions, parameter a depends only on the soil type and ring radius being maximum for small ring radii or soils with high capillarity (a = 1) and minimum for large rings or coarse soils (a = 0.467).

An optimization procedure, with a constraint among the four infiltration constants, was applied to fit the SA model to both analytical and experimental infiltration data to derive  τ_crit and the associated value of a.

The analytical data confirmed that the ratio τ_crit/t_max was constant and equal to 1.495, regardless the combination of soil, ring diameter and initial water saturation. The calculated a values varied between 0.706 and 0.904, with a mean equal to a = 0.807, and were independent of the initial water content for saturation degrees up to approximately 0.50.

Application of the optimization procedure to field data was problematic given it was successful only in 29 out of 70 infiltration tests. Fixing τ_crita-priori could be advisable in this case and it was shown that two alternative empirical criteria for selecting τ_crit yielded a values differing by a nearly negligible mean factor of 1.10 and significantly correlated to one another (R² = 0.997).

However, a rather high percentage of a values (45.5%) were greater than the theoretical maximum value (a = 1), and therefore were physically implausible. Excluding these values from the analysis, the mean a parameter (a = 0.735) was close to that estimated by the successful applications of the optimization procedure (a = 0.673).

Therefore, consistent results were obtained by field and analytical data with a values intermediate between the suggested values in the literature (a = 0.45 and 0.91). These findings can inform parameterization choices for others working with infiltration models, and should reduce uncertainty during interpretation of infiltration measurements.

How to cite: Iovino, M., Abou Najm, M. R., Angulo-Jaramillo, R., Bagarello, V., Castellini, M., Concialdi, P., Di Prima, S., Lassabatere, L., and Stewart, R. D.: Explicit comprehensive models for single ring infiltration: suggestions for model application and parameterization, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1712, https://doi.org/10.5194/egusphere-egu21-1712, 2021.

The study deals with the unsaturated hydraulic conductivity of soils within the scope of the Diverfarming-Project, funded by the EU commission (Horizon 2020 grant agreement no 728003). For this reason, the field work took place in the examined vineyard of the Wawerner Jesuitenberg near Kanzem in the Saar-Mosel valley (Rhineland-Palatinate, Germany). The mentioned parameter is one of the most important specific factors of the hydrological cycle to characterize soil hydraulic properties in the unsaturated soil zone. A mini disc infiltrometer was used to measure the conductivity values at different suctions. The purpose of this study is to determine the plausibility of the fundamentals and the analytical expression of the unsaturated conductivity models in a nearly skeletal soil of schist. In this regard, the mathematical expressions of Mualem (1976), van Genuchten (1980) and Zhang (1997) are focused on calculating the unsaturated hydraulic conductivity. The two variables α and n are analysed in order to better compare between literature specifications and the explicit calculated data of the vineyard’s soil. As a result, the various developments of α are similar thus the significant difference is based on the value of n. Nevertheless, in consideration of these frame conditions the models represent a suitable mathematical expression of the unsaturated hydraulic conductivity. Furthermore, a range of parameters affecting this conductivity is analysed, particularly with regard to the applied variable soil and cultivation management under the grapevines in the vineyard. Also, the rock fragment cover and the pore size distribution are taken into account. In this context the soil compaction and modified pore size distribution in the wheel tracks stand out due to salient unsaturated hydraulic conductivities at higher tensions. In particular, the stone cover of the contact surface influence the characteristics of the analysed conductivity. Additionally, the connection of stone cover, management and pore size distribution creates a mixture of affected parameters of the unsaturated hydraulic conductivity.

Mualem, Y.: A new model for predicting the hydraulic conductivity of unsaturated porous media, Water Resour. Res, 12, 513–522, https://doi.org/10.1029/WR012i003p00513, 1976.

Van Genuchten, M.T.: A Closed-form Equation for Predicting the Hydraulic Conductivity of Unsaturated Soils, Soil Sci. Soc. Am. J., 44, 892–898, https://doi.org/10.2136/sssaj1980.03615995004400050002x, 1980.

Zhang, R.: Determination of Soil Sorptivity and Hydraulic Conductivity from the Disk Infiltrometer, Soil Sci. Soc. Am. J., 61, 1024–1030, https://doi.org/10.2136/sssaj1997.03615995006100040005x, 1997.

How to cite: Walle, S., Iserloh, T., and Seeger, M.: Unsaturated hydraulic conductivity of vineyard soils with high rock fragment content in the Mosel area, Germany, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2572, https://doi.org/10.5194/egusphere-egu21-2572, 2021.

