Measurements Of Saturated Hydraulic Conductivity Research Articles

Designing and Evaluating the Quality and Cost-effectiveness of Saturated Sediment Permeameters

Many simulations require accurate measurements of saturated hydraulic conductivity, a sediment property that governs the speed at which water flows through sediments relative to head differences. The goal of our project is to design and build an inexpensive permeameter capable of producing accurate hydraulic conductivity values. We tested four permeameters; a standard research grade constant-head permeameter, a falling-head permeameter modeled off of an <em>in situ</em> stream method, a constant-head permeameter made out of 4” PVC pipe, and a similar constant-head permeameter made out of 2” PVC pipe. Our custom-built constant-head permeameters both utilized a U-shaped design, two tubes which form a manometer, and multiple output overflows. Despite significant differences in design, method, and cost, we found that all four of the permeameters yielded relatively consistent mean hydraulic conductivities with low standard deviations (0.004-0.019). We also compared the attributes: price, weight, and number of parts. Our conclusion is that because the average K-value and standard deviation of each design is within reason, the best choice depends on the practitioner’s situation and intention. <strong>Faculty Sponsor</strong>: Susa H. Stonedahl

ASSESSMENT OF GROUNDWATER RESPONSE AND SOIL MOISTURE FLUCTUATIONS IN THE MUGELLO BASIN (CENTRAL ITALY)

Extreme meteorological events such as heavy rainstorms are considered to increase due to global warming. The consequences of such events can be manifold, and might cause massive interferences of the hydrological system of a landscape. Particularly the intramontane basins of the Apennine in Italy are frequently threatened by extreme rainfall events that cause severe damage on buildings and infrastructure. Moreover, the lithological and geomorphological settings of these basins, which depict the products of a complex landscape history, amplify these threats. In order to develop possible mitigation strategies, it is crucial to assess landscape functioning by analysing hydrological processes of the landscape system. In this study, we conducted spatially distributed and dynamic hydrological modelling on a catchment in the intramontane basin of the Mugello valley in Tuscany, Italy. Foremost, measurements of saturated hydraulic conductivity and texture analyses were performed to estimate both infiltration and hydraulic conductivity of the surface and topsoil, respectively. We regionalised the collected data with a stochastic gradient treeboost method for the whole catchment. Soil depth was estimated with a simple sine-cosine-slope relation, whereas, hydropedologic parameters for the hydrological model were estimated with pedotransferfunctions applied on the collected infiltration data. We modelled a period of 100 days, representing each day per time step. A synthetic rainfall period was compiled based on measured data from meteorological stations within the Mugello basin. To produce a reliable synthetic rainfall data set, the estimated precipitation values were set in comparison to calculated return periods for extreme events of all available meteorological station. To assess the diversity of the hydrological response of several locations in the catchment, six semirandom test locations were located on hillslopes and spots were sedimentation is apparent. The results show that groundwater and soil moisture fluctuations appear to be significantly different for both hillslopes and areas were sediments are deposited. The differences cannot be explained by the topographical settings but rather by the approximated thickness of the weathered zone and the spatial diversity of the hydropedological properties of the soil.

Estimating Soil Saturated Hydraulic Conductivity using the Kostiakov and Philip Infiltration Equations

Core Ideas We analyzed the relationships between the parameters of the Kostiakov and Philip infiltration equations. A new method was proposed to estimate the Ks by parameters in Kostiakov equation. Effect of soil hydraulic parameters on a new method was discussed. Direct measurement of saturated hydraulic conductivity (Ks) is time and labor consuming, so Ks is often estimated by mathematical models describing soil–water infiltration. The Kostiakov infiltration equation is widely used to estimate soil–water infiltration rate (i) with only two empirical parameters m and n, which are highly dependent on Ks. The objective of this study was to develop a new method to estimate Ks using the parameters of the Kostiakov equation for short infiltration times. The new method is based on the relationship between the Kostiakov equation and Philip's two‐term equation. We used infiltration data of one‐dimensional vertical water absorption simulated by Hydrus‐1D to evaluate the applicability of the new method. The parameters of Philip's equation were applied to estimate the parameters of the Kostiakov equation with relative errors for m and n &lt;1.15 and 4.20%, respectively. The estimated Ks agreed closely with the parameters of the Kostiakov equation, and the optimal depth of the wetting front (zf) was used as an indicator to confirm the infiltration time in the one‐dimensional vertical water‐absorption simulation. The inverse of the air‐entry value, the saturated hydraulic conductivity, the pore‐size distribution index, and the initial water content had significant effects on zf, which could be described as a power function of these parameters, with an R2 of 0.9717. The parameters of the Kostiakov equation can thus be used to calculate Ks for most soils when the depth of the wetting front is approximately 20 ± 5 cm.

