A software package to calibrate preferential flow models

ERT and salinity – A method to determine whether ERT-detected preferential pathways in brackish water-irrigated soils are water-induced or an artifact of salinity

Abstract. Subsurface flow in preferential pathways in soils may transport water more rapidly than the soil matrix, may be quickly activated during precipitation events and enhance infiltration or interflow. Vertical pathways are particularly important for runoff generation. However, identifying these pathways is challenging because traditional methods such as piezometers, soil moisture sensors, or hillslope trenches do not adequately capture the spatial scale and frequency of preferential flow features, while other experimental techniques like dye tracing are labor-intensive and invasive. In this study, we introduce a novel method to identify the locations of preferential flow by analysing vertical soil profiles of stable water isotopes. Across four catchments, we drilled 100 soil cores (1–3 m deep) per catchment and analyzed the stable isotope composition of the soil water in 10–20 cm depth intervals to construct depth profiles. We employed clustering techniques to group soil-water isotope profiles and selected those that matched to a seasonal sampling date to establish a reference profile for each catchment using LOESS regression, representing profiles influenced solely by matrix infiltration. Deviations from these reference profiles were then used as indicators of being influenced by vertical or lateral preferential flow. Our results revealed evidence of preferential flow in all studied catchments. Especially in the alpine catchment with highly heterogeneous soils, many profiles showed distinct preferential flow features, including multiple, vertically independent pathways occurring at variable depths, even among adjacent profiles. These findings demonstrate the feasibility of using soil water isotope profiles to assess preferential flow pathways and highlight the substantial spatial and vertical variability of preferential flowpaths at hillslope and catchment scales.

<p>Research over the past several decades has shown that preferential flow is more the rule than the exception. However, our collective understanding of preferential flow processes has been limited by a lack of suitable methods to detect and visualize the initiation and evolution of non-uniform wetting at high spatial and temporal resolutions, particularly in real-world settings. In this study, we investigate water infiltration initiation by tree trunk and root systems. We carried out time-lapse ground penetrating radar (GPR) surveys in conjunction with a simulated stemflow event to provide evidence of root-induced preferential flow and generate a three-dimensional representation of the wetted zone.</p><p>We established a survey grid (3.5 m × 5 m, with a local slope of 10.3°), consisting of ten horizontal and thirteen vertical parallel survey lines with 0.5 m intervals between them. The horizontal lines were downslope-oriented. The grid was placed around a Quercus suber L. We collected a total of 46 (2 GPR surveys × 23 survey lines) radargrams using an IDS (Ingegneria Dei Sistemi S.p.A.) Ris Hi Mod v. 1.0 system with a 900-MHz antenna mounted on a GPR cart. Two grid GPR surveys were carried out before and after the artificial stemflow experiment. In the experiment, we applied 100 L of brilliant blue dye (E133) solution on the tree trunk. The stemflow volume of 100 L corresponded to 63.2 mm of incident precipitation, considering a crown projected area of 201 m<sup>2</sup> and a 1.3% conversion rate of rainfall to stemflow. Trench profiles were carefully excavated with hand tools to remove soil and detect both root location and size and areas of infiltration and preferential pathways on the soil profile.</p><p>The majority (84.4%) of artificially applied stemflow infiltrated into the soil, while the remaining 15.6% generated overland flow, which was collected by a small v-shaped plastic channel placed into a groove previously scraped on the downhill side of the tree. The 3D diagram clearly demarcated the dimension and shape of the wetted zone, thus providing evidence of root-induced preferential flow along coarse roots. The wetted zone extended downslope up to a horizontal distance of 3 m from the trunk and down to a depth of approximately 0.7 m. Put all together, this study shows the importance of accounting for plant and trees trunk and root systems when quantifying infiltration.</p>

Soils derived from different lithologies and their controls on preferential flow remain underexplored in forested landscapes. In the same lithology, the propensity for preferential flow occurrence at different hillslope positions also remains largely elusive. By utilizing a soil moisture response time method, we compared preferential flow occurrence between a shale site (Shale Hills, silt loam soils) and a sandstone site (Garner Run, sandy loam soils) at four hillslope positions: ridge‐top, North‐ and South‐facing mid‐slopes and toe slope, for over 2 years. The catchments are neighbouring and covered by temperate forest. For the four hillslope positions, Shale Hills had higher preferential flow frequencies compared to Garner Run. Between these two catchments, the South‐facing mid‐slope sites showed the highest contrasts in preferential flow frequency (33.5% of events at Shale Hills vs. 8.8% at Garner Run) while the ridge‐top sites showed the lowest contrasts (18.7 vs. 13.2%). Additionally, over the unfrozen period, for seven out of eight monitoring sites, drier antecedent conditions tended to be more favourable for preferential flows to occur, with significant (p < .01) relationships at two sites. Except for the South‐facing mid‐slope sites, both Shale Hills and Garner Run had two preferential flow pathways. The characteristic preferential flow pathways at Shale Hills were the Bwand C horizons, and for Garner Run, preferential flow moved from the E/AE horizon to the Bwhorizon. This study shows that shale‐derived soils tended to have higher preferential flow occurrence than sandstone soils, but hillslope positions exhibit different levels of contrasts. More effort should be paid to study the impact of lithology on preferential flows in the context of land surface modelling and biogeochemical reactions to improve ecosystem services of headwater catchments.