The soil hydraulic properties controlling infiltration are dynamic depending on interrelated factors such as soil texture and structure, climate (rainfall intensity), land use, vegetation cover and plant root systems. These physical and biological factors directly influence the size and geometry of the conductive pores, and therefore the bulk density, soil structure and finally water infiltration at surface. In the Sahelian zone, the slightest modification of the physical properties of the soil has severe consequences on the soil properties and thus on hydrological processes. It is therefore essential to improve knowledge on the spatial distribution of the hydraulic behavior of soils for optimization of agricultural uses.

We used the BEST method (Beerkan Estimation of Soil Transfer parameters) on a toposequence of the Senegalese groundnut basin (Fatick region) in the Faidherbia-Flux observatory[1] where the average rainfall is 590 mm/yr. The studied toposequence (400 m long) is representative of a common agroforestry zone with annual cultivation of millet and peanuts and a sparse density of Faidherbia albida. The slope is low (1%) with small lowland areas made up of sandy soil with more clay (clay soil), while the glacis is represented by more or less compacted sand. The infiltrometry measurements were made with the automatic single-ring infiltrometer developed by Di Prima et al. (2016), used here for the first time in West Africa. The explicative variables tested are the type of soils, including: clay soils under tree (CLUT) and outside tree (CLOT), sandy soils under tree (SSUT) and outside trees (SSOT), and cattle trampled soils outside trees (TSOT) particularly compacted and largely present in the study area. BEST algorithms were applied to the experimental data to determine the hydraulic properties of the soils of the different variables and to draw water retention and hydraulic conductivity curves.

There are significant differences in infiltration rates between the sampled zones and in relation with the studied factors. The highest infiltration rate is found on sandy soils under tree (SSUT) with an average infiltration rate of 14.0 mm/min, followed by SSOT with 11.6 mm/min. Then the clay soils CLUT and CLOT are characterized by similar lower hydraulic responses with average infiltration rates of 6.9 mm/min and 6.2 mm/min, respectively. The average infiltration rate is the lowest on the compacted sandy soils TSOT, with only 5.4 mm/min. The study of the variability of the infiltration rates measured by class of variable shows a large variability for CLOT, CLUT and SSUT (decreasing order of variability). These results are in agreement with the measured values of dry soil bulk density. The high infiltration rates in the clay soils outside and under trees can be explained by the higher content of organic matter observed on the sampling, and probably by the existence of preferential flow activated by the macropores particularly present on clay soils (CLOT and CLUT) and on sandy soils under tree (SSUT).

Di Prima, S., et al., 2016. Testing a new automated single ring infiltrometer for Beerkan infiltration experiments. Geoderma, 262, 20–34. doi:10.1016/j.geoderma.2015.08.006

How to cite: Faye, W., Orange, D., Diongue, D. M. L., Do, F., Jourdan, C., Roupsard, O., Niang Fall, A., Kane, A., Faye, S., Di Prima, S., Angulo-Jaramillo, R., and Lassabatère, L.: Potential impact of Faidherbia albida tree on soil infiltration in a semi-arid agroforestry system of the Senegalese groundnut basin: role of preferential flows? , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8768, https://doi.org/10.5194/egusphere-egu21-8768, 2021.

Plastic bottles can be used in irrigation fields which introduces a sustainable low-cost alternative for irrigation methods. Until now, plastic bottles were used in small irrigation fields since there are limited scientific measurements of it is performance. The aim of this study is to predict the size and shape of the wetting patterns generated by inverted opened plastic bottles. Hydraulic simulations for 48 design cases of 12 different soil types and 4 sizes of subsurface source were accomplished using 2D-Hydrus. The simulation outputs were validated using experimental results. Multi regression analysis was used to identify the general formulae of the dependent variables of hydraulic conductivity, area of subsurface source, depth of the source, head of application, and time of application. The statistical analysis was formulated by the R-studio program. Results show that the maximum width and depth of wetting patterns occurred in sandy soil which were 34.1 and 96.8 cm, respectively. The minimum values were in silty clay with the width and depth of wetting patterns of 4.3 and 19 cm, respectively. The standard deviation of the width and depth were 9.02 and 22.58, respectively.  In conclusion, the soil type is a vital factor that impacts the flow in the soil profile and the size and shape of the wetting patterns. In addition, the size and depth of the subsurface source impact the size and location of the wetting patterns. The Head of the water in IOPB can be used to specify the size of the wetting patterns. The statistical model can be used to predict the size of the wetting patterns created by IOPBs accurately.

How to cite: Alrubaye, Y., Yusuf, B., and AL-Sammak, A.: Prediction of Size and Shape of Wetting Patterns Created by Inverted Open Plastic Bottles (IOPB), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11187, https://doi.org/10.5194/egusphere-egu21-11187, 2021.