Prediction of biopore- and matrix-dominated flow from X-ray CT-derived macropore network characteristics

Abstract. Prediction and modeling of localized flow processes in macropores is of crucial importance for sustaining both soil and water quality. However, currently there are no reliable means to predict preferential flow due to its inherently large spatial variability. The aim of this study was to investigate the predictive performance of previously developed empirical models for both water and air flow and to explore the potential applicability of X-ray computed tomography (CT)-derived macropore network characteristics. For this purpose, 65 cylindrical soil columns (6 cm diameter and 3.5 cm height) were extracted from the topsoil (5 cm to 8.5 cm depth) in a 15 m × 15 m grid from an agricultural field located in Silstrup, Denmark. All soil columns were scanned with an industrial X-ray CT scanner (129 µm resolution) and later employed for measurement of saturated hydraulic conductivity, air permeability at −30 and −100 cm matric potential, and gas diffusivity at −30 and −100 cm matric potential. Distribution maps for saturated hydraulic conductivity, air permeability, and gas diffusivity reflected no autocorrelation irrespective of soil texture and organic matter content. Existing empirical predictive models for saturated hydraulic conductivity and air permeability showed poor performance, as they were not able to realistically capture macropore flow. The tested empirical model for gas diffusivity predicted measurements at −100 cm matric potential reasonably well, but failed at −30 cm matric potential, particularly for soil columns with biopore-dominated flow. X-ray CT-derived macroporosity matched the measured air-filled porosity at −30 cm matric potential well. Many of the CT-derived macropore network characteristics were strongly interrelated. Most of the macropore network characteristics were also significantly correlated with saturated hydraulic conductivity, air permeability, and gas diffusivity. The predictive Ahuja et al. (1984) model for saturated hydraulic conductivity, air permeability, and gas diffusivity performed reasonably well when parameterized with novel, X-ray CT-derived parameters such as effective percolating macroporosity for biopore-dominated flow and total macroporosity for matrix-dominated flow. The obtained results further indicate that it is crucially important to discern between matrix-dominated and biopore-dominated flow for accurate prediction of macropore flow from X-ray CT-derived macropore network characteristics.

Hydraulic properties of dune sand–bentonite mixtures of insulation barriers for hazardous waste facilities

This paper presents a study on the valorization of local materials such as desert dune sand obtained from Laghouat region in the South Algeria and mine bentonite intended for the realization of liner base layers in the conception of insulation barriers for hazardous waste facilities. In practice, an economical mixture satisfying the hydraulic requirements is generally concerned. First, in order to get an adequate dune sand–bentonite mixture compacted to the optimum Proctor condition, an investigation on saturated hydraulic behavior is carried out in this study for different mixtures. Using oedometer test (indirect measurement), the adequate mixture of 85% dune sand and 15% bentonite satisfies the conditions of saturated hydraulic conductivity (k < 10−9 m/s). This result of the adequate mixture is also confirmed by direct measurement of saturated hydraulic conductivity using triaxial cell. Second, the unsaturated hydraulic conductivity of the adequate mixture is measured with an original vapor equilibrium technique (VET) used for Sr < 30% (very high suction s > 3 MPa). This technique is conducted based on the exploitation of the water retention curve in order to establish the relationships between hydraulic conductivity, degree of saturation, and suction. It shows that the hydraulic conductivity increases with the degree of saturation and decreases with the suction. However, the hydraulic conductivity has a constant value for suctions larger than 20 MPa. The selected dune sand–bentonite mixture satisfies the regulation requirements and hence constitutes a good local and economical material for the conception of barrier base liners.