Core Ideas In the 20th century there was a general denial of the evidence for preferential flow. The development of models of preferential flow is reviewed. Some critical areas for future research in this field are identified. This review provides a historical summary of the development of knowledge on preferential and non‐equilibrium flows in soils, particularly in the period 1864 to 1984. It is pointed out that preferential flows were recognized long before the equilibrium concepts of Edgar Buckingham and Lorenzo A. Richards were developed and became the dominant underpinnings of soil physics in the 20th century, effectively in denial of all the evidence that the Buckingham–Richards theory was inadequate to deal with water flows in field soils. Summaries of evidence from soil microscopy, tracing and breakthrough curve experiments, infiltration and throughflow observations, and studies of natural pipes are presented. Approaches to modeling preferential flows at soil profile and hillslope responses are also reviewed, including viscous flow theory and particle tracking methods. Further research is required into the integration of viscosity and capillarity‐dominated flows, into nonlaminar flows in larger pores, and into upscaling from core and profile experiments to larger hillslope scales. Also, more rigorous hypothesis testing is required in soil physics and hillslope hydrology using both flow and tracer data. This might lead to a new paradigm in representing the detail complexity of flow in soils in applications at larger scales.

Structural pores associated with macropore‐type preferential flow pathways can accelerate chemical transport in unsaturated soils, thereby potentially causing groundwater contamination. To predict chemical transport through these pathways, classical deterministic models depend on soil hydraulic conductivity, which effectively lumps flow contributions from all individual pathways. We contend, however, that quantifying the pore spectrum of preferential pathways, without lumping the contributions of individual pores, is the appropriate method for simulating convective chemical transport through macropore‐type preferential pathways. In this study, we conducted field‐scale experiments by using an improved tile drain monitoring protocol to measure the mass flux breakthrough patterns of conservative tracers. The tails of these patterns suggested that the impact of preferential pathways on contaminant transport can be conceptualized as that occurring through cylindrical capillary tubes. We then proposed a distribution function bracketed by sharp cut‐off points to represent the pore spectrum of these tubes. Finally, we used the measured tracer breakthrough curves (BTCs) as data sources to find the parameters of the proposed function. Our results, based on the best fitting, showed that the preferential pathways are naturally clustered into domains; preferential pathways with a wide range of pore radii could become active simultaneously when infiltration rate increases. Because the derived pore spectra simultaneously satisfy both water movement and solute transport, pore spectra can be used to (i) calculate soil hydraulic conductivity of preferential pathways in deterministic approaches, and (ii) construct multiple probability density functions (PDFs) for the transfer function approach, to accommodate different infiltration patterns.

Data assimilation is predominantly used for state estimation, combining observational data with model predictions to produce an updated model state that most accurately approximates the true system state whilst keeping the model parameters fixed. This updated model state is then used to initiate the next model forecast. Even with perfect initial data, inaccurate model parameters will lead to the growth of prediction errors. To generate reliable forecasts, we need good estimates of both the current system state and the model parameters. This article presents research into data assimilation methods for morphodynamic model state and parameter estimation. First, we focus on state estimation and describe implementation of a three‐dimensional variational (3D‐Var) data assimilation scheme in a simple 2D morphodynamic model of Morecambe Bay, UK. The assimilation of observations of bathymetry derived from synthetic aperture radar (SAR) satellite imagery and a ship‐borne survey is shown to significantly improve the predictive capability of the model over a 2‐year run. Here, the model parameters are set by manual calibration; this is laborious and is found to produce different parameter values depending on the type and coverage of the validation dataset. The second part of this article considers the problem of model parameter estimation in more detail. We explain how, by employing the technique of state augmentation, it is possible to use data assimilation to estimate uncertain model parameters concurrently with the model state. This approach removes inefficiencies associated with manual calibration and enables more effective use of observational data. We outline the development of a novel hybrid sequential 3D‐Var data assimilation algorithm for joint state‐parameter estimation and demonstrate its efficacy using an idealised 1D sediment transport model. The results of this study are extremely positive and suggest that there is great potential for the use of data assimilation‐based state‐parameter estimation in coastal morphodynamic modelling. Copyright © 2012 Royal Meteorological Society