The infiltration capacity of soil under variably saturated conditions is critical knowledge that is essential in order to design effective engineered systems that rely on the percolation of water and/or wastewater as an integral part of the overall treatment process; for example, solutions for on-site wastewater treatment (septic tanks systems etc.) and storm water runoff systems associated with Sustainable Urban Drainage Systems. Such treatment systems are increasingly seen as appropriate sustainable solutions, which aim to attenuate both hydraulic and pollutant loads in order to protect surface and groundwater resources. Several different approaches can be taken to determine a soil’s hydraulic conductivity, either using percolation tests (carried out in the laboratory or in the field) or via soil size distribution. Each method yields different estimates of (saturated) hydraulic conductivity which can then be combined with knowledge of soil moisture retention curves, from which predictions can then be made of water flow under transient unsaturated conditions using, for example, the commonly adopted Richards equation.

In Ireland, falling head percolation tests are used to assess whether a site is suitable for an on-site wastewater treatment process for new developments in areas which lack access to centralised wastewater treatment systems. These tests are relatively easy to carry out, but suffer from lack of rigorous, standardised conditions during the test and so prove challenging when trying to convert the results into a rigorous metric that can be used for infiltration design. This research therefore carried out an international review of testing methods used in other countries of percolation characteristics of soils, including those based on constant head tests and/or soil texture. The advantages and disadvantages of each method are compared, as well as how the results are incorporated into soil treatment unit design.

In parallel to the international review, this research has evaluated results from over 800 falling head field tests carried out across a range of different subsoil types in Ireland. The data from each percolation test (water level drop and/or volume infiltrated) has then been modelled using both 2-D and 3-D numerical modelling code (Hydrus 2D) to derive saturated hydraulic conductivity values (K_s). The relationship between the field derived falling head saturated hydraulic conductivity results (K_sat)againstthe model derived K_s values has been plotted across the range of soil textural classes from fast percolating sandy soils to very slow clayey soils. Equally, the same comparison has been made between K_sat from a more limited number of field permeameter (constant head) tests against the model derived K_s values to allow direct comparison to be made between the two field methods via the same numerical modelling approach. Integrating the learnings gained from the assessment of different approaches and the modelling of field results in Ireland, the research concludes by recommending a staged move away from the falling head tests used in Ireland to a more hybrid approach based on soil texture and constant head (permeameter) tests.

How to cite: Gill, L., Mac Mahon, J., Knappe, J., and Morrissey, P.: An investigation into the validity of using falling head percolation tests as part of a field assessment procedure for the design of on-site wastewater treatment systems , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12355, https://doi.org/10.5194/egusphere-egu21-12355, 2021.

While the basic processes of water infiltration into soil are well understood, their details are difficult to quantify due to the opaque nature of soil. In this study, we investigated the potential and limitations of X-ray radiography to measure the water front progression in a narrow sample (15 × 15 × 1 cm) filled with dry soil under simulated rainfall of high intensity (53 mm/h). The temporal resolution of the acquired infiltration movies was 133 milliseconds. We evaluated three different kinds of soil samples. i) Bare samples filled with a 1:1 mixture of a sandy loam and peat. ii) The same soil-peat mixture, but cropped with Trifolium incarnatum, Trifolium repens, Lathyrus odoratus and Lupinus regalis, all of them plants that have been reported to induce water repellency; prior to the experiments, the plants were harvested and only the roots remained in place. iii) Sandy loam soil that had been incubated for four weeks in an outside garden plot. Due to time limitations of the project, the incubation period was in early spring, which meant that plant growth in the samples was negligible. All three sample types were replicated five times, resulting in 15 individual samples. We carried out the infiltration experiments in four-fold replications, from which it follows that we collected 60 individual infiltration movies. After each infiltration round, the samples were placed in a drying room to reset them to a similar initial moisture content. The experiments aimed at testing i) whether the infiltration patterns of the four consecutive infiltration runs conducted on each sample remained identical and ii) to document differences in infiltration patterns between bare, cropped and incubated samples. We found that increasing X-ray scattering with increasing soil water contents made it challenging to evaluate the image data quantitatively. Advantages of our setup are that X-ray captures the complete water content at a specific depth and that sample boxes with irregularly shaped walls can be used to prevent preferential flow along the walls. Preliminary analyses of the data showed that the infiltration fronts in the bare and the incubated samples were less uniform than the ones for the cropped samples. In contrast, the likelihood of observing the same infiltration pattern in all four consecutive infiltration runs was larger for the bare and the incubated samples. The latter fact may have been caused by the interaction with root exudates in the cropped samples that may have been redistributed or mineralized during the wetting-drying cycles. We conclude that the here presented setup has large potential to investigate unstable infiltration phenomena into soil after an image correction approach has been developed that removes X-ray scattering artifacts. Alternatively, scattering may be suppressed by using a collimator during image acquisition.