Runoff initiation in a preserved semiarid Caatinga small watershed, Northeastern Brazil

AbstractThis study analyses some hydrological driving forces and their interrelation with surface‐flow initiation in a semiarid Caatinga basin (12 km2), Northeastern Brazil. During the analysis period (2005 – 2014), 118 events with precipitation higher than 10 mm were monitored, providing 45 events with runoff, 25 with negligible runoff and 49 without runoff. To verify the dominant processes, 179 on‐site measurements of saturated hydraulic conductivity (Ksat) were conducted. The results showed that annual runoff coefficient lay below 0.5% and discharge at the outlet has only occurred four days per annum on average, providing an insight to the surface‐water scarcity of the Caatinga biome. The most relevant variables to explain runoff initiation were total precipitation and maximum 60‐min rainfall intensity (I60). Runoff always occurred when rainfall surpassed 31 mm, but it never occurred for rainfall below 14 mm or for I60 below 12 mm h−1. The fact that the duration of the critical intensity is similar to the basin concentration time (65 min) and that the infiltration threshold value approaches the river‐bank saturated hydraulic conductivity support the assumption that Hortonian runoff prevails. However, none of the analysed variables (total or precedent precipitation, soil moisture content, rainfall intensities or rainfall duration) has been able to explain the runoff initiation in all monitored events: the best criteria, e.g. failed to explain 27% of the events. It is possible that surface‐flow initiation in the Caatinga biome is strongly influenced by the root‐system dynamics, which changes macro‐porosity status and, therefore, initial abstraction. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Effects of subsoil compaction on hydraulic properties and preferential flow in a Swedish clay soil

Soil compaction by vehicular traffic modifies the pore structure and soil hydraulic properties. These changes potentially influence the occurrence of preferential flow, which so far has been little studied. Our aim was to study the effect of compaction on soil hydraulic and transport properties in subsoil. A randomized block design trial at two sites on a well-structured clay soil in central Sweden was established. Plots with two levels of compaction were created at both sites, in the following referred to as trafficked and control. The trafficked treatment was created by 4 passes track-by-track with a three-axle dumper with a maximum wheel load of 5.8Mg. After one year, undisturbed soil columns (20cm height×20cm diameter) from both trafficked and control plots at a depth of 30–50cm were sampled. The columns were analyzed using X-ray CT imaging, together with measures of the degree of preferential transport derived from non-reactive tracer breakthrough curves and measurements of saturated hydraulic conductivity (Ks) and air permeability at the field moisture content (Ka).Although the traffic treatment did not cause any compaction effects at one of the two sites, it did result in significant reductions in saturated hydraulic conductivity, air permeability and number of macropores at the second site. At this site, the traffic also significantly reduced the strength of preferential flow, presumably due to compaction-induced disruption of macropore continuity. In apparent contrast, some previous studies have shown increases in the strength of preferential flow as a result of compaction. We propose a conceptual model to explain these apparently contradictory results, which suggests that preferential flow should be strongest at some intermediate level of compaction.

Sample dimensions effect on prediction of soil water retention curve and saturated hydraulic conductivity

Soil water retention curve (SWRC) and saturated hydraulic conductivity (SHC) are key hydraulic properties for unsaturated zone hydrology and groundwater. In particular, SWRC provides useful information on entry pore-size distribution, and SHC is required for flow and transport modeling in the hydrologic cycle. Not only the SWRC and SHC measurements are time-consuming, but also scale dependent. This means as soil column volume increases, variability of the SWRC and SHC decreases. Although prediction of the SWRC and SHC from available parameters, such as textural data, organic matter, and bulk density have been under investigation for decades, up to now no research has focused on the effect of measurement scale on the soil hydraulic properties pedotransfer functions development. In the literature, several data mining approaches have been applied, such as multiple linear regression, artificial neural networks, group method of data handling. However, in this study we develop pedotransfer functions using a novel approach called contrast pattern aided regression (CPXR) and compare it with the multiple linear regression method. For this purpose, two databases including 210 and 213 soil samples are collected to develop and evaluate pedotransfer functions for the SWRC and SHC, respectively, from the UNSODA database. The 10-fold cross-validation method is applied to evaluate the accuracy and reliability of the proposed regression-based models. Our results show that including measurement scale parameters, such as sample internal diameter and length could substantially improve the accuracy of the SWRC and SHC pedotransfer functions developed using the CPXR method, while this is not the case when MLR is used. Moreover, the CPXR method yields remarkably more accurate soil water retention curve and saturated hydraulic conductivity predictions than the MLR approach.

