Abstract
In structured soils, clay-organic coatings are spatially distributed along macropore surfaces. Information on thickness and volume of coating material is essential for macropore-matrix mass exchange of water and solutes. However, their determination is difficult and fraught with uncertainty due to irregular shapes of macropore surfaces. The objective of this study was to test the use of a three-dimensional (3D) confocal laser scanning microscope for gauging soil macropore coatings. For this test, coating material was manually separated from the surface of intact soil clods (5 cm edge length) sampled from Bt horizons (with illuvial clay) of till-derived and loess-derived Haplic Luvisols. The resulting coating thicknesses ranged between approx. 0.1 and 0.5 mm and volumes between 0.057 and 2.9 mm3 with mean densities of 1.96 g cm−3 for loess-derived and 2.67 g cm−3 for till-derived coatings. The 3D laser microscopic method yielded data based on micro-topographic information. The key benefits of the 3D confocal laser scanning microscope are the information on the mm-scale spatial distribution of coating thickness and bulk density. These additional parameters of the clod surface or aggregate micro-topography can be useful for improved quantification of accessibility of sorption surfaces and for describing macropore-matrix mass transfer of reactive solutes.
[40] Beck-Broichsitter S., Fleige H., Dusek J., Gerke H.H. (2020), Anisotropy of unsaturated hydraulic properties of compacted mineral capping systems seven years after construction, Soil and Tillage Research, 204, 104702. doi: 10.1016/j.still.2020.104702
Abstract
The mechanical compaction of soil material of mineral landfill systems affects the continuity and connectivity of the complex soil pore network. A horizontally-oriented layering is intended to generate a slope-induced lateral water flow out of mineral capping systems that is sufficient to minimise the statutory required vertical percolation through the underlying waste body and the potential leachate. In this case, soil compaction affects both the porosity and water retention as capacity values and the hydraulic conductivity as intensity parameter. The idea of this study was to combine information on both soil properties in an extended anisotropy factor based on the soil water diffusivity. The analysis is focused on the directiondependent soil hydraulic properties of a mechanically compacted landfill capping system. In particular, the volume fractions were related to the fractional capillary potential for each of the characteristic pore size classes. Three different soil profiles of top, middle, and bottom slopes of the mineral capping system of the Rastorf landfill in Northern Germany were sampled seven years after construction. Undisturbed soil cores of 100 cm3 and 438 cm3 were extracted in vertical (ver) and horizontal directions (hor) in depths of 20, 50, and 80 cm representing the main layers. The soil water retention and unsaturated hydraulic conductivity, K, functions were determined by suctions plates, permeameter, and the evaporation method. In the coarse pores range (pressure head values of h ≥ −300 hPa), the standard anisotropy ratio, AR, (K(Se)hor/K(Se)ver) as a function of effective saturation, Se, in the sealing liner in 80 cm depth was larger than 1, indicating higher horizontal than vertical K(Se) values. Thus, AR-values above 1 in the range close to water saturation especially in 80 cm depth suggest the tendency of lateral water flow out of mineral capping system due to a sufficient hydraulic potential and thus its reasonable functionality, even seven years after construction. The anisotropy factor was extended in two steps; for AR* and AR**, the pore size class-related matric flux potential, ϕ, and the soil water diffusivity, D(θ), were proposed to combine intensity parameters with capacitybased volume fractions of pore size classes and the fractional capillary potential. The ϕ- and D(θ)-weighted anisotropy ratios, AR* and AR**, indicate that anisotropy increases with the volume fraction of macropores (r2 AR* of 0.69−0.77; r2 AR** of 0.71−0.80) and wide coarse pores (r2 AR* of 0.57−0.78; r2 AR** of 0.79−0.89) in both directions. The results suggest that by combining both the intensity and the capacity parameters of the soil hydraulic properties in an extended anisotropy ratio improves the information on compacted mineral capping systems.
[39] Beck-Broichsitter S., Gerriets M.R., Gerke H.H., Sobotkova M., Dusek J., Dohrmann R., Horn R. (2020), Brilliant Blue sorption characteristics of clay-organic aggregate coatings from Bt horizons, Soil and Tillage Research, 201, 104635. doi: 10.1016/j.still.2020.104635
Abstract
In structured soils, water and reactive solutes can move preferentially through larger inter-aggregate macropores (biopores and cracks) and smaller intra-aggregate pores. Especially clay-organic coating material is of major importance for the exchange of water and solutes between macro- and micropores and the soil matrix by affecting the reactive transport in a yet largely unknown way. The objective of this study was to compare the adsorption and desorption behaviour of clay-organic coatings from samples of till- (T-Bt) and loess- (L-Bt) derived Bt horizons of Haplic Luvisols with those of the soil matrix and a hillslope loam-derived Bsh (HL-Bsh) horizon of a forest Cambisol-Podzol (CM-PZ) without coatings. These coatings are characterized by qm-values of up to 1100 μmol cm−3 for clay contents of up to 330 g kg−1. The values are significantly higher than those of mixed samples without coatings (qm of 180 μmol cm−3 for clay content of 115 g kg−1). The results indicate two different adsorption mechanisms, i) sorption on siloxane surfaces of the alumosilicates (clay minerals) and ii) adsorption controlled by hydrophobic interaction with soil organic material which is possibly attached to clay-organic complexes. The great difference in sorption properties between coatings and matrix suggests that mean values obtained from analyzing mixed samples cannot be used to describe the retardation of dissolved reactive substances on the surfaces of biopores and larger cracks during preferential flow events.
Abstract
Green roofs, as an element of the green infrastructure, contribute to the urban heat island effect mitigation and the urban drainage outflow reduction. To achieve the desired functions, it is essential to understand the role of the individual roof layers and ensure their proper design. A physically-based model was used to assess the hydrological and thermal regime of two experimental green roof test beds containing distinct soil substrates (a local Technosol and a more permeable commercial substrate “Optigreen”). The test beds together with a meteorological station were built on the building green roof. Each test bed has an effective area of one square meter and is equipped with a soil temperature sensor and an outflow gauge; one of the test beds is continuously weighed. The observed conditions were simulated using one-dimensional numerical model describing the water flow in variably saturated porous medium by Richards’ equation and the heat transport by the advection-conduction equation. The model was able to satisfactorily reproduce the measured outflow and soil temperature. The water-potential- gradient based root water uptake module effectively captured the water storage depletion between the rainfall events. The difference between the two soil substrates tested is demonstrated by the contrasting ability of the soil layers to retain water. Model representation of the thermal conditions within the green roof soils was achieved using independently evaluated thermal properties of the soils and drainage board. The model was also used to analyze the effects of the substrate depth and type of vegetation cover on the transpiration and soil water regime of the green roofs. Increasing the substrate depth causes a rise of root water uptake and induces a significant reduction of the maximal temperature. The thinner soil profiles are more sensitive to the plant species selection.
[37] Vogel T., Dusek J., Dohnal M., Snehota, M. (2020), Moisture regime of historical sandstone masonry — A numerical study, Journal of Cultural Heritage, 42, 99–107. doi: 10.1016/j.culher.2019.09.005
Abstract
Rising damp causes deterioration of masonry walls in many historical buildings. Although the phenomenon of capillary rise in porous structures is relatively well understood, reliable numerical modeling of the moisture regime, applicable to the assessment of current state as well as to the predictions of changes induced by various corrective moisture-reduction measures, remains a challenge. This paper presents the results of a numerical modeling study dealing with the moisture regime of a masonry wall of the baroque Church of All Saints in Hermankovice. The numerical approach used is based on general concepts of mass conservation and Darcian flow of capillary water in porous media. The model was parameterized and validated using experimental data obtained by on-site survey, laboratory analysis and monitoring. The modeling results confirmed our hypothesis that, in a long-term perspective, the second-stage evaporation—controlled by the hydraulic properties of the masonry—prevails over the first-stage evaporation—controlled by the atmospheric conditions—for most simulation scenarios conducted. Of the two corrective moisture-reduction measures considered, i.e. a closed drain versus open drain installation, the latter was found to be significantly more effective, leading to a greater reduction of moisture in the masonry.