How to cite: Koestel, J., Garbari, L., and Iseskog, D.: Using X-ray radiography to image rapid water infiltration into soil, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9479, https://doi.org/10.5194/egusphere-egu21-9479, 2021.

Time-lapse ground penetrating radar (GPR) surveys in conjunction with automated single-ring infiltration experiments can be used for non-invasive monitoring of the spatial distribution of infiltrated water and for generating 3D representations of the wetted zone. In this study we developed and tested a protocol to quantify and visualize water distribution fluxes under unsaturated and saturated conditions into layered soils. We carried out a gridded GPR survey on a 0.3-m thick sandy clay loam layer underlain by a restrictive limestone layer at the Ottava experimental station of the University of Sassari (Sardinia, IT). We firstly established a survey grid (1 m × 1 m), consisting of six horizontal and six vertical parallel survey lines with 0.2 m intervals between them. The field survey then consisted of six steps, including i) a first GPR survey, ii) a tension infiltration experiment conducted within the grid and aimed at activating only the soil matrix, iii) a second GPR survey aimed at highlighting the amplitude fluctuations between repeated GPR radargrams of the first and second surveys, due to the infiltrated water moving within the matrix flow region, iv) a single-ring infiltration experiment of the Beerkan type carried out within the grid on the same infiltration surface using a solution of brilliant blue dye (E133) and aimed to activate the whole pore network, v) a third GPR survey aimed to highlight the amplitude fluctuations between repeated GPR radargrams of the first and third surveys, due to the infiltrated water moving within the whole pore network (both matrix and fast-flow regions), and vi) the excavation of the soil to expose the wetted region. The shapes of the 3D diagrams of the wetted zones facilitated the interpretation of the infiltrometer data, allowing us to resolve water infiltration into the layered system. Finally, we used the infiltrometer data in conjunction with the Beerkan estimation of soil transfer parameter (BEST) method to determine the following capacitive indicators of soil physical quality of the upper soil layer: air capacity AC (m³ m^–3), plant-available water capacity PAWC (m³ m^–3), relative field capacity RFC (–), and soil macroporosity p_MAC (m³ m^–3). Results showed that the investigated soil was characterized by high soil aeration and macroporosity (i.e., AC and p_MAC) along with low values for indicators associated with microporosity (i.e., PAWC and RFC). These findings suggest that the upper soil layer facilitates root proliferation and quickly drains excess water towards the underlying limestone layer, and, on the contrary, has limited ability to store and provide water to plant roots. In addition, the 3D diagram allowed the detection of non-uniform downward water movement through the restrictive limestone layer. The detected difference between the two layers in terms of hydraulic conductivity suggests that surface ponding and overland flow generation occurs via a saturation-excess mechanism. Indeed, percolating water may accumulate above the restrictive limestone layer and form a shallow perched water table that, in case of extreme rainfall events, could rise causing the complete saturation of the soil profile.

How to cite: Di Prima, S., Giannini, V., Ribeiro Roder, L., Stewart, R. D., Abou Najm, M. R., Longo, V., Winiarski, T., Angulo-Jaramillo, R., Pirastru, M., Lassabatere, L., and Roggero, P. P.: Using GPR surveys and infiltration experiments for assessing soil physical quality of an agricultural soil, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2034, https://doi.org/10.5194/egusphere-egu21-2034, 2021.

Benard P.^1*, Bachmann J.², Bundschuh U.³, Cramer A.¹, Kaestner A.⁴, Carminati A.¹

¹Physics of Soils and Terrestrial Ecosystems, Department of Environmental Systems Science, ETH Zürich, Universitätstrasse 16, 8092 Zürich, Switzerland

²Institute of Soil Science, Leibniz Universität Hannover, Herrenhäuser Strasse 2, 30419 Hannover, Germany

³Soil Physics, Faculty for Biology, Chemistry, and Earth Sciences, University of Bayreuth, Universitätsstraße 30, 95447 Bayreuth, Bavaria, Germany

⁴Paul Scherrer Institute, Lab. for Neutron Scattering and Imaging, Forschungsstrasse 111, 5232 Villigen, Switzerland

*corresponding author; pascal.benard@usys.ethz.ch

Plant roots and microorganisms engineer soil physical properties on the pore scale. The accumulation of organic residues in forest soils and the release of exudates alter local soil wettability and by that impact soil rewetting. We captured the capillary driven infiltration of water and ethanol in forest soils and model rhizosphere using time-series neutron radiography. Information on the evolution of local soil water and ethanol content were used to estimate the distribution of wettability employing a 3D pore-network model. Estimates derived by inverse modelling were compared to classic measures of soil wettability and a set of contrasting scenarios regarding their impact on soil rewetting.