Are we getting accurate measurements of Ksat for sodic clay soils?

In this paper we discuss the use of a series of column experiments to improve understanding of the effect irrigation water chemistry (saline solutions) has on measurements of saturated hydraulic conductivity (Ksat) of a sodic clay soil. We highlight in particular the use of extended leaching periods to determine whether the duration of leaching affects the results.In the experiments, mixed cation solutions of two different salinity levels, 50meq/L and 100meq/L, were applied under constant head to columns of a repacked sodic clay soil using three replicates for each treatment. The maximum Ksat measured during leaching with the 100meq/L solution was approximately double the maximum Ksat measured during leaching with the 50meq/L solution. Measured flow rates were found to increase rapidly after flow commenced then decrease gradually until flow rates became stable. The final, stable flow rate was roughly 80% less than the maximum flow rate measured. Reasons for these changes in saturated hydraulic conductivity are discussed. The key finding from these experiments is that long term leaching, involving significantly more pore volumes than is commonly reported in the literature, is required to obtain a ‘stable’ Ksat. We recommend that further studies be carried out to (1) determine whether similar behaviour in Ksat occurs in a wide range of sodic clay soils and (2) to help build a better understanding of the causes and implications of the observed behaviour in Ksat.

Factors affecting the spatial pattern of bedrock groundwater recharge at the hillslope scale

AbstractThe spatial patterns of groundwater recharge on hillslopes with a thin soil mantle overlying bedrock are poorly known. Complex interactions between vertical percolation of water through the soil, permeability contrasts between soil and bedrock and lateral redistribution of water result in large spatial variability of water moving into the bedrock. Here, we combine new measurements of saturated hydraulic conductivity of soil mantle and bedrock of the well‐studied Panola Mountain experimental hillslope with previously collected (sub)surface topography and soil depth data to quantify the factors affecting the spatial pattern of bedrock groundwater recharge.We use geostatistical characteristics of the measured permeability to generate spatial fields of saturated hydraulic conductivity for the entire hillslope. We perform simulations with a new conceptual model with these random fields and evaluate the resulting spatial distribution of groundwater recharge during individual rainstorms and series of rainfall events. Our simulations show that unsaturated drainage from soil into bedrock is the prevailing recharge mechanism and accounts for 60% of annual groundwater recharge. Therefore, soil depth is a major control on the groundwater recharge pattern through available storage capacity and controlling the size of vertical flux. The other 40% of recharge occurs during storms that feature transient saturation at the soil‐bedrock interface. Under these conditions, locations that can sustain increased subsurface saturation because of their topographical characteristics or those with high bedrock permeability will act as hotspots of groundwater recharge when they receive lateral flow. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Comparison of empirical models and laboratory saturated hydraulic conductivity measurements

Numerous methods for estimating soil saturated hydraulic conductivity exist, which range from direct measurement in the laboratory to models that use only basic soil properties. A study was conducted to compare laboratory saturated hydraulic conductivity (Ksat) measurement and that estimated from empirical models. Soil samples for the study were collected from four sites at varying depths (15cm, 30cm, 45cm and 60cm) at the Faculty of Agriculture Teaching and Research Farm, University of Maiduguri. The Ksat value for each sample was determined in the laboratory using the falling head permeameter method. Soil physical properties (bulk density, porosity, gravimetric water content, % sand and % silt) required by the models were also determined. A refined Kozeny-Carman model and model developed from multiple regression analysis were used to predict Ksat which were compared with the results obtained from laboratory measurement. The developed model predicted values of 0.0065, 0.0010, 0.0965 and 0.0048cm/s at 15cm, 30cm, 45cm and 60cm, respectively, that is closer to the value of Ksat measured in the laboratory (0.0061, 0.0054, 0.0050 and 0.0048cm/s at 15cm, 30cm, 45cm and 60cm, respectively) while Kozeny-Carman model predicted a value of 0.2208, 0.2161, 0.2020 and 0.1974cm/s at 15cm, 30cm, 45cm and 60cm, respectively, that is far above the one measured in the laboratory. Therefore, Ksat estimating models could not fit for all locations very well.Key Words: Saturated hydraulic Conductivity, Empirical models, Laboratory measurements