[36] Dusek J., Vogel T. (2019), Modeling travel time distributions of preferential subsurface runoff, deep percolation and transpiration at a montane forest hillslope site, Water, 11, 2396. doi: 10.3390/w11112396
Abstract
Residence and travel times of water in headwater catchments, or their smaller spatial units, such as individual hillslopes, represent important descriptors of catchments’ hydrological regime. In this study, travel time distributions and residence times were evaluated for a montane forest hillslope site. A two-dimensional dual-continuum model, previously validated on water flow and oxygen-18 data, was used to simulate the seasonal soil water regime and selected major rainfall–runoff events observed at the hillslope site. The model was subsequently used to generate hillslope breakthrough curves of a fictitious conservative tracer applied at the hillslope surface in the form of the Dirac impulse. The simulated tracer breakthroughs allowed us to estimate the travel time distributions of soil water associated with the episodic subsurface stormflow, deep percolation and transpiration, thus yielding partial travel time distributions for the individual discharge processes. The travel time distributions determined for stormflow were dominated by the lateral component of preferential flow. The stormflow median travel times, calculated for nine selected rainfall–runoff events, varied considerably—ranging from 1 to 17 days. The estimated travel times were significantly affected by the temporal rainfall patterns and antecedent soil moisture distributions. The residence times of soil water, evaluated for three consecutive growing seasons, ranged from 29 to 37 days. The analysis reveals the interplay of soil water storage and discharge processes at the hillslope site of interest. The applied methodology can be used for the evaluation of runoff dynamics at the hillslope and catchment scales as well as for the quantification of biogeochemical transformations of dissolved chemicals.
Abstract
A new modeling approach was developed to facilitate simulations of soil water flow and energy transport during sporadic freezing–thawing episodes typical for the winter regime of humid-temperate continental climate. The approach is based on an accurate non-iterative algorithm for solving the highly non-linear energy balance equation during phase transitions. The new algorithm was successfully verified against the analytical solution for idealized freezing and thawing conditions. Two examples of the model application – under hypothetical and real field conditions – are given.
Abstract
Parameterization of transformation and transport processes of dissolved organic carbon (DOC) in soils is challenging especially under variable hydrological conditions. In this study, DOC concentrations in stormflow were analysed with a physically based modelling approach. A one-dimensional dual-continuum vertical flow and transport model was applied to simulate subsurface processes in a macroporous forest hillslope soil over a period of 4.5 years. Microbially mediated transformations of DOC were assumed to depend primarily on soil moisture and soil temperature. Two conceptually different descriptions of the sorption of DOC to soil were examined with equilibrium and kinetic approaches. In order to quantify the uncertainties associated with the model parameterization, Monte Carlo analyses in conjunction with Latin hypercube sampling was performed. Despite the complexity of microbial transformations, the simulated temporal patterns of DOC concentrations in stormflow showed similar behaviour to those reflected in the observed DOC fluxes. Due to preferential flow, the hillslope DOC export (5.0 ± 0.5 g C m−2 year−1) was higher than the amounts usually reported in the literature. Overall DOC transport in hillslope scenarios could be described appropriately using the equilibrium sorption assumption. The performed analyses showed that the inclusion of the kinetic description of DOC sorption only slightly improved the predictions of the DOC hillslope export. Moreover, influences of seasonal hydro-climatological conditions on hillslope export of DOC could be observed. Reduced DOC transport during an extreme warm and dry summer was described with lower accuracy, thus indicating the difficulties in the representation of DOC transformations under dry conditions.
[33] Sobotkova M., Dusek J., Alavi G., Sharma L., Ray C. (2018), Assessing the feasibility of soil infiltration trenches for highway runoff control on the island of Oahu, Hawaii, Water, 10, 1832. doi: 10.3390/w10121832
Abstract
The coastal waters of Hawaii are extremely important for recreation as well as for the health of the marine environment. Non-point source pollution from storm runoff poses a great threat to surface water quality in Hawaii. The State of Hawaii Department of Transportation (HDOT) includes infiltration trenches as a best management practice (BMP) option to reduce pollution caused by stormwater runoff. HDOT guidelines state that the implementation of BMPs is needed to reduce sediment and pollutant loads to streams and the ocean. In this study, the suitability of soils adjacent to highways on Oahu for the siting of infiltration trenches was examined. In addition to field surveys and in-situ tests, laboratory investigations on soil properties, infiltration experiments on undisturbed soil columns, and mathematical modeling of hydraulic functioning of the infiltration trench were conducted. Dissolved metal concentrations in highway stormwater runoff were observed to exceed the groundwater environmental action levels for all heavy metals tested, but the soils had high sorption capacity for these metals. The results of the simulations indicated that all the sampled Oahu soils, with one exception, would require less than two hours to drain a filled hypothetical trench. Therefore, these soils are suitable for construction of infiltration trenches as a possible BMP, even when clogging of soil is considered in the simulation.
[32] Dusek J., Vogel T. (2018), Hillslope hydrograph separation: The effects of variable isotopic signatures and hydrodynamic mixing in macroporous soil, Journal of Hydrology, 534, 590–605. doi: 10.1016/j.jhydrol.2018.05.054
Abstract
The prevailing opinion on the temporal origin of water in a hillslope stormflow hydrograph is that the pre-event water represents a dominant fraction. Such conclusion is usually based on hydrograph separation techniques using stable water isotopes (or other conservative tracers) in conjunction with a mass balance approach. In this study, a two-dimensional dual-continuum model was used to study preferential flow of water and transport of Oxygen-18 (O-18) in a vertical cross-section of a hillslope located in a temperate spruce forest. The effects of hydrodynamic mixing and the spatiotemporal variability of isotopic signatures on estimated pre-event/event water fractions in the hillslope discharge were studied by means of numerical simulation experiments. Pre-event and event water contributions to hillslope stormflow were evaluated using a two-component mass balance approach combined with the 2D flow and transport simulations involving real as well as synthetic O-18 signatures. Long-term simulations of O-18 transport in the hillslope segment were compared with the observed O-18 content in soil water and in the hillslope effluent. The results of the long-term simulations indicated significant mixing of pre-event and event water occurring near the subsurface trench and in the soil above the soil–bedrock interface where the transfer of O-18 from the soil matrix to the preferential pathways takes place. Despite the dominant role of preferential flow in the generation of hillslope stormflow, the pre-event water formed 52–84% of total subsurface stormflow. The mass balance approach failed in partitioning the hillslope discharge into the pre-event/event water components for two thirds of the selected rainfall–runoff episodes due to similar natural isotopic signatures of pre-event and event water. The analysis showed that spatially and temporally variable exchange of O-18 between the soil matrix and preferential pathways exerted a primary control on the estimates of the temporal origin of water in the hillslope runoff. It was demonstrated that the degree of hydrodynamic mixing in the flow domain played an important role in the interpretation of the isotope-based hydrograph separation.
