Soil Science Society of America Journal

Sadeghi, M.; Ghahraman, B.; Ziaei, A.N.; Davary, K.; Reichardt, K.

doi: 10.2136/sssaj2011.0275pmid: N/A

Scaling methods allow a single solution to Richards' equation (RE) to suffice for numerous specific cases of water flow in unsaturated soils. During the past half‐century, many such methods were developed for similar soils. In this paper, a new method is proposed for scaling RE for a wide range of dissimilar soils. Exponential‐power (EP) functions are used to reduce the dependence of the scaled RE on the soil hydraulic properties. To evaluate the proposed method, the scaled RE was solved numerically considering two test cases: infiltration into relatively dry soils having initially uniform water content distributions, and gravity‐dominant drainage occurring from initially wet soil profiles. Although the results for four texturally different soils ranging from sand to heavy clay (adopted from the UNSODA database) showed that the scaled solution were invariant for a wide range of flow conditions, slight deviations were observed when the soil profile was initially wet in the infiltration case or deeply wet in the drainage case. The invariance of the scaled RE makes it possible to generalize a single solution of RE to many dissimilar soils and conditions. Such a procedure reduces the numerical calculations and provides additional opportunities for solving the highly nonlinear RE for unsaturated water flow in soils.

Moysey, Stephen M.J.; Liu, Zuolin

doi: 10.2136/sssaj2010.0413pmid: N/A

This study evaluated whether electrical resistivity monitoring could be used as a tool to detect the activation of macropores in soils. Experiments were performed where soil resistivity was measured using a Wenner array while individual soil macropores were sequentially saturated to simulate changes in macropore activation in a hydrologically active soil. The measured apparent resistivity was found to be highly dependent on the activated macropore porosity. An increase in macropore porosity from 0 to 4% decreased the apparent resistivity of the soil by up to 30%. A simple two‐domain model was developed to evaluate the controls on apparent resistivity in a macroporous soil by conceptualizing the macropores as fluid‐filled tubes embedded within a homogeneous soil matrix. The model qualitatively reproduced the trends of the observed data and provided reasonable bounds on the expected response of soils, suggesting that it captures first‐order influences of macropores on apparent resistivity measurements. The model indicates that the magnitude of the reduction in apparent resistivity is strongly dependent on the contrast in resistivity between the soil (ro) and macropores (rm), which can be readily estimated in field monitoring scenarios. The ro/rm ratio is therefore suggested as a useful diagnostic for evaluating conditions where changes recorded in resistivity monitoring data might be used to detect the onset of macropore flow in field‐based studies.

Arthur, Emmanuel; Moldrup, Per; Schjønning, Per; Jonge, Lis W.

doi: 10.2136/sssaj2011.0125pmid: N/A

Accurate estimation of soil gas diffusivity (Dp/Do, the ratio of gas diffusion coefficients in soil and free air) and air permeability (ka) from basic texture and pore characteristics will be highly valuable for modeling soil gas transport and emission and their field‐scale variations. From the topsoil of two Danish arable fields representing two natural clay gradients, Dp/Do and ka were measured at soil water matric potentials between −1 and −100 kPa on undisturbed soil cores. The Rosin–Rammler particle size distribution parameters α and β (characteristic particle size and degree of sorting, respectively) and the Campbell water retention parameter b were used to characterize particle and pore size distributions, respectively. Campbell b yielded a wide interval (4.6–26.2) and was highly correlated with α, β, and volumetric clay content. Both Dp/Do and ka followed simple power‐law functions (PLFs) of air‐filled porosity (εa). The PLF tortuosity–connectivity factors (X*) for Dp/Do and ka were both highly correlated with all basic soil characteristics, in the order of volumetric clay content = Campbell b > gravimetric clay content > α > β. The PLF water blockage factors (H) for Dp/Do and ka were also well (but relatively more weakly) correlated with the basic soil characteristics, again with the best correlations to volumetric clay content and b. As a first attempt at developing a simple Dp/Do model useful at the field scale, we extended the classical Buckingham Dp/Do model (εa2) by a scaling factor based on volumetric clay content. The scaled Buckingham model provided accurate predictions of Dp(εa)/Do across both natural clay gradients.