A study on the effect of compaction on transport properties of soil gas and water. II: Soil pore structure indices

Experimental data on the effects of compaction and applied organic matter (OM) on macropore structure indices, more particularly on pore continuity, have yet rarely been documented. In this study, static compaction was simulated in the laboratory at 150, 225, and 300kPa upon rice husk, rice straw, compost, sawdust, and wood bark-mixed soils and control. Measurements of relative gas diffusivity (Dp/D0)100 and air permeability (ka100) were conducted at −100cm H2O soil matric suction after measurement of saturated hydraulic conductivity (ks). Corresponding dry bulk density (ρd), total porosity (f), and air content (ɛ100) values were also determined. Volume of macropores (ϕ≥30μm) and micropores (ϕ<30μm) were expressed as volume of air and water at −100cm H2O soil matric suction, respectively, relative to the volume of soil solid. Specific gas diffusivity (SD100) and specific air permeability (Ska100) were calculated as (Dp/D0)100/ɛ100 and ka100/ɛ100, respectively. Analogous to the SD100 and Ska100, specific hydraulic conductivity (Sks) was defined as ks/ɛ100. The results showed that compaction significantly increased ρd, which was followed by a reduction in f, and the mixed OM resulted in a significantly lower ρd and higher f than the control. The volume of macropores was reduced by compaction whereas the volume of micropores remained unaffected, for which the mixed OM tended to result in a higher volume of macropores than the control. Compaction resulted in more tortuous macropores for gas diffusion (lower SD100) and less continuous macropores for gas convection (lower Ska100) for which a significant difference was more pronounced between the 300 and 150kPa compactions. Compaction also resulted in fewer continuous macropores for water movement as indicated by lower Sks. The mixed OM was likely to result in a lower SD100, but except for rice straw tended to result in a higher Ska100 than the control. In addition, the mixed OM also seemed to result in a higher Sks than the control. Of the OM-mixed soils, the decrease in (Dp/D0)100 and ka100 was more sensitive to compaction (i.e., decrease in ɛ100) than that of the control whereas the decrease in ks acted conversely. Discussion of the measured (Dp/D0)100, ka100, and ks is presented in the companion paper.

Macropores and Macropore Transport

Preferential transport of water through soil macropores is a governing process in the facilitated transport of strongly sorbing compounds. The aim of this study was to investigate the relationships between macropore density and the hydraulic conductivity of the soil and to test the sampling representativeness of soil columns for the measurement of saturated hydraulic conductivity. Macropore density was determined in three horizons in four typical Danish soil types (third year of pasture), and saturated hydraulic conductivity and near-saturated hydraulic conductivity were measured in the laboratory on undisturbed soil columns (6,280 cm) in the same three horizons. A strong relationship between macropore density and the mean particle diameter was found in the B and C horizons. A poor relationship was found between macropore density and the hydraulic conductivity at a matric potential of −10 hPa, whereas a stronger relationship was found at saturation. Results of the test of the number of soil columns needed for a representative distribution of macropores for the measurement of saturated hydraulic conductivity indicated that approximately six soil columns were enough. Our work suggests that integrating knowledge of the abundance of macropores in relation to soil type and land use will increase the performance of pedotransfer functions in predicting saturated hydraulic conductivity.

Using the KINEROS2 Modeling Framework to Evaluate the Increase in Storm Runoff from Residential Development in a Semiarid Environment

The increase in runoff from urbanization is well known; one extreme example comes from a 13-ha residential neighborhood in southeast Arizona where runoff was 26 times greater than in an adjacent grassland watershed over a 40-month period from 2005 to 2008. Rainfall-runoff modeling using the newly described KINEROS2 urban element, which simulates a contiguous row of houses and the adjoining street as a series of pervious and impervious overland flow planes, combined with tension infiltrometer measurements of saturated hydraulic conductivity (Ks), indicate that 17±14% of this increase in runoff is due to a 53% decrease in Ks in constructed pervious areas as compared to the undeveloped grassland. Ks in the urban watershed identified from calibrating the rainfall-runoff model to measured runoff is higher than measured Ks but much lower than indicated by a soil texture–based KINEROS2 parameter look-up table. Tests using different levels of discretization found that watershed geometry could be represented in a simplified manner, although more detailed discretization led to better model performance.