[31] Marx A., Dusek J., Jankovec J., Sanda M., Vogel T., van Geldern R., Hartmann J., Barth J.A.C. (2017), A review of CO2 and associated carbon dynamics in headwater streams: a global perspective, Reviews of Geophysics, 55, 560–585. doi: 10.1002/2016rg000547
Abstract
Terrestrial carbon export via inland aquatic systems is a key process in the global carbon cycle. It includes loss of carbon to the atmosphere via outgassing from rivers, lakes or reservoirs and carbon fixation in the water column as well as in sediments. This review focuses on headwater streams that are important because their stream biogeochemistry directly reflects carbon input from soils and groundwaters that becomes superimposed by additional inputs further downstream. Major drivers of carbon dioxide partial pressures (pCO2) in streams and mechanisms of terrestrial dissolved inorganic, organic and particulate organic carbon (DIC, DOC, and POC) influxes are summarized in this work. Our analysis indicates that the global river average pCO2 of 3,100 ppmV is more often exceeded by contributions from small streams when compared to rivers with larger catchments (> 500 km2). Because of their large proportion in global river networks (> 96 % of the total number of streams), headwaters contribute large – but still poorly quantified – amounts of CO2 to the atmosphere. Conservative estimates imply that globally 36 % (i.e. 0.93 Pg C yr-1) of total CO2 outgassing from rivers and streams originate from headwaters. We also discuss challenges in determination of CO2 sources, concentrations and fluxes. To overcome uncertainties of CO2 sources and its outgassing from headwater streams on the global scale, new investigations are needed that should include groundwater data. Such studies would also benefit from applications of integral CO2 outgassing isotope approaches and multi-scale geophysical imaging techniques.
[30] Marx A., Hintze S., Sanda M., Jankovec J., Oulehle F., Dusek J., Vitvar T., Vogel T., van Geldern R., Barth J.A.C. (2017), Acid rain footprint three decades after peak deposition: Long-term recovery from pollutant sulphate in the Uhlirska catchment (Czech Republic), Science of the Total Environment, 598, 1037–1049. doi: 10.1016/j.scitotenv.2017.04.109
Abstract
The granitic Uhlirska headwater catchment with a size of 1.78 km2 is located in the JizeraMountains in the northern Czech Republic and received among the highest inputs of anthropogenic acid depositions in Europe. An analysis of sulphate (SO4 2–) distribution in deposition, soil water, stream water and groundwater compartments allowed to establish a SO4 2– mass-balance (deposition input minus surface water export) and helped to evaluate which changes occurred since the last evaluation of the catchment in 1997. The determined SO4 2– concentrations decreased in the following order: wetland groundwater > groundwater from 20 m below ground level (bgl) > groundwater from 30 m bgl > stream water > groundwater from10 m bgl > hillslope soil water > wetland soil water > bulk deposition with median values of 0.24, 0.21, 0.17, 0.15, 0.11, 0.07, 0.03 and 0.01 mmol L−1, respectively. Our results showthat average deposition reductions of 62% did not result in equal changes of the sulphate mass-balance, which changed by only 47%. This difference occurs because sulphate originates from internal sources such as the groundwater and soil water. The Uhlirska catchment is subject to delayed recovery from anthropogenic acid depositions and remains a net source of stored sulphur even after three decades of declining inputs. The wetland groundwater and soil water provide environmental memories of legacy pollutant sulphate. Elevated stream water sulphate concentrations after the unusually dry summer 2015 imply importance of weather and climate patterns for future recovery from acidification.
[29] Vogel T., Votrubova J., Dohnal M., Dusek J. (2017), A simple representation of plant water storage effects in coupled soil water flow and transpiration stream modeling, Vadose Zone Journal, 16. doi: 10.2136/vzj2016.12.0128
Abstract
When describing the movement of water in variably saturated plant root zone, most existing hydrological models employ the assumption of quasi-steady-state flow to relate root water uptake to canopy transpiration, thereby neglecting the effect of changing plant water storage. This approach is known to be problematic, especially when considering relatively large volumes of water stored in the tissues of tall trees. We propose a simple algorithm, based on the concept of whole-plant hydraulic capacitance, to deal with the problem. The algorithm is implemented in a one-dimensional soil water flow model involving vertically distributed macroscopic root water uptake. In this study, the proposed transient storage approach was compared with the quasi-steady-state approach. Both approaches are used to simulate soil water flow and diurnal variations of transpiration at a forest site covered with Norway spruce [Picea abies (L.) H. Karst]. The key parameter of the transient storage approach, the plant hydraulic capacitance, is estimated by comparing the variations of potential transpiration rate, derived from micrometeorological measurements, with observed sap flow intensities. The application of the proposed algorithm leads to more realistic predictions of root water uptake rates at the site of interest. The algorithm can be easily implemented into existing soil water flow models and used to simulate transpiration stream responses to varying atmospheric and soil moisture conditions including isohydric and anisohydric plant responses to drought stress.
[28] Dusek J., Vogel T., Dohnal M., Barth J.A.C., Sanda M., Marx A., Jankovec J. (2017), Dynamics of dissolved organic carbon in hillslope discharge: Modeling and challenges, Journal of Hydrology, 546, 309–325. doi: 10.1016/j.jhydrol.2016.12.054
Abstract
Reliable quantitative prediction of water movement and fluxes of dissolved substances – specifically organic carbon – at both the hillslope and the catchment scales remains a challenge due to complex boundary conditions and soil spatial heterogeneity. In addition, microbially mediated transformations of dissolved organic carbon (DOC) are recognized to determine the balance of DOC in soils. So far, only few studies utilized stable water isotope information in modeling and even fewer linked dissolved carbon fluxes to mixing and/or transport models. In this study, stormflow dynamics of 18O/16O ratios in the water molecules (expressed as d18O) and DOC were analyzed using a physically-based modeling approach. A one-dimensional dual-continuum vertical flow and transport model was used to simulate the subsurface transport processes in a forest hillslope soil over a period of 2.5 years. The model was applied to describe the transformation of input signals of d18O and DOC into output signals observed in the hillslope stormflow. To quantify uncertainty associated with the model parameterization, Monte Carlo analysis in conjunction with Latin hypercube sampling was applied. d18O variations in hillslope discharge and in soil pore water were predicted reasonably well. Despite the complex nature of microbial transformations that caused uncertainty in model parameters and subsequent prediction of DOC transport, the simulated temporal patterns of DOC concentration in stormflow showed similar behavior to that reflected in the observed DOC fluxes. Due to preferential flow, the contribution of the hillslope DOC export was higher than the amounts that are usually found in the available literature.
[27] Dohnal M., Vogel T., Dusek J., Votrubova J., Tesar M. (2016), Interpretation of ponded infiltration data using numerical experiments, Journal of Hydrology and Hydromechanics, 64, 289–299. doi: 10.1515/johh-2016-0020
Abstract
Ponded infiltration experiment is a simple test used for in-situ determination of soil hydraulic properties, particularly saturated hydraulic conductivity and sorptivity. It is known that infiltration process in natural soils is strongly affected by presence of macropores, soil layering, initial and experimental conditions etc. As a result, infiltration record encompasses a complex of mutually compensating effects that are difficult to separate from each other. Determination of sorptivity and saturated hydraulic conductivity from such infiltration data is complicated. In the present study we use numerical simulation to examine the impact of selected experimental conditions and soil profile properties on the ponded infiltration experiment results, specifically in terms of the hydraulic conductivity and sorptivity evaluation. The effect of following factors was considered: depth of ponding, ring insertion depth, initial soil water content, presence of preferential pathways, hydraulic conductivity anisotropy, soil layering, surface layer retention capacity and hydraulic conductivity, and presence of soil pipes or stones under the infiltration ring. Results were compared with a large database of infiltration curves measured at the experimental site Liz (Bohemian Forest, Czech Republic). Reasonably good agreement between simulated and observed infiltration curves was achieved by combining several of factors tested. Moreover, the ring insertion effect was recognized as one of the major causes of uncertainty in the determination of soil hydraulic parameters.