Zhu, Jianting; Warrick, A. W.

doi: 10.2136/sssaj2011.0028pmid: N/A

This study examines the appropriateness of the harmonic mean function of unsaturated hydraulic conductivities in representing structured heterogeneity for a finitely deep profile typical of many applications such as water fluxes between the ground surface and saturated zone. The heterogeneity is constructed by forming repeated unit cells of homogeneous sublayers. We address the significance of water flux (both infiltration and evaporation), structural arrangement and compositional fraction within the unit cell, and number of repeated units on the applicability of the harmonic mean function in simulating fluxes through heterogeneous layered media. We demonstrate that both structural location and the fraction of coarse material in the unit cell are important indicators of whether the harmonic mean function will over‐ or underpredict fluxes. The harmonic mean function represents infiltration fluxes better than evaporation in layered heterogeneous soils. Under the evaporation scenario, the errors from using the harmonic mean function generally decrease with increasing coarse layer fraction. On the other hand, the errors generally increase with increasing coarse layer fraction for the infiltration scenario. As the number of layers becomes large, the fluxes through the heterogeneous layered soils converge to the flux predicted by the harmonic mean function; however, the convergence is from opposite directions for evaporation and infiltration.

Kelleners, T. J.; Norton, J. B.

doi: 10.2136/sssaj2011.0222pmid: N/A

The soil freezing characteristic, defined as the relationship between freezing soil temperature and unfrozen water content, can be used to determine the soil water retention curve in situ. The objective of this study was to investigate whether freezing characteristics measured with Hydra impedance sensors (Stevens Water Monitoring Systems, Portland, OR) result in accurate depthwise soil water retention curves. Hydra sensors measuring the complex permittivity and soil temperature were installed at five depths at five sites in southeastern Wyoming and monitored for approximately 2 yr. A dielectric mixing model was calibrated using unfrozen soil data to predict the liquid water content and ice content in frozen soils. The total water potential in the frozen soils was calculated from the Hydra sensor soil temperature measurements using the Clapeyron equation. Soil water pressure heads were calculated from the total soil water head by subtracting the osmotic head. Comparison of the resulting Hydra sensor water retention data with depthwise laboratory retention data using a dew point potentiometer and a pressure plate extractor showed mixed results (coefficient of determination 0 ≤ R2 ≤ 0.94). The best results were obtained for the shallowest sensors at the five sites (0.74 ≤ R2 ≤ 0.93) because of the more significant and more prolonged soil freezing at these depths, resulting in relatively wide ranges for the calculated soil water pressure heads. Fitted curves for the Hydra sensor water retention data yielded unreliable parameters because of insufficient information on the wet end of the water retention curves.

Laegdsmand, Mette; Moldrup, Per; Schjønning, Per

doi: 10.2136/sssaj2011.0043pmid: N/A

Solute diffusivity in soil plays a major role in many important processes with relation to plant growth and environmental issues. Soil solute diffusivity is affected by the volumetric water content as well as the morphological characteristics of water‐filled pores. The solute diffusivity in intact soil samples from two different tillage treatments (soil from below the depth of a harrow treatment and soil from within a moldboard plowed plow layer) was estimated based on concentration profiles using a newly developed method. The method makes use of multiple tracers (two sets of counterdiffusing tracers) for a better determination of the diffusivity. The diffusivity was higher in the below‐till soil than the plowed soil at the same soil water matric potential due to higher water content but also due to higher continuity and lower tortuosity of the soil pores. We measured identical solute diffusivities independent of the tracer set used. We analyzed the whole data set using Archie's law and found a linear relation between Archie's exponent and the logarithm of the soil water matric suction in centimeters of water (pF). An analysis of seven data sets from the literature showed that this was a general trend for soils with moderate to low clay contents.

Rosenzweig, Ravid; Shavit, Uri; Furman, Alex

doi: 10.2136/sssaj2011.0155pmid: N/A

This study investigated the effect of biofilms and, in particular, that of extracellular polymeric substances (EPS) on the hydraulic properties of porous media under unsaturated conditions. The quantitative understanding of the way biological activity alters hydraulic properties is a major key in understanding and engineering relevant systems such as soil aquifer treatment, bioremediation, and wastewater irrigation. Using an EPS analog (xanthan) we explored the effect of EPS on the water retention function of two sandy soils. The result was a significant increase in the water content at any given matric head that could reach 270% of its value within pure soil. For most of the water content range, we successfully modeled the effect of the EPS as a linear superposition of the original pure soil and the xanthan retention curves. Finally, we examined two mechanisms that can attribute to modification of the water retention curve: the EPS holding capacity and alteration of the soils' pore‐size distribution; in our case, it appears that the first mechanism was dominant.