<p>Preferential flow pathways (PFPs) are key contributors for the ecological status of the hydrosphere in high mountain environments, as the precipitation will transfer to PFPs with rapid solute transport from soil to groundwater. This particularly refers to nutrient allocation from soils to groundwater and surface waters.</p><p>To understand the effects of the pedogenesis and forest types on the soil PFPs, the soil preferential flow was studied by <em>in situ</em> dye tracing image analysis and elemental chemical analysis at the Hailuogou glacier chronosequence, Gongga Mountain on the eastern Tibetan Plateau. A soil chronosequence and a vegetation primary succession following the retreat of the Hailuogou glacier has been forming since ~1890. Three sites representing different exposure age (45, 85 and 125 years) in the Hailuogou glacier retreat area chronosequence and two sites typical forest lands (deciduous broadleaf forest and coniferous forest) were selected to carry out a brilliant blue dyeing experiment to visualize the distribution of water infiltration in soil.</p><p>The tracer-infiltration patterns were parameterized by dye coverage (DC), preferential flow fraction (PF), length index (L<sub>i</sub>) and the variation coefficient of DC in the PFPs (C<sub>V</sub>). Furthermore, the distribution of PFPs, transported solute of soil PFPs was analyzed including Hailuogou glacier chronosequence and vegetation succession.</p><p>According to the comparison of PFPs parameters, soil PFPs at the 125-year-old site extremely more developed than that at the younger site due to the fracture development between rock and soil on the process of soil development. The soil PFPs under broadleaf forest is more pronounced than that in coniferous forest soils, largely depending on the different root system.</p><p>In general, PFPs in Gongga Mountain were important contributors to the potential translocation of bioavailable inorganic P (PBPi) and organic P translocation to the hydrosphere. The elements transported with PFPs could be divided into three categories, reactive, conservative, and both reacted and conservative elements for the concentration of the elements remain in the PFPs. The results indicated that Mg and Al are the reactive elements, while Na, K, Ca and Mn are the conservative elements in the PFPs. Iron is both reacted and conservative element in the PFPs. Zn, Na, K, Mg, PBPi, had a significant correlation with the variation coefficient of DC in the PFPs (C<sub>V</sub>).</p><p>The results highlight the effects of the pedogenesis and forest types on the distribution of PFPs and solute transfer. Preferential flow contributes largely to elements flow at the Hailuogou glacier chronosequence and vegetation succession, Gongga Mountain.</p><p>The financial support of this work was obtained from National Natural Science Foundation of China (Grant No. 41272220) and Natural Scientifc Foundation for Young Scientists of Guangxi Zhuang Autonomous Region of China (Grant No. 2017GXNSFBA198162). The first author was financially supported by the Sino-German (CSC-DAAD) Postdoc Scholarship Program funded by China Scholarship Council (CSC) and Deutscher Akademischer Austausch Dienst (DAAD).</p>

A method is proposed to localize preferential fluid flow pathways in earthen embankments with time-lapse self-potential fluctuations associated with a salt tracer injection upstream. This method is first tested using laboratory data then demonstrated at a field site. In the laboratory, a network of non-polarizing electrodes is connected to a highly sensitive voltmeter to record the electrical field fluctuations occurring over time after a salt injection into a sand tank containing a preferential flow channel through a fine sand matrix. The electrical potential distribution is monitored at the surface of the tank to localize the pulse of saline water through the channel over time, and to determine the flow velocity. A finite element model allows us to reproduce the time-lapse electrical potential distribution over the channel, although some discrepancies were observed on the banks. This method applies in real time and can be used to map high permeability flow pathways in earthen embankments that may be spatially localized. We demonstrate the use of this method at a 12 m high, 100 m long earthen embankment by injecting a salt tracer at the crest and monitoring the self-potential and DC resistivity through time on the surface of the downstream slope. Our results clearly display a high permeability preferential flow path that was discovered in the center of embankment during a reconnaissance survey.

Water is a crucial factor controlling the fate and processing of soil organics. Water commonly flows through the vadose zone via preferential flow pathways, resulting in nonuniform and rapid infiltration. Hence, a large portion of the soil matrix is bypassed. Preferential flow paths, often associated with well-connected macropore networks (>300 μm Ø), offer a unique balance between water availability, nutrient delivery, and re-oxygenation upon drainage. The heightened concentrations of moisture, nutrients, and oxygen make these locations optimal for high rates of microbial activity. Flow paths often display temporal stability. This stability results in repeated wetting and biogeochemical reactivation through time creating a lasting impact on micro-environmental conditions relevant to microbial functioning and carbon cycling in soil. Despite decades of research on preferential flow, there is still a need to link flow paths and the resultant heterogeneous moisture distributions to soil function. In this review, we discuss how preferential flow can serve as a framework of reference for the spatially and temporally heterogeneous biogeochemical cycling of soil carbon. We highlight the importance of combining current knowledge of pore-scale carbon dynamics with an appreciation of connected networks of hydraulically active pores/paths within the soil profile. Such combination opens new possibilities for upscaling pore-scale processes with the inclusion of resource heterogeneity at the macroscale. Working within this hydraulically connected framework can provide insight for the mechanistic representation of hot moments, which are temporally isolated large pulses of CO2 after rewetting or thawing events. We conclude with suggestions on knowledge gaps and stress the critical need of linking soil physics with biology to mechanistically understand soil functions.