[26] Dusek J., Vogel T. (2016), Hillslope-storage and rainfall-amount thresholds as controls of preferential stormflow, Journal of Hydrology, 534, 590–605. doi: 10.1016/j.jhydrol.2016.01.047
Abstract
Shallow saturated subsurface flow is a dominant runoff mechanism on hillslopes of headwater catchments under humid temperate climate. Its timing and magnitude is significantly affected by the presence of preferential pathways. Reliable prediction of runoff from hillslope soils under such conditions remains a challenge. In this study, a quantitative relationship between rainfall, stormflow, and leakage to bedrock for hillslopes, where lateral preferential runoff represents a dominant part of the overall response, was sought. Combined effects of temporal rainfall distribution and initial hillslope saturation (antecedent moisture conditions) on stormflow, leakage to bedrock, and overall water balance were evaluated by conducting simulations with synthetic rainfall episodes. A two-dimensional dual-continuum model was used to analyze hydrological processes at an experimental hillslope site located in a small forested headwater catchment. Long-term seasonal simulations with natural rainfall indicated that leakage to bedrock occurred mostly as saturated flow during major runoff events. The amount of rainfall needed to initiate stormflow appeared as a dynamic hillslope property, depending on temporal rainfall distribution, initial hillslope storage, and the spatial distribution of soil water within the hillslope. No single valued rainfall threshold responsible for triggering stormflow was found. Rainfall–stormflow as well as rainfall–leakage relationships were found highly nonlinear for low initial hillslope saturations. Temporal rainfall distribution affected the amount of rainfall necessary to initiate stormflow more than it did the amounts of stormflow or leakage to bedrock. In spite of a simple hillslope geometry with constant slope and parallel soil–atmosphere and soil–bedrock interfaces considered in the analysis, the applied model predicted a hysteretic behavior of storage–discharge relationship. The results showed a mutual interplay of components of hillslope water balance exposing a nonlinear character of the hillslope response. The study provided a quantitatively coherent insight in the hydraulic functioning of hillslopes where preferential flow constitutes a dominant part of stormflow.
[25] Vogel T., Votrubova J., Dusek J., Dohnal M. (2016), Mesoscopic aspects of root water uptake modeling – Hydraulic resistances and root geometry interpretations in plant transpiration analysis, Advances in Water Resources, 88, 86–96. doi: 10.1016/j.advwatres.2015.12.006
Abstract
In the context of soil water flow modeling, root water uptake is often evaluated based on water potential difference between the soil and the plant (the water potential gradient approach). Root water uptake rate is modulated by hydraulic resistance of both the root itself, and the soil in the root vicinity. The soil hydraulic resistance is a function of actual soil water content and can be assessed assuming radial axisymmetric water flow toward a single root (at the mesoscopic scale). In the present study, three approximate solutions of mesoscopic root water uptake – finite difference approximation, steady-state solution, and steady-rate solution – are examined regarding their ability to capture the pressure head variations in the root vicinity. Insignificance of their differences when implemented in the macroscopic soil water flow model is demonstrated using the critical root water uptake concept. Subsequently, macroscopic simulations of coupled soil water flow and root water uptake are presented for a forest site under temperate humid climate. Predicted soil water pressure heads and actual transpiration rates are compared with observed data. Scenario simulations illustrate uncertainties associated with estimates of root geometrical and hydraulic properties. Regarding the actual transpiration prediction, the correct characterization of active root system geometry and hydraulic properties seems far more important than the choice of a particular mesoscopic model.
[24] Dusek J., Dohnal M., Snehota M., Sobotkova M., Ray C., Vogel T. (2015), Transport of bromide and pesticides through an undisturbed soil column: A modeling study with global optimization analysis, Journal of Contaminant Hydrology, 175–176, 1–16. doi: 10.1016/j.jconhyd.2015.02.002
Abstract
The fate of pesticides in tropical soils is still not understood as well as it is for soils in temperate regions. In this study, water flow and transport of bromide tracer and five pesticides (atrazine, imazaquin, sulfometuron methyl, S-metolachlor, and imidacloprid) through an undisturbed soil column of tropical Oxisol were analyzed using a one-dimensional numerical model. The numerical model is based on Richards’ equation for solving water flow, and the advection-dispersion equation for solving solute transport. Data from a laboratory column leaching experiment were used in the uncertainty analysis using a global optimization methodology to evaluate the model’s sensitivity to transport parameters. All pesticides were found to be relatively mobile (sorption distribution coefficients lower than 2 cm3 g–1). Experimental data indicated significant non-conservative behavior of bromide tracer. All pesticides, with exception of imidacloprid, were found less persistent (degradation half-lives smaller than 45 days). Three of the five pesticides (atrazine, sulfometuron methyl, and S-metolachlor) were better described by the linear kinetic sorption model, while the breakthrough curves of imazaquin and imidacloprid were more appropriately approximated using nonlinear instantaneous sorption. Sensitivity analysis suggested that the model is most sensitive to sorption distribution coefficient. The prediction limits contained most of the measured points of the experimental breakthrough curves, indicating adequate model concept and model structure for the description of transport processes in the soil column under study. Uncertainty analysis using a physically-based Monte-Carlo modeling of pesticide fate and transport provides useful information for the evaluation of chemical leaching in Hawaii soils.
Abstract
The heterogeneity of water flow was evaluated in sandy loam soil covered by grass. The radioactive tracer infiltration experiment was performed at two parallel plots with different irrigation intensities. Effective cross section and degree of preferential flow parameters were used to evaluate flow regime during the experiment. For both plots, the heterogeneity of water flow increased with depth. The differences in irrigation intensity did not result in different values of the effective cross section and degree of preferential flow, indicating similar flow regime within the two experimental plots. The heterogeneity of water flow in shallower depths (0–50 cm) did not change with cumulative infiltration except for early times/small cumulative infiltrations, when the flow paths of preferential flow were formed. In deeper depths (60–70 cm) the flow paths of preferential flow were formed later, and therefore, the heterogeneity of water changed with cumulative infiltration.
[22] Dusek J., Vogel T. (2014), Modeling subsurface hillslope runoff dominated by preferential flow – 1D versus 2D approximation, Vadose Zone Journal, 13. doi: 10.2136/vzj2013.05.0082
Abstract
Shallow saturated subsurface flow, frequently observed on hillslopes of headwater catchments in humid temperate climate, often dominates hydrological responses of the catchments to major rainfall events. Typically, these responses are significantly affected by the presence of preferential flow. Reliable prediction of runoff from hillslope soils under such conditions remains a challenge. In this study, two approaches to modeling hillslope responses to rainstorms, which differ in dimensionality and thus also in the complexity of geometric, material, and boundary conditions, are tested and confronted with the hillslope discharge data observed in an experimental trench. In the one-dimensional approach, 1D variably saturated vertical soil water flow is combined with 1D lateral saturated flow above the soil–bedrock interface. In this approach, vertical flow is modeled by means of a dual-continuum concept involving two coupled Richards’ equations (representing flow in the soil matrix and in the preferential pathways), while lateral flow is described by the diffusion wave equation. In the two-dimensional approach, the movement of water in a variably saturated hillslope segment is modeled as vertical planar flow (i.e., the vertical and lateral flow components are fully integrated into one flow system). Similar to the 1D approach, the preferential flow effects are implemented in the 2D model by means of the dual-continuum concept. The two model approaches (1D and 2D) resulted in similar hillslope discharge hydrographs, characterized by short-term runoff peaks followed by zero discharge periods, but the 2D model showed closer agreement between observed and simulated soil water pressure heads near the trench. The sensitivity analysis of soil and bedrock properties confirmed a significant influence of the bedrock saturated hydraulic conductivity on simulated hillslope discharge. The simpler 1D approach, based on the combination of 1D vertical flow and 1D lateral flow, was found to provide a useful approximation of the more complex and flexible 2D system and to be far more efficient in terms of computing time.