Nouwakpo, Sayjro K.; Huang, Chi-hua

doi: 10.2136/sssaj2011.0148pmid: N/A

Surface reconstruction using digital photogrammetry offers a great advantage for soil erosion research. The technology can be cumbersome for field application because it relies on the accurate measurement of control points, often using a survey‐grade instrument. Also, even though digital photogrammetry has been used in much soil erosion research, its sensitivity in detecting soil elevation changes has rarely been assessed. This study aimed at simplifying the digital photogrammetric procedure for soil erosion research and assessing the sensitivity of this technology to detect soil erosion. To simplify the technology, we propose to combine a photogrammetric procedure for control point generation in a first step, followed by a conventional photogrammetric digital elevation model (DEM) extraction procedure. The performance of the method was assessed in the laboratory and tested in the field to digitize ephemeral gullies. In the accuracy test of photogrammetric survey step, we found that the maximum length measurement error was 3.4 mm, while the maximum angular deviation from the vertical or horizontal axes was 0.93°. The maximum error between control point coordinates generated by photogrammetry and those generated by a survey‐grade total station was 26 mm on the horizontal axes and 10 mm on the vertical axis. We also found that the sensitivity of digital photogrammetry in detecting soil surface elevation changes was similar to that of a laser scanner when the detection was performed on smooth soil surfaces and when the standard deviation of the elevation changes was approximately three times the precision of the photogrammetric DEM.

Callegary, James B.; Ferré, Ty P. A.; Groom, R. W.

doi: 10.2136/sssaj2011.0003pmid: N/A

There is an ongoing effort to improve the understanding of the correlation of soil properties with apparent soil electrical conductivity as measured by low‐induction‐number electromagnetic‐induction (LIN FEM) instruments. At a minimum, the dimensions of LIN FEM instruments' sample volume, the spatial distribution of sensitivity within that volume, and implications for surveying and analyses must be clearly defined and discussed. Therefore, a series of numerical simulations was done in which a conductive perturbation was moved systematically through homogeneous soil to elucidate the three‐dimensional sample volume of LIN FEM instruments. For a small perturbation with electrical conductivity similar to that of the soil, instrument response is a measure of local sensitivity (LS). Our results indicate that LS depends strongly on the orientation of the instrument's transmitter and receiver coils and includes regions of both positive and negative LS. Integration of the absolute value of LS from highest to lowest was used to contour cumulative sensitivity (CS). The 90% CS contour was used to define the sample volume. For both horizontal and vertical coplanar coil orientations, the longest dimension of the sample volume was at the surface along the main instrument axis with a length of about four times the intercoil spacing (s) with maximum thicknesses of about 1 and 0.3 s, respectively. The imaged distribution of spatial sensitivity within the sample volume is highly complex and should be considered in conjunction with the expected scale of heterogeneity before the use and interpretation of LIN FEM for mapping and profiling.

Minacapilli, M.; Cammalleri, C.; Ciraolo, G.; D'Asaro, F.; Iovino, M.; Maltese, A.

doi: 10.2136/sssaj2011.0122pmid: N/A

We are proposing a new method for estimating soil surface water content from thermal inertia distributions retrieved from visible–near infrared (VIS‐NIR) and thermal infrared (TIR) images. A drying experiment was conducted on three fine‐textured soils while acquiring multispectral VIS‐NIR and TIR images. Simultaneous measurements of soil water content and thermal inertia were conducted by the thermogravimetric method and the heat pulse technique, respectively. Direct measurements were used to test the thermal inertia approach proposed by Murray and Verhoef that requires only knowledge of soil porosity and can be easily inverted to derive soil water content from thermal inertia. For the three considered soils, the performance of the Murray and Verhoef model was practically equal to that of the traditional approach based on the direct estimation of thermal conductivity and heat capacity, which requires more detailed information about soil properties. With the aim of simplifying the estimation of thermal inertia from remotely sensed images, a modified Kersten function was proposed in which the normalized thermal inertia is substituted by the normalized apparent thermal inertia. Comparison between the two modified Kersten functions was satisfactory. The proposed approach allowed predictions of the surface soil moisture from apparent thermal inertia distributions with an acceptable level of accuracy for practical purposes (0.028 ≤ RMSE ≤ 0.043 m3 m−3) and therefore it can be considered a simple and effective tool for estimating the spatial and temporal distribution of surface soil moisture from VIS‐NIR and TIR remotely sensed data.