How to cite: Benard, P.: Impact of wettability distribution on soil rewetting, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12244, https://doi.org/10.5194/egusphere-egu21-12244, 2021.

Within the European project Diverfarming (Horizon 2020, no 728003), which investigates crop diversification and low-input farming across Europe, we study the aggregate stability variability of soils with high rock fragment content on steep sloping vineyards in the upper Saar valley of the Mosel area (Wawern, Rhineland-Palatinate, Germany).

In the framework of the case study researched by Trier University and their partners, aromatic herbs (Oregano and Thyme) are planted in rows underneath the grapevines to minimize soil erosion, suppress unwanted weeds and to be harvested for further use. Additionally, this cultivation affects different soil characteristics such as aggregate stability.

We analyse the aggregate stability using and comparing three different methods:

• wet sieving which is executed in two different ways – slaked and rewetted treatment,
• percolation method and
• single drop technique.

Aim of the study is to understand the effect of soil treatments underneath the grapevines, and to identify the method(s) being able to quantify the differences best.

Regarding the different methods, first results indicate that the quantified aggregate stabilities of each method are comparable. With this, we could identify differences between uncultivated rows (control areas), and the rows intercropped with aromatic herbs. In the latter ones, the aggregate stability underneath the grapevines is affected positively. Furthermore, there is a clear difference between slaked and rewetted treatment within the wet sieving method, where less stable aggregates are isolated.

The results indicate that the accomplished management (vine intercropped with Oregano and Thyme) improves the aggregate stability and therefore it improves the soil quality in general.

How to cite: Benzing, T., Hauter, P., Iserloh, T., and Seeger, M.: Aggregate stability of cultivated vineyard soils with high rock fragment content in the Mosel area, Germany, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5167, https://doi.org/10.5194/egusphere-egu21-5167, 2021.

The integral suspension pressure method (ISP) is an automated method to evaluate sedimentation experiments for particle size analysis of soil materials. In contrast to the traditional pipette and hydrometer methods, it is based on the continuous measurement of the suspension pressure at one depth in the sedimentation cylinder. The particle size distribution is determined by inverse simulation (Durner et al., 2017). The ISP is promising because it is semi-automated, continuous, based on process simulation, and does not hinge on oversimplifying assumptions. Most importantly, disturbance of the settling particles is minimized whereas disturbance is unavoidable when applying the traditional methods. ISP has been made commercially available by the METER Group AG (Munich) with a device called PARIO^TM. This implementation of ISP leads to a computerized system which yields quasi-continuous particle-size distribution curves.

Practical experience with PARIO has shown that, despite cutting-edge pressure sensor technology with a resolution of 0.1 Pa, the accuracy of the particle-size analysis was less than expected from a theoretical analysis, and that the time required to determine the clay content exceeded theoretical expectations. In this contribution, we analyze the reasons for disturbances of the methodology in practical applications and show ways to improve accuracy by compensating different errors. Furthermore, we show how an extended version of ISP called ISP+, which considers a single additional measurement in the objective function (Durner and Iden, 2019), leads to stable estimates of the clay fraction while considerably reducing the measurement time.

References:

Durner, W., S.C. Iden, and G. von Unold (2017): The integral suspension pressure method (ISP) for precise particle-size analysis by gravitational sedimentation., Water Resources Research, 53, 33-48, doi:10.1002/2016WR019830.

Durner, W., and S.C. Iden: ISP+: improving the Integral Suspension Pressure method by an additional measurement, Geophysical Research Abstracts Vol. 21, EGU2019-12761, 2019.

How to cite: Durner, W., Alina, M., Thomas, P., and Sascha C., I.: Determining the particle-size distribution of soil materials with the integral suspension pressure (ISP) method – lessons learned from PARIO measurements, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9092, https://doi.org/10.5194/egusphere-egu21-9092, 2021.

Quality control of the measurement of soil hydraulic properties (water retention curve, saturated hydraulic conductivity) using soil cores is not very common in soil physics laboratories. The missing quality control in the labs might be due to the lack of a suitable reference material for the measurement of soil hydraulic properties (SHP). However, a standardized quality of these measurements is needed, especially when generated data from different laboratories are used.

So far no satisfying reference material has been presented that can be used for quality control during the measurement of SHP. Reference material should have a rigid pore system and pore surfaces properties that do not change over time. Additionally, the reference material should be very sensitive to provide a sufficient quality control for the measurement of SHP.

We present sintered glass cylinders with a defined pore size distribution that were tested in the laboratory for reproducibility. After a standardized cleaning procedure of the glass cylinders, water contents after equilibration at -63 hPa (field capacity) showed reasonably low standard deviations. Thus, it seems promising that these cylinders can be used as reference material for the measurement of the water retention curve.