[21] Lichner L., Dusek J., Dekker L.W., Zhukova N., Fasko P., Holko L., Sir M. (2013), Comparison of two methods to assess heterogeneity of water flow in soils, Journal of Hydrology and Hydromechanics, 61, 299–304, doi: 10.2478/johh-2013-0038
Abstract
The heterogeneity of water flow and solute transport was assessed during radioactive tracer infiltration experiment in a black clay loam soil using modified methods to estimate the effective cross section (ECS) and the degree of preferential flow (DPF). The results of field and numerical experiments showed that these parameters characterized the heterogeneity of water flow in the soils unequivocally. The ECS decreases non-linearly and the DPF increases linearly with an increase of the bypassing ratio (ratio of macropore flow rate to total flow rate). The ECS decreased and the DPF increased with depth, which suggests an increase in the heterogeneity of water flow with depth. The plot of the DPF against ECS values calculated from the tracer experiment data was consistent with the relationship obtained by the numerical simulation assuming preferential flow in the neighbourhood of three probes.
[20] Dusek J., Lichner L., Vogel T., Stekauerova V. (2013), Transport of iodide in structured soil under spring barley during irrigation experiment analyzed using dual-continuum model, Biologia, 68, 1094–1098, doi: 10.2478/s11756-013-0249-4
Abstract
Transport of radioactive iodide 131I– in a black clay loam soil under spring barley in an early ontogenesis phase was monitored during controlled field irrigation experiment. It was found that iodide bound in the soil matrix could be mobilized by the surface leaching enhanced by mechanical impact of water drops and transported below the root zone of crops via soil cracks. The iodide transport through structured soil profile was simulated by the one-dimensional dual-continuum model, which assumes the existence of two inter-connected flow domains: the soil matrix domain and the preferential flow domain. The model predicted relatively deep percolation of iodide within a short time, in a good agreement with the observed vertical iodide distribution in soil. The dual-continuum approach proved to be an adequate tool for evaluation of field irrigation experiments conducted in structured soils.
[19] Vogel T., Dohnal M., Dusek J., Votrubova J., Tesar M. (2013), Macroscopic modeling of plant water uptake in a forest stand involving root-mediated soil-water redistribution, Vadose Zone Journal, 12, doi: 10.2136/vzj2012.0154
Abstract
One of the principal components of the mass exchange within the soil-plant-atmosphere system is the soil water extraction by plant roots. Its adequate evaluation is a prerequisite for correct predictions of plant transpiration and soil water distribution in root zone. The main objective of the present study is to contribute to the development of sufficiently realistic, yet algorithmically simple models of water exchange between soil and plant roots applicable for numerical simulation of soil water responses to atmospheric forcing. In our case, a simple macroscopic vertically distributed plant root water uptake approximation based on traditional water-potential-gradient (WPG) formulation was adopted and implemented in a one-dimensional dual-continuum model of soil water flow based on Richards’ equation. This combined model was used to simulate soil water movement at a forested site. The results were compared with observations (sap flow, soil water pressure, and soil water content) as well as with results of simulation produced using the standard semi-empirical model of Feddes. Principal aspects of the WPG prediction, such as root-mediated soil water redistribution, compensation for local water scarcity, and transpiration reduction, are exposed and discussed.
[18] Gerke H.H., Dusek J., Vogel T. (2013), Solute mass transfer effects in 2D dual-permeability modeling of bromide leaching from a tile-drained field, Vadose Zone Journal, 12, doi: 10.2136/vzj2012.0091
Abstract
Preferential flow (PF) depends on processes and structures in soil at the small scale and can affect flow and transport processes at much larger scales. For studying PF processes, the discharge and solute effluent from subsurface drained experimental fields has frequently been used as a field-integrated signal that included combined effects of macropore flow and lateral transport towards the drain. The objective of this study was to better understand effects of the mass transfer coefficients on bromide (Br) leaching in a 2D dual-permeability concept. The Br leaching was simulated for data of a Br tracer irrigation experiment on a drained field (5000 m2 area) at Bokhorst (Germany), where soils developed from glacial till sediments. Flow and transport in 2D vertical cross-sections was described using a numerical 2D dual-permeability model. For applied Br, influx of Br only in the soil matrix (SM) domain, only in the preferential flow (PF) domain, and proportional to the water influx in both domains was considered to assess the impact of small-scale redistribution processes occurring at the structured soil surface on field and plot scale transport. Three values of the water and four of the solute mass transfer rate coefficients were tested to imitate local effects (e.g., of clay-organic coatings) on the inter-domain mass transfer. The local scale solute mass transfer between the PF and SM domains had a clear impact on Br concentrations in drain effluent at the field-scale; concentrations mainly increased more rapidly with smaller values of the diffusive mass transfer coefficient. In the 2D flow domain, representing the plot scale, mass transfer rates were temporally and spatially variable with varying importance of diffusive and advective components depending on the influx rates; local effects were still significant at the field-scale. Small-scale properties and processes such as domain-specific infiltration and mass transfer in structured soil seem important for improving the description of larger scale flow and transport processes.
[17] Dohnal M., Jelinkova V., Snehota M., Dusek J., Brezina J. (2013), Three-dimensional numerical analysis of water flow affected by entrapped air: Application of non-invasive imaging techniques, Vadose Zone Journal, 12, doi: 10.2136/vzj2012.0078
Abstract
Recurrent ponded infiltration experiments on undisturbed samples of coarse sandy loam reveal a significant flow instability characterized by a decrease of the steady state flow rate of the second infiltration run, conducted into wet soil, as compared to the first infiltration run, conducted into drier soil. It has been hypothesized that this decrease is caused by air entrapment during the second run with subsequent blocking of the preferential pathways. In the present study, entrapped air distribution and its impact on water flow was studied through a novel combination of magnetic resonance (MR) imaging and numerical modeling. An undisturbed sample of coarse sandy loam was subject to the recurrent ponded infiltration while being monitored by MR. Internal structure of the soil sample was visualized by X-ray computed tomography (CT). A parallel version of three-dimensional (3D) water flow model based on Richards’ equation was employed to simulate the water flow through the heterogeneous soil sample. Information from the CT was used to describe internal heterogeneity of the soil sample via scaling factors. MR relaxometry imaging data were utilized to derive 3D maps of air entrapped. These were implemented into simulation of the second infiltration run as regions of no flow. MR images were employed to assess distribution of water content within the sample. The 3D model was able to describe measured outflow rates and pressure heads and also to reproduce the heterogeneous distribution of water content within the sample. The results obtained support the assumption that the observed decrease of the outflow rate could be caused by entrapped air in large pores of the soil sample.