Significant quantities of agricultural chemicals can be transported rapidly through preferential flow pathways in soil. Time domain reflectometry (TDR) has been used to characterize solute transport in soil. However, previous TDR studies have not fully addressed the preferential flow of reactive solutes. A TDR method was tested for its ability to measure the preferential flow of nitrate and phosphate in soil. Saturated miscible displacement experiments were conducted using three undisturbed soil cores and a tracer solution containing chloride, phosphate, and nitrate. An inverse curve fitting method (CXTFIT) was used to estimate mobile-immobile model (MIM) parameters using the TDR and observed effluent data. The parameters fitted to the time-varying TDR-determined relative resident concentration were similar to the estimates from measured effluent chloride breakthrough curves (BTCs). Predicted BTCs were obtained from the parameters fitted to the TDR data. The predicted BTCs were comparable to the measured effluent nitrate BTCs, with a root mean square error (RMSE) of 0.0054. The peak times were 0.18, 0.16, and 0.12 pore volumes for the predicted BTCs and 0.26, 0.19, and 0.21 for the effluent nitrate BTCs. Phosphate BTCs differed distinctly from chloride and nitrate BTCs in our study; thus, the TDR method did not work for phosphate. However, the vertical TDR probe technique proved to be a practical method for a first approximation of nitrate preferential flow in soil.

Several counties in eastern West Virginia have extensive animal agriculture and generate considerable animal waste. Often these manures are surface applied to satisfy crop N demands. This leads to over application of P, and the environmental soundness of this practice is dependent on a soil's ability to sorb any excess P. However, preferential flow may bypass significant portions of the soil matrix and lead to a deeper distribution of P in the soil profile. To examine this phenomenon, three benchmark soil series common to pastures in West Virginia were selected for study. At multiple sites, FD&C Blue no. 1 dye was applied in a ponding fashion to identify active macropores. Then soil pits were excavated, and composite soil samples were collected from the stained and unstained matrix within each horizon. The soil test P (STP) levels of the samples were determined by Mehlich‐1 extractions in the stained and unstained soil within each horizon. In general, the STP levels declined with depth. However, the STP levels were significantly higher in the stained matrix samples (preferential flow pathways) than the surrounding unstained matrix samples. We identified no significant effect from soil series on the relationship between STP and preferential flow. Phosphorus sorption at the surface does not appear to prevent P movement along preferential flow pathways. Additional research with consideration given to P saturation status and detailed management histories is needed to adequately describe the potential risk of P loss to surface waterways.

Click to increase image sizeClick to decrease image size 2Paper given at the 3rd International Summer School on Problems of Model Choice and Parameter Estimation in Regression Analysis, Mühlhausen (G.D.R.), May 1977 Notes 2Paper given at the 3rd International Summer School on Problems of Model Choice and Parameter Estimation in Regression Analysis, Mühlhausen (G.D.R.), May 1977

Modeling of dual-permeability gas and solute reactive transport in macroporous agricultural soils with a focus on GHG cycling and emissions

Leaching risks of surface -applied agrichemicals in water repellent soils can only be quantified with an acceptable degree of accuracy if knowledge of the underlying principles and an appropriate simulation model are available. The present study aimed to investigate water flow and solute transport processes in a water repellent sandy soil, and to introduce and apply new modeling approaches. Automated TDR measurements revealed that preferential pathways develop rapidly during severe rain storms, causing infiltrating water to be preferentially transported to the deeper subsoil. Furthermore, preferred pathways recurred at the same sites during all rain events. Simulations with a 2-D, numerical finite element flow and transport model indicate that preferential flow paths will only form during infiltration into dry water repellent soils, i.e. in the range below the so-called critical soil water content. Incorporation of hysteresis is essential to generate the formation and recurrence of preferential flow paths with the model. The process of preferential flow and transport has been incorporated in the well-known SWAP model also, and applied to field data of tracer transport through a water repellent sandy soil in the Netherlands. Results indicate early arrival times of bromide in the subsoil in case preferential flow is taken into account.