First Results of repeated saturated hydraulic conductivity measurements (Ks) of the same sintered glass cylinders showed larger variability and an increasing trend over the time. Currently the reason for this trend is unknown. Therefore, it is worked on standardizing procedures of using the reference cylinders and on cleaning the cylinders to improve the reproducibility. The results show how sensitive the measurement of saturated hydraulic conductivity is and that we need to put more emphasis on quality control in our work.

How to cite: Lamparter, A. and Stange, C. F.: Reference material for the measurement of soil hydraulic properties, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12164, https://doi.org/10.5194/egusphere-egu21-12164, 2021.

To study the effect of drought on soil water dynamics, we need an accurate description of water retention and hydraulic conductivity from saturation to complete dryness. Recent studies have demonstrated the inaccuracy of conventional soil hydraulic models, especially in the dry end. Likewise, current pedotransfer functions (PTFs) for soil hydraulic properties are based on the classical Mualem-van Genuchten functions.

This study will evaluate models that estimate soil water retention and unsaturated hydraulic conductivity curves in full soil moisture ranges. An example is the Fredlund-Xing scaling model coupled with the hydraulic conductivity model of Wang et al. We will develop pedotransfer functions that can estimate parameters of the model. We will compare it with existing PTFs in predicting water retention and hydraulic conductivity.

The results show that a new suite of PTFs that used sand, silt, clay, and bulk density can be used successfully to predict water retention and hydraulic conductivity over a range of moisture content. The prediction of hydraulic properties is used in a soil water flow model to simulate soil moisture dynamics under drought. This study demonstrates the importance of accurate hydraulic model prediction for a better description of soil moisture dynamics.

How to cite: Minasny, B., Rudiyanto, R., and Maggi, F.: Estimating soil hydraulic properties from saturation to complete dryness, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10481, https://doi.org/10.5194/egusphere-egu21-10481, 2021.

The sedimentation (pipette) (SP) method has been in use for a long time as a solid reference method to estimate particle size distribution (PSD) in soil. The procedure is demanding, not the least concerning the manual extraction of soil fractions at given depth and time intervals during the sedimentation process and their subsequent drying and weighing. The more recent laser diffraction (LD) and integral suspension pressure (ISP) methods are promising alternatives. They have the advantage that the extraction-drying-weighing procedure for the finer soil fractions (clay and silt) is replaced by automatic registration of particle volumes (for LD) and pressures at given depth during the sedimentation process (for ISP). Due to these differences in measurement technics, PSD:s determined with LD and ISP methods often deviate more or less from PSD:s by SP method, which have implications for the matching with historical SP soil databases. We present some draft results of studies comparing the three methods on samples from agricultural soils in Sweden. The results show that there is still a need for further fine-tuning in the methodologies to align PSD composition from one method to the other.

How to cite: Messing, I., Mingot Soriano, A. M., Nimblad Svensson, D., and Barron, J.: Soil particle size distribution – comparison between laser diffraction, integral suspension pressure and sedimentation (pipette) methods, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10869, https://doi.org/10.5194/egusphere-egu21-10869, 2021.

Surfactants have been long used to aid water infiltration into hydrophobic soils since it can reduce the surface tension of water and consequently, the contact angle (CA) form at the solid-liquid-air interface. The degree of soil hydrophobicity is commonly engaged with direct or indirect measurements of the apparent initial advancing CA which is not necessarily correlated with infiltration characteristics of aqueous surfactant solutions. The main objective of this study was to quantify the dynamics of surfactant drop penetration into hydrophobic soils. Three surfactants were examined: anionic (SDS), cationic (CTAB), and nonionic (TX100) at aqueous concentrations of 0.4, 0.8, 1, and 2 C/CMC (where C is the bulk concentration and CMC is critical micelle concentration). Sand with the particle size distribution of 100-210, 425-500, and 600-700 μm was hydrophobized using Leonardite (IHSS). Each run was initiated by placing a 30 μL droplet on the soil surface that was packed into quartz cuvette (2.5×2.5×4 cm). The droplet infiltration dynamics were monitored by an optical goniometer (OCA 20, DataPhysics, Germany), specifically, the drop height, drop base diameter, and CA as a function of time. Notable differences between droplet infiltration characteristics of the three surfactants could be observed. For a given particle fraction, the TX100 and SDS, at concentrations above and below the CMC, the CA and drop height decreased while the drop base diameter increased, suggesting that spreading took place during infiltration. For the CTAB, a significant lag-phase could be observed for all quantities, ranging from 100 to 1000. Following this phase, the drop height and CA showed a relatively gradual decrease while the base diameter exhibited minor changes, suggesting minor changes in solution spreading on the soil surface. Additional observation and interpretation on infiltration characteristics of aqueous surfactants solution will be presented and their implications for enhanced infiltration rate in hydrophobic soils will be discussed.    