[16] Dusek J., Vogel T., Sanda M. (2012), Hillslope hydrograph analysis using synthetic and natural oxygen-18 signatures, Journal of Hydrology, 475, 415–427, doi: 10.1016/j.jhydrol.2012.10.025
Abstract
Shallow subsurface runoff is one of the most important mechanisms determining hydrological responses of headwater catchments to rainstorms. In this study, a simplified approach combining one-dimensional dual-continuum vertical flow in a variably saturated soil profile and one-dimensional saturated flow along the soil-bedrock interface was used to study rainfall-runoff events at an experimental hillslope. A dual set of Richards’ equations was used to predict vertical flow of water in the soil matrix and preferential pathways. Subsurface flow along the soil-bedrock interface was described by diffusion wave (Boussinesq-type) equation. The observed subsurface runoff and its 18O composition were compared with the model predictions. Contributions of pre-event and event water to hillslope runoff during major rainfall-runoff episodes were evaluated by means of numerical experiments involving synthetic 18O rainfall signatures. Although preferential flow played an important role in the hillslope runoff formation, pre-event water was found to be significant runoff component in most events (it formed 47-74% of total subsurface runoff). The simulation results confirmed the hypothesis of significant mixing between infiltrating rainwater and water stored in the hillslope soil profile. The modeling approach presented in this study was successful in describing both vertical and lateral mixing of water.
[15] Dusek J., Vogel T., Dohnal M., Gerke H.H. (2012), Combining dual-continuum approach with diffusion wave model to include a preferential flow component in hillslope scale modeling of shallow subsurface runoff, Advances in Water Resources, 44, 113–125, doi: 10.1016/j.advwatres.2012.05.006
Abstract
In the absence of overland flow, shallow subsurface runoff is one of the most important mechanisms determining hydrological responses of headwater catchments to rainstorms. Subsurface runoff can be triggered by preferential flow of infiltrating water frequently occurring in heterogeneous and structured soils as a basically one-dimensional (1D) vertical process. Any attempt to include effects of preferential flow in hydrological hillslope studies is limited by the fact that the thickness of the permeable soil is mostly small compared to the length of the hillslope. The objective of this study is to describe preferential flow effects on hillslope-scale subsurface runoff by combining a 1D vertical dual-continuum approach with a 1D lateral flow equation. The 1D vertical flow of water in a variably saturated soil is described by a coupled set of Richards’ equations and the 1D saturated lateral flow of water on less permeable bedrock by the diffusion wave equation. The numerical solution of the combined model was used to study rainfall-runoff events on the Tomsovska hillslope by comparing simulated runoff with observed trench discharge data. The dual-continuum model generated the observed rapid runoff response, which served as an input for the lateral flow model. The diffusion wave model parameters (i.e., length of the contributing hillslope, effective porosity, and effective hydraulic conductivity) indicate that the hillslope length that contributed to subsurface drainage is relatively short (in the range of 25-50 m). Significant transformation of the 1D vertical inflow signal by lateral flow is expected for longer hillslopes, smaller effective conductivities, and larger effective porosities. The physically-based combined modeling approach allows for a consistent description of both preferential flow in a 1D vertical soil profile and lateral subsurface hillslope flow in the simplest way.
[14] Dusek J., Dohnal M., Vogel T., Ray C. (2011), Field leaching of pesticides at five test sites in Hawaii: Modeling flow and transport, Pest Management Science, 67, 1571–1582, doi: 10.1002/ps.2217
Abstract
BACKGROUND: Physically-based tier-II models may serve as possible alternatives to expensive field and laboratory leaching experiments required for pesticide approval and registration. The objective of this study was to predict pesticide fate and transport at five different sites in Hawaii using data from an earlier field leaching experiment and a one-dimensional tier-II model. Since the predicted concentration profiles of pesticides did not provide close agreement with data, inverse modeling was used to obtain adequate reactive transport parameters. The estimated transport parameters of pesticides were also utilized in a tier-I model, which is currently used by the state authorities to evaluate the relative leaching potential. RESULTS: Water flow in soil profiles was simulated by the tier-II model with acceptable accuracy at all experimental sites. The observed concentration profiles and center of mass depths predicted by the tier-II simulations based on optimized transport parameters provided better agreements than did the non-optimized parameters. With optimized parameters, the tier-I model also delivered results consistent with observed pesticide center of mass depths. CONCLUSION: Tier-II numerical modeling helped to identify relevant transport processes in field leaching of pesticides. The process-based modeling of water flow and pesticide transport coupled with inverse procedure can contribute significantly to the evaluation of chemical leaching in Hawaii soils.
[13] Dusek J., Ray C., Alavi G., Vogel T., Sanda M. (2010), Effect of plastic mulch on water flow and herbicide transport in soil cultivated with pineapple crop: A modeling study, Agricultural Water Management, 97, 1637–1645, doi: 10.1016/j.agwat.2010.05.019
Abstract
In Hawaii, pineapple is typically grown in raised beds covered with impervious plastic mulch. Field measurements of a commonly used herbicide (bromacil) mass beneath mulch-covered pineapple beds and inter-bed open areas revealed that open areas contained a mass of bromacil about 3.5 times greater than was originally applied, based on label instructions, to the entire field. The broadcast bromacil ended up in the inter-bed open areas through water runoff from the plastic mulch covering the pineapple beds. The objective of this study was to evaluate the impact of surficial management on water dynamics and bromacil concentration in the soil on a pineapple plantation using the one- (1D) and two-dimensional (2D) flow and transport models. Flow and transport processes were simulated in a 2D vertical cross-section perpendicular to the plant rows. The 1D simulation was limited to the open inter-bed areas. Several simulation scenarios were proposed to evaluate the effect of plastic mulch on bromacil transport in soil. In our simplified approach, the water and solute boundary fluxes for the non-covered areas were increased to simulate the water and solute contribution from the plastic mulch surface. The simulation results were compared with field observations of soil water potentials and resident bromacil concentration profiles. The field and laboratory-measured hydraulic and transport parameters were used for all simulation scenarios. Reasonably good agreement between the model-predicted and observed soil water potentials and bromacil concentration profiles was obtained. Biased 1D and 2D results were predicted when the water runoff from plastic mulch was neglected. The 1D approach to quantify bromacil transport beneath the inter-bed open areas seemed to be sufficient in case the water runoff from the mulch was taken into account.
[12] Vogel T., Brezina J., Dohnal M., Dusek J. (2010), Physical and numerical coupling in dual-continuum modeling of preferential flow, Vadose Zone Journal, 9, 260–267, doi: 10.2136/vzj2009.0091
Abstract
Dual-continuum models are useful for describing flow in porous systems with significant local pressure disequilibrium between slow moving water, contained in the porous matrix, and fast moving water in preferential pathways. The formation and intensity of preferential flow depends on the contrast between hydraulic properties of the two flow domains as well as on the properties of their interface. In the present study, we focus on both physical coupling of the flow domains through the mass transfer term and numerical coupling of the respective governing equations. The set of governing equations was alternatively solved using sequentially coupled approach (SC) and fully coupled approach (FC). The SC approach is shown to be computationally more efficient for strongly developed preferential flow in systems with high interfacial resistance. However, it becomes numerically unstable for weak preferential flow associated with low interfacial resistance. The FC approach represents computationally more expensive yet numerically more robust alternative, capable of simulating a complete class of intermediate flow regimes ranging from strongly preferential flow in a dual-continuum system to non-preferential flow in a single-continuum system. To illustrate the performance of the numerical coupling approaches in conjunction with the effect of different interfacial resistances, we present a simple example problem involving one dimensional near-saturated flow in a vertical soil column.