Numerical Evaluation of Depth Effects of Double‐Ring Infiltrometers on Soil Saturated Hydraulic Conductivity Measurements

Previous research has indicated that the measurements of soil saturated hydraulic conductivity (Ks) are influenced by both the radial size and depth of a double‐ring infiltrometer. Few studies are available, however, on how the depth to which an infiltrometer is driven affects Ks measurement. In this study, six inner ring depths (di = 5, 10, 15, 25, 40, and 60 cm), six outer ring depths (do = 5, 10, 15, 25, 40, and 60 cm), and their combinations were designed. Thus 24 sets of driven depths of infiltrometer were involved. For each infiltrometer set, 31 stochastic fields of Ks were randomly generated in the simulation domain by adopting six correlation lengths (L = 0, 10, 20, 50, 100, and 200 cm) and six standard deviations (SD = 0, 0.1, 0.25, 0.5, 0.75, and 1.0). Therefore, a total of 7224 simulations were conducted to investigate the driven‐depth effect of the double‐ring infiltrometer on the measurements of soil saturated hydraulic conductivity of the wetted zone (Kw). Results demonstrated that the inner ring depth plays a more important role than the outer ring depth in Kw measurements by double‐ring infiltrometer, and increasing the inner ring depth could obtain a better approximation of accurate Kw measurements. Meanwhile, the outer ring added to a single‐ring infiltrometer was necessary in guaranteeing vertical water flow in the inner ring and improving measurement accuracy. Considering that the disturbance of soil caused by ring installation will increase with increasing ring depth, the ring depth of 5 to 15 cm that has been adopted in most ring‐infiltrometer‐related field experiments is acceptable and recommended for Kw measurements.

Evaluation of Different Methods for Measuring Field Saturated Hydraulic Conductivity under High and Low Water Table

In this research, four methods of in situ measurement of saturated hydraulic conductivity (Ks) including the Guelph permeameter (GP), auger hole (AH), original Porchet (OP), and saturated Porchet methods were compared for a silt loam soil. The representative Ks in a drainage system was also determined as a reference value. The mean values of Ks for the GP, AH, OP, SP, and drainage system methods were 1.18, 1.06, 1.85, 1.18, and 1.08 m d−1, respectively. Furthermore, the GP and OP methods had the highest and lowest coefficients of variation at 42.1 and 24.2%, respectively. The difference in Ks values among all methods except SP and GP and the difference in CV values between GP and OP were statistically significant at the 0.01 level of probability. The direct measurement from the drainage system resulted in Ks values of 0.584, 0.915, 1.019, and 0.915 times that of the OP, SP, AH, and GP methods, respectively. The results of the SP, GP, and AH methods were very similar to that obtained from direct measurement of the drainage system. Results showed that the AH and GP methods are the best methods for measuring Ks in the presence and absence of a water table, respectively, with −1.85 and 9.26% difference, respectively, compared with the reference method. Therefore, in regions with a high water table, the AH method is suitable, and for a low water table, the GP method is recommended.

A Comparison of In Situ Constant and Falling Head Permeameter Tests to Assess the Distribution of Clogging Within Horizontal Subsurface Flow Constructed Wetlands

Clogging is the main operational problem associated with horizontal subsurface flow constructed wetlands (HSSF CWs). The measurement of saturated hydraulic conductivity has proven to be a suitable technique to assess clogging within HSSF CWs. The vertical and horizontal distribution of hydraulic conductivity was assessed in two full-scale HSSF CWs by using two different in situ permeameter methods (falling head (FH) and constant head (CH) methods). Horizontal hydraulic conductivity profiles showed that both methods are correlated by a power function (FH = CH0.7821, r2 = 0.76) within the recorded range of hydraulic conductivities (0–70 m/day). However, the FH method provided lower values of hydraulic conductivity than the CH method (one to three times lower). Despite discrepancies between the magnitudes of reported readings, the relative distribution of clogging obtained via both methods was similar. Therefore, both methods are useful when exploring the general distribution of clogging and, specially, the assessment of clogged areas originated from preferential flow paths within full-scale HSSF CWs. Discrepancy between methods (either in magnitude and pattern) aroused from the vertical hydraulic conductivity profiles under highly clogged conditions. It is believed this can be attributed to procedural differences between the methods, such as the method of permeameter insertion (twisting versus hammering). Results from both methods suggest that clogging develops along the shortest distance between water input and output. Results also evidence that the design and maintenance of inlet distributors and outlet collectors appear to have a great influence on the pattern of clogging, and hence the asset lifetime of HSSF CWs.