How to cite: Nguyen, T. and Arye, G.: Drop infiltration dynamics of anionic, cationic, and nonionic surfactants into hydrophobic soils: effect of the particle size distribution , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14394, https://doi.org/10.5194/egusphere-egu21-14394, 2021.

In order identify the impact of chemicals on water quality and related risks, an understanding of soil infiltration processes in the unsaturated zone is required. In this work, two lysimeters installed at a test field south of Munich, Germany, were investigated. Maize was cultivated at the test field, and lysimeter soil cores are characterized by sandy gravels (lysimeter 1) and sandy-clayey silt (lysimeter 2). For three years, stable water isotopes in precipitation and seepage water were measured in 1-2 week intervals. Observations were interpreted by modeling in order to identify mean transit times of water and dispersion properties. A lumped-parameter model (LPM) implementing an analytical solution was applied. By subdividing the whole observation period into seasonal and vegetative periods with quasi steady-state flow the LPM was improved. Mean transit time of water, dispersion parameters and the contribution of preferential flow paths for all sub-periods with constant conditions were estimated. The improved LPM allows to mimic transient flow conditions and was able to describe the stable water isotope observations more accurately. Hence, the improved LPM approach could reduce model uncertainties as compared to the consideration of steady-state flow. In order to validate the findings from the improved LPM and enhance process understanding, unsaturated flow was also modeled numerically using Hydrus 1D. Soil hydraulic parameters were deducted from laboratory experiments and further adjusted by inverse modeling. Findings from applying the improved LPM could generally be confirmed by numerical modeling. Advantages of the improved LPM over numerical models include the need of a lower number of fitting parameters, which are often associated with higher uncertainties and required efforts concerning model input data. Combing the measurement of stable water isotopes in precipitation and seepage water with the improved LPM revealed a promising approach that could also be applied to support decision making, such as for agricultural practices that aim at minimizing chemical impacts to soil and groundwater quality. Investigations are currently continued for improving simulations by the consideration of mobile and immobile water and root water uptake.

How to cite: Imig, A., Shajari, F., Einsiedl, F., and Rein, A.: Characterization of soil infiltration by stable water isotopes and an improved lumped-parameter model approach, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12595, https://doi.org/10.5194/egusphere-egu21-12595, 2021.

Measuring and modelling of water and solute fluxes in the Critical Zone across soil-vegetation-atmosphere system is nowadays a very important challenge because of the complexity of both soil and plants. Considering the one-dimensional problem, we implement a virtual lysimeter model, LysimeterGEO, in which we coupled infiltration and evapotranspiration by using stress factor (Jarvis, 1976; Ball et al., 1987), with which we can compute effective evapotranspiration and remove it from Richards’ equation balance (Casulli and Zanolli, 2010).

As regards the IT implementation, LysimeterGEO is a system of components built upon the Object Modelling System v3 (OMS3). The infiltration component of the virtual lysimeter is WHETGEO 1D - Water, Heat and Transport in GEOframe (Tubini N. 2021), which solves the mass and energy balance for the one-dimensional case. The mass balance is represented by the Richards equation and the non-linear system is solved using the nested Newton algorithm (Casulli and Zanolli, 2010). Evapotranspiration flows are instead estimated using the GEOframe-Prospero model (Bottazzi M. 2020) which estimates the effective transpiration through the equilibrium temperature of the canopy as a function of the stomatal conductance. Finally, the transpiration is calculated starting from the method of Schymanski and Or (2017) and modified by including the dependence on the transpiring surface, the model of conductance of the stomata, as well as the conservation of mass. In LysimeterGEO the interaction between infiltration and evapotranspiration is made possible by BrokerGEO component (D’Amato C. 2021), which computes the water stress factor for vegetation by using Jarvis or Ball-Berry model. BrokerGEO computes the water stress factor considering the water content information by WHETGEO in each control volumes of the soil column discretization. Moreover, it computes a representative water stress factor for the whole column of soil for the evapotranspiration component. Finally, the density root distribution is considered to remove water into the soil used for evapotranspiration flows.

The modelling of water and solute fluxes across soil-vegetation-atmosphere is made possible by implementation of travel times of waters within vegetation, the growing of the roots and in general the growing of the plants. The idea of a joint infiltration-evapotranspiration model allows us to investigate also problems related to radical growth and the different effect of roots on vegetation. Furthermore, the implementation of travel times on a vegetation scale allows a careful analysis of the behaviour of the same as the soil moisture conditions vary.