[11] Vogel T., Sanda M., Dusek J., Dohnal M., Votrubova J. (2010), Using oxygen-18 to study the role of preferential flow in the formation of hillslope runoff, Vadose Zone Journal, 9, 252–259, doi: 10.2136/vzj2009.0066
Abstract
Soil water dynamics at an experimental hillslope site is studied by means of one-dimensional dual-continuum model. The model is based on Richards’ equation for vertical soil water flow and the advection-dispersion equation for transport of stable oxygen isotope 18O. The water body contained in the soil matrix pore space and the one transmitted through the system of preferential pathways are treated as two separate, mutually communicating soil water continua. The 18O isotope, monitored in precipitation, subsurface hillslope discharge, and soil water, is used as a natural tracer to study the role of preferential flow in the formation of shallow subsurface runoff. It is shown that the dual-continuum approach can, in principle, explain the observed variations of 18O content in the subsurface hillslope discharge. The model successfully describes mixing of new water, which reflects the isotope signatures of the individual precipitation events, with old water, reflecting the seasonal variability of the isotope signal.
[10] Dohnal M., Dusek J., Vogel T. (2010), Improving hydraulic conductivity estimates from minidisk infiltrometer measurements for soils with wide pore-size distributions, Soil Science Society of America Journal, 74, 804–811, doi: 10.2136/sssaj2009.0099
Abstract
Disk infiltrometers are established as standard devices for measuring soil surface hydraulic properties. This study explored the validity of a semiempirical approach that is used to obtain estimates of the near-saturated hydraulic conductivity from disk infiltrometer data. The approach was compared with two other estimation expressions. The analysis was based on three-dimensional numerical modeling of the infiltration process, i.e., on synthetic data. The results of the validation procedure showed that the original expression performed best among the compared methods, but still failed for fine-textured soils and the selected Cambisols. This is due to the overwhelming importance of lateral soil water movement by capillarity, which is not adequately addressed by any of the models. The study showed that improved estimates, specifically for fine-textured soils and Cambisols and for small infiltrometer radii (minidisks), can be obtained by extending the original approach. This is achieved primarily (i) by using the modified van Genuchten parameterization of soil hydraulic functions instead of the original one, and (ii) by including a more representative set of soils in the objective function when optimizing the estimation formula.
[9] Dusek J., Sanda M., Loo B., Ray C. (2010), Field leaching of pesticides at five test sites in Hawaii: Study description and results, Pest Management Science, 66, 596–611, doi: 10.1002/ps.1914
Abstract
BACKGROUND: Following the discovery of pesticides in wells, the Hawaii Department of Agriculture (HDOA) supported research to evaluate the likelihood of pesticide leaching to the groundwater in Hawaii. The aim of this study was to evaluate the relative leaching pattern of five pesticides at five different sites on three islands and to compare their leaching behavior with bromide and a reference chemical (atrazine) that is known to leach in Hawaiian conditions. Laboratory measurements of sorption and degradation of the pesticides were made. RESULTS: Most of the applied mass of pesticides was still present in the top 80 cm after the sixteen week study period. The aggregated oxisol at Kunia showed the most intensive leaching among the five sites. The revised attenuation factor screening approach used by the HDOA indicated that all chemicals, with the exception of trifloxystrobin, had the potential to leach. Similarly, the ground-water ubiquity score ranked trifloxystrobin as a non-leacher. The field leaching data, however, suggested that trifloxystrobin was the most mobile compound among the pesticides tested. CONCLUSION: Although the results were variable among the sites, the field and laboratory experiments provided useful information for regulating use of these pesticides in Hawaii.
[8] Dusek J., Vogel T., Lichner L., Cipakova A. (2010), Short-term transport of cadmium during a heavy-rain event simulated by a dual-continuum approach, Journal of Plant Nutrition and Soil Science, 173, 536–547, doi: 10.1002/jpln.200800281
Abstract
The transport of solutes in soils, and its intensification due to preferential flow, plays crucial role when problems related to the groundwater pollution are dealt with. The objective of this study was to examine transport of cadmium (Cd) in response to an extreme rainfall event for three different soils using numerical modeling. The 115mCd2+ concentration profile had been measured in the Bodiky reference site (Danubian Lowland, Slovakia) by the radioactive-tracer technique, and used for the calibration of the dual-continuum model S1D. The Cd transport during a single rain event was predicted with the S1D model for light, medium-heavy and heavy soil in the same region. The cadmium transport through the soil profile was simulated by the one-dimensional dual-permeability model, which assumes the existence of two pore domains: the soil-matrix domain and the preferential-flow domain. The model is based on Richards’ equation for water flow and advection-dispersion equation for solute transport. A modified batch technique enables to distinguish process of adsorption in the matrix domain and the preferential pathways. Modeling with classical single-permeability approach and dual-continuum approach without considering the particle-facilitated transport led to negligible Cd penetration. The rainfall event with extremely high rainfall intensity induced deep penetration of cadmium in the medium-heavy and heavy soil, which may indicate increased vulnerability to shallow groundwater pollution for the respective sites in Danubian Lowland region. The highest cadmium leaching was predicted for heavy clay soil, where the preferential particle-facilitated transport of cadmium through the soil profile was significant due to the contrasting properties of the soil-matrix domain and the preferential-flow domain. The results of the sensitivity analysis suggested only slight effect of the transfer rate coefficients on simulated Cd leaching.
[7] Dohnal M., Dusek J., Vogel T., Cislerova M., Lichner L., Stekauerova V. (2009), Ponded infiltration into soil with biopores - field experiment and modeling, Biologia, 64, 580–584, doi: 10.2478/s11756-009-0078-7
Abstract
Preferential movement of water in macropores plays an important role when the process of ponded infiltration in natural porous systems is studied. For example, the detailed knowledge of water flow through macropores is of a major importance when predicting runoff responses to rainfall events. The main objectives of this study are to detect preferential movement of water in Chernozem soil and to employ numerical modeling to describe the variably saturated flow during a field ponded infiltration experiment. The infiltration experiment was performed at the Macov experimental station (Calcari-Haplic Chernozem in Danubian Lowland, Slovakia). The experiment involved single ring ponded infiltration. At the quasi steady state phase of the experiment dye tracer was added to the infiltrating water. Then the soil profile was excavated and the penetration pattern of the applied tracer was recorded. The abundance of biopores as a product of fauna and flora was found. To quantify the preferential flow effects during the infiltration experiment, three-dimensional axisymmetric simulations were carried out by a two-dimensional dual-continuum numerical model. The water flow simulations based on measured hydraulic characteristics without consideration of preferential flow effects failed to describe the infiltration experiment adequately. The 3D axisymmetric simulation based on dual-permeability approach provided relatively realistic space-time distribution of soil water pressure below the infiltration ring.
[6] Dusek J., Gerke H.H., Vogel T. (2008), Surface boundary conditions in 2D dual-permeability modeling of tile drain bromide leaching, Vadose Zone Journal, 7, 1241–1255, doi: 10.2136/vzj2007.0175
Abstract
The simulation of preferential flow in structured soil using dual-porosity models requires separate sets of the hydraulic, transport, and mass transfer parameters and of the boundary conditions. Analyses of tracer experiments with two-domain models are limited by constraints in specifying separate boundary conditions (BCs) for each pore domain. The appropriate boundary conditions for dual-permeability models have not been systematically studied or addressed in experiments. The objective of this study was to numerically evaluate the effects on Br leaching from a tile-drained field of different surface boundary conditions for a two-dimensional dual-permeability model. For the previously described Br tracer experiment at the Bokhorst site (Germany), effects of irrigation intensities, flux-and resident-type solute BCs, and Br application domain on drain discharge and Br effluent concentrations were compared. The two-dimensional dual-permeability numerical model divides the soil into soil matrix (SM) and preferential flow (PF) domains. In case of ponding at the surface of the SM domain, water is redistributed toward the PF domain surface. The combination of detailed irrigation record, flux-type solute BCs, and solute application to the PF domain resulted in the largest Br leaching. In contrast, the lowest Br leaching was predicted for averaged irrigation rates, flux-type solute BCs and Br addition to the SM domain. For flux-type (third-type) solute BC application to both pore domains, enhanced Br leaching was obtained due to surface redistribution effects. Detailed irrigation patterns with realistic intensities predicted higher Br leaching than averaged intensities. In addition to surface effects, the temporal availability of Br in the preferential pathways seemed to control Br leaching patterns. The results suggest that the formulation of the upper BCs strongly affects two-dimensional dual-permeability Br leaching predictions. Proper experimental consideration of domain-specific BCs may help improve descriptions of preferential flow.