Buffer Index Effects on Hydraulic Conductivity Measurements Using Numerical Simulations of Double‐Ring Infiltration

The double‐ring infiltrometer is widely used to measure soil saturated hydraulic conductivity in the field. Both the inner ring size and outer ring size (two factors in the buffer index) of an infiltrometer affect the measurements of saturated hydraulic conductivity. Few systematic studies have been conducted to investigate the combined effects of the inner and outer ring sizes of a double‐ring infiltrometer on the measurements of field saturated hydraulic conductivity. A total of 7224 numerical simulations were conducted to investigate the optimum combination of inner and outer ring sizes for reliable saturated hydraulic conductivity measurements by using 24 infiltrometers with six inner ring diameters (10, 20, 40, 80, 120, and 200 cm) and, for each ring diameter, four buffer indices (b = 0.2, 0.33, 0.5, and 0.71). Results demonstrated that the inner ring size is a more important factor to be considered than the buffer index itself (or the outer ring size) in practice, and a larger inner ring diameter assembled with an outer ring (in most cases, with diameter ≥80 cm and b ≥ 0.33) is recommended to obtain reliable in situ measurement of soil field saturated hydraulic conductivity.

Testing laboratory methods to determine the anisotropy of saturated hydraulic conductivity in a sandy–loam soil

Anisotropy, a (the log of the ratio of horizontal to vertical conductivity, log 10( K h/ K v)), of saturated soil hydraulic conductivity, K s, affects transport processes in soil but is not routinely measured, probably because practical and validated methods are lacking. The objective of this investigation was to determine the effects of different constant-head laboratory and sampling procedures on anisotropy of saturated hydraulic conductivity measurements. The sequence of K s measurements was varied (vertical conductivity, K v, first, then horizontal, K h, second and vice versa) for an experimental set-up considering five variables: 1) water ponding type (Mariotte or siphon); 2) saturation state prior to experiment start (unsaturated or saturated); 3) experiment duration (long or short); 4) sample geometry or extraction (cube or core); and 5) sample volume. The Mariotte, unsaturated/saturated, long experiment for a single soil cube resulted in unreliable mean anisotropy results, where a differed in sign. Generally, the sequence of measurements had a negligible impact on a for a siphon, saturated, short experiment for a soil cube. Furthermore, different a were obtained by varying the undisturbed soil sample collection procedure (cube vs. core). The conclusion of this investigation was that using a siphon and a short-duration run on an initially saturated cube of soil encased in foam is generally expected to yield reliable bi-directional K s results. However, a check of the independence of the estimated anisotropy on the order of measurements for the sampled soil is recommended. An alternative procedure to determine a mean anisotropy for an area of interest would be to measure K v and K h on different soil cubes. Finally, the ratio between the mean K h and K v results varied from a not statistically significant factor of 1.02 to a statistically significant factor of 1.95 during the one-year investigation period (five sampling dates). Therefore, anisotropy of this sandy–loam soil varied with time but it was always low or negligible.

Prediction of unsaturated soil hydraulic conductivity with electrical conductivity

Soil hydraulic conductivity (K) varies greatly with matric potential (h) and exhibits a high variability at the field scale. However, this key property for estimating water flux in soils is difficult to measure. In contrast, soil electrical conductivity (σ) is easier to measure and is influenced by the same parameters affecting K. We derive a simple relationship between σ and K(h) and test it against laboratory and literature data. Importantly, we show that parameters of this σ‐K(h) relationship can be completely determined with accessible measurements of saturated hydraulic conductivity, electrical conductivity of the soil solution, and clay content. This results in K(h) estimation with a RMSE ranging between 0.4 and 0.5 for log K, i.e., of the order of most experimental determinations of K. A further test of the σ‐K(h) relationship on the large UNSODA hydraulic database shows good agreement and the robustness of the relationship. Such a relation could be useful in the spatial monitoring of water fluxes at the field scale using electrical resistivity tomography if the σ(h) relationship can be obtained.