How to cite: D'Amato, C., Tubini, N., and Rigon, R.: LysimeterGEO for modelling soil-vegetation-atmosphere 1D system in the Critical Zone, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14865, https://doi.org/10.5194/egusphere-egu21-14865, 2021.

Having a numerical model able to predict soil water content correctly is a very useful tool for many different objectives. However, it depends on the correct election of the soil hydraulic properties (SHP) on which the simulations are based. Measuring SHP in laboratory is time and economic-consuming and their inference by soil water monitoring and inverse modelling is a smart alternative. 

However, the resources needed to obtain copious data are sometimes unavailable and questions arise regarding what is the best monitoring strategy that let to obtain the best SHP with the fewest number of sensors.  When null or scarce data is present for some soil layers several solutions of the same problem are encountered. SHP estimations by inverse modeling could vary according to the data available and the vertical distribution of the measurement points.  The aim of this work is to evaluate different monitoring strategies to obtain an accurate hydraulic model with a limited number of observations depths. For this purpose, data monitored in an experimental plot in Bahía Blanca (Argentina) was used to run several inverse numerical simulations with the HYDRUS software.  Several scenarios of available data were considered: (1) six monitoring depths (6-MD) (30 cm, 60 cm, 90 cm, 120 cm, 150 cm, and 180 cm); (2) five monitoring depths (5-MD) subtracting the information from one soil depth at a time; (3) four monitoring depths (4-MD) subtracting the information from two soil depths, simultaneously. Each depth included soil water content, ϴ, and soil pressure head, h, measurements.

The best fit was achieved with the 6-MD strategy. The Nash-Sutcliffe coefficient of efficiency (EF) were 0.784 and 0.665 for the ϴ and h, respectively. Besides, the relative root mean square error (rRMSE) was 0.134 for ϴ and 0.127 for h. For the 5-MD strategy the best performance was achieved by removing the information from depths of 90 cm, 120 cm, or 150 cm. In those cases, EF was between 0.715-0.717 and rRMSE ranged from 0.132-0.133. Statistics reported a worse fit when removing data from the upper and the lower layers. For the 4-MD strategy, the best performance was accomplished by suppressing data from 90 cm and 120 cm (EF=0.707; rRMSE=0.135).

The observation points that had less weight in parameter prediction corresponded to the intermedium vadose zone. If data from the upper and lower boundaries of the soil profile are available, ϴ and h from the middle section could be predicted reasonably well, anyway. The inversely model SHP from the 5-MD and 4-MD strategies correctly represent field retention data points θ (h). If the optimal monitoring depths are recognized, the time, cost, and labor needed to a correctly soil manage practice will be greatly reduced. 

How to cite: Scherger, L. E., Valdes-Abellan, J., and Lexow, C.: Evaluation of vertical monitoring strategies to predict soil hydraulic characteristics and water contents by inverse modeling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9999, https://doi.org/10.5194/egusphere-egu21-9999, 2021.

The representation of land surface processes in hydrologic and climatic models is critically dependent on the soil water characteristics curve (SWCC) that defines the hydrologic behavior of unsaturated soil. The SWCC depends not only on soil texture, but it is also shaped by biopores, soil structure, and clay type. To capture climate, vegetation and other soil formation processes on SWCC in spatial context, we predict how SWCC parameter values vary with local environmental covariates using a machine learning approach. The model was trained using (i) a novel and comprehensive compilation of global dataset of soil water retention measurements collected from the literature (approximately 13,000 pairs of water content and matric potential data) and (ii) global maps of environmental covariates and soil texture developed at 250 m resolution. Because in many cases only few measurements per sample are available to fit the SWCC, the estimated parameters are often highly uncertain and could yield unrealistic predictions of related physical quantities. To address these limitations, we added constraints to the values of residual and saturated water content based on clay content and mineralogy and ensured that the shape parameters related to air-entrance and pore size distribution honor other physical constraints, such as the characteristic length of evaporation and the ponding time. The resulting global maps of SWCC parameters are compared with predictions using pedotransfer functions (PTFs) based on soil information alone that were trained on data mainly collected for samples from arable land in temperate regions. We anticipate that our model including environmental covariates and geospatial data (covariate-based geotransfer functions CoGTFs) would enable us to provide more reliable predictions (compared to traditional PTFs) of SWCC that can be implemented in Earth system models.

How to cite: Gupta, S., Papritz, A., Lehmann, P., Hengl, T., Bonetti, S., and Or, D.: Global mapping of the soil water characteristics curve using machine-learning, a comprehensive dataset and spatial covariates, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12894, https://doi.org/10.5194/egusphere-egu21-12894, 2021.