[5] Alavi G., Dusek J., Vogel T., Green R.E., Ray C. (2007), Evaluation of dual-permeability models for chemical leaching assessment to assist pesticide regulation in Hawaii, Vadose Zone Journal, 6, 735–745, doi: 10.2136/vzj2006.0139
Abstract
Groundwater is the primary source of drinking water for all the islands of Hawaii. Past agricultural practices have led to the contamination of groundwater in certain locations. As a result, the state of Hawaii emphasizes the prevention of contamination of groundwater from the leaching of pesticides. Hawaii currently uses a simple ( Tier I) screening assessment model to evaluate the leaching potential of pesticides. This model is only capable of indicating if a chemical is likely to leach; it can estimate neither the concentration profile in soil nor the concentration in leachate water. The USEPA is seeking partnership with the state of Hawaii for examining the feasibility of using Tier II models in Hawaii conditions for pesticide registration. Two pesticide leaching models, MACRO 4.3 and S1D DUAL, were tested using leaching data for five pesticides from a field site on the island of Oahu. Despite deficiencies, it is one of the best data sets currently available for tropical soils. Both MACRO 4.3 and S1D DUAL models explicitly include preferential flow components but use different concepts in model formulations. The performances of the two models were generally similar. The results show that preferential flow had a minor role in transporting the chemicals compared with micropore flow because of the high saturated conductivity of micropores (matrix). We conclude that a process-based model will contribute substantially to the evaluation of chemical leaching risk and complement the Tier I model that currently is used for pesticide registration in Hawaii.
[4] Gerke H.H., Dusek J., Vogel T., Kohne J.M. (2007), Two-dimensional dual-permeability analyses of a bromide tracer experiment on a tile-drained field, Vadose Zone Journal, 6, 651–667, doi: 10.2136/vzj2007.0033
Abstract
Preferential flow has been hypothesized as an important factor for chemical leaching from tile-drained agricultural fields with structured soils originating from glacial till sediments. Previous studies showed that one-dimensional single-porosity models (1D-SPM) failed and that one-dimensional dual-permeability models (1D-DPERM) were limited in explaining both Br leaching and residual Br distribution, although tile water outflow peaks could somehow be reproduced. The objective of this paper was to analyze the tile outflow and leaching patterns using a two-dimensional (2D)-DPERM and a standard 2D-SPM for comparison. Flow and transport were simulated in a 2D vertical cross-section of 5.9 m length and 2 m depth using previously tested parameters. Simulated drainage rates and Br-effluent concentrations were made comparable with collector data from a field experiment by weighing results for irrigated and nonirrigated plots according to their area fractions. The 2D-DPERM simulations for surface application of Br in dissolved form in both domains overestimated the observed initial outflow concentration peaks, in contrast to closer approximation of observations assuming Br application in the soil matrix domain only. The simulated 2D mass transfer rate distribution showed most intensive exchange between domains near the water table and in the topsoil. Results from the 2D-DPERM analyses suggest that conditions at the soil surface, near the water table, and of the field-scale mixing are significantly affecting leaching patterns, in addition to local nonequilibrium effects. Here, the description of preferential flow toward tile drain could be strongly improved with the 2D-DPERM compared with the 2D-SPM. Further improvements remain challenging with respect to DPERM numerical modeling and field experimentation, with special attention toward soil structure and soil surface conditions.
[3] Vogel T., Lichner L., Dusek J., Cipakova A. (2007), Dual-continuum analysis of a cadmium tracer field experiment, Journal of Contaminant Hydrology, 92, 50–65, doi: 10.1016/j.jconhyd.2007.01.001
Abstract
Cadmium penetration into a sandy-loam soil during the field ponded infiltration at Kralovska luka site in Southern Slovakia was observed in a controlled experiment. Adsorption of cadmium was examined using the radioactive tracer techniques in laboratory batch tests. Quite deep Cd-115m penetration during the field experiment (65 cm beneath the soil surface) gives evidence of the particle-facilitated transport of cadmium through preferential pathways. More than 40% of the applied cadmium moved deeper than 10 cm. The field experiment was analyzed using both conventional single-continuum approach (based on Richards' equation for water flow and advection-clispersion equation for Cd transport) and dual-continuum approach. The latter approach allowed us to simulate more adequately the observed movement of cadmium.
Abstract
Numerical modelling is used to analyze the transport of cadmium in response to an extreme rainfall event. The cadmium transport through the soil profile was simulated by the one-dimensional dual-permeability model, which assumes the existence of two mutually communicating domains: the soil matrix domain and the preferential flow domain. The model is based on Richards' equation for water flow and advection-dispersion equation for solute transport. A modified batch technique allowed us to consider domain specific sorption, i.e. each of the domains has its own distribution coefficient. The dual-permeability model predicts that the cadmium can be transported substantially below the root zone after the storm. On the other hand, classical single permeability approach predicted that almost all applied cadmium stays retained near the soil surface.
[1] Ray C., Vogel T., Dusek J. (2004), Modeling depth-variant and domain-specific sorption and biodegradation in dual-permeability media, Journal of Contaminant Hydrology, 70, 63–87, doi: 10.1016/j.jconhyd.2003.08.009
Abstract
A dual-permeability model (S-ID-DUAL) was developed to simulate the transport of land-applied pesticides in macroporous media. In this model, one flow domain was represented by the bulk matrix and the other by the preferential flow domain (PFD) where water and chemicals move at faster rates. The model assumed the validity of Darcian flow and the advective-dispersive solute transport in each of the two domains with inter-domain transfer of water and solutes due to pressure and concentration gradients. It was conceptualized that sorption and biodegradation rates vary with soil depth as well as in each of the two flow domains. In addition to equilibrium sorption, kinetic sorption was simulated in the PFD. Simulations were conducted to evaluate the combined effects of preferential flow, depth- and domain-variant sorption, and degradation on leaching of two pesticides: one with strong sorption potential (trifluralin) and the other with weak sorption potential (atrazine). Simulation results for a test case showed that water flux in the PFD was three times more than in the matrix for selected storm events. When equilibrium sorption was considered, the simulated profile of trifluralin in each domain was similar; however, the atrazine profile was deeper in the PFD than in the bulk matrix under episodic storm events. With an assumption of negligible sorption in the PFD, both the atrazine and the trifluralin profiles moved twice deeper into the PFD. The simulated concentrations of the chemicals were several orders higher in the PFD than in the matrix, even at deeper depths. The volume fraction of the macropores and the sorption and biodegradation properties of the chemicals could also affect the amount of pesticides leaving the root zone. For an intense storm event, slow sorption reaction rates in the PFD produced higher breakthrough concentrations of atrazine at the bottom of the simulated soil profile, thus posing the risk for breakthrough of chemicals from the root zone.