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Soil hydraulic properties have a predominating impact on soil physical quality (SPQ) because they directly or indirectly control air and water storage, infiltration and drainage, nutrient leaching, microbial activity, greenhouse gas generation, and carbon sequestration. The hydraulic properties of many soils are often better described using “bimodal” water content and hydraulic conductivity (θ-K-h) functions, where the θ-K-h of a large-pore “structure domain” is combined with the θ-K-h of a small-pore “matrix domain”. This study uses closed-form bimodal van Genuchten θ-K-h functions to characterize SPQ from the perspective of storage and transmission of water and air in soils containing distinct structure and matrix domains. Consistently good fits were achieved between the soil water content function, θ(h), and water content data from intact soil, repacked diatomite pellets, and repacked soil aggregates (R 2 ≥ 0.9854, RMSE ≤ 0.0223 m 3 m − 3 ), but variable fits were attained between the hydraulic conductivity function, K(h), and hydraulic conductivity data. It was found that even though the SPQ of bulk soil may be optimal or near-optimal, the SPQ of the corresponding structure and matrix domains could be limited or poor in one or more categories. The structure domain tended to be water-limited and potentially prone to leaching, while the matrix domain tended to be aeration-limited and potentially prone to greenhouse gas generation. It was concluded that maximizing the economic and environmental performance of field crop production would likely require selective improvement of structure or matrix SPQ, rather than bulk soil SPQ. [ABSTRACT FROM AUTHOR]

The variable-head borehole permeameter (VHBP) method is a long-standing international standard for in-situ measurement of field-saturated hydraulic conductivity, K FS , in natural and engineered porous media. Its applicability is restricted, however, because traditional VHBP theory does not apply for unsaturated or deformable porous media, and because precise knowledge of the boundary condition on the interface of the borehole outlet (screen) is required for accurate K FS determination. This study extends the traditional VHBP theory to include saturated, unsaturated, rigid and deformable porous media, and also clarifies the boundary condition at the screen interface. Using a recent VHBP analysis developed for rigid, unsaturated porous media, it is shown (via flow conservation and the total increment theorem) that change in porous medium water content, ∆θ, can be extended to include change in porosity (deformation) as well as change in degree of saturation. It is also shown that the appropriate boundary condition on the borehole screen is antecedent pore water pressure head, H a , for saturated porous media, but effective wetting front pressure head, ψ f (or sorptive number, α*), for unsaturated porous media. The K FS , ∆θ, H a , and ψ f (or α*) parameters can be determined using numerical optimization (e.g. “Solver” in the Excel spreadsheet) to curve-fit the extended VHBP analysis directly to borehole head versus time measurements; however, fitting to the velocity graph (borehole head plotted against change in head with time) is generally less problematic. In a cursory assessment of the extended VHBP analysis, K FS was determined with ≤ 7% error, ∆θ with ≤ 15% error, H a with ≤ 1.4% error, and ψ f with ≤ 0.5% error, which is more than sufficient accuracy for most applications. It was concluded that the VHBP method was successfully extended for application to saturated, unsaturated, rigid and deformable porous media. [ABSTRACT FROM AUTHOR]

Abstract: Single-borehole infiltration tests are widely used for in-situ measurement of field-saturated hydraulic conductivity, Kfs, in the vadose or unsaturated zone. United States Bureau of Reclamation (USBR) methods are used extensively in geologic and water management engineering to analyse single-borehole infiltration data, while borehole permeameter (BP) methods are preferred in the environmental and agricultural sciences. Little is known about how well the USBR and BP analyses correspond; hence, the objectives of this study were to compare and provide usage recommendations for some of the most common constant head and falling head USBR and BP analyses in terms of range of validity and accuracy for Kfs determination. HYDRUS-2D numerical simulations of variably saturated, axisymmetric flow for a range of specified Kfs values, porous media capillarities (as represented by sorptive number, α*, and change in water content, ∆θ) and outflow geometries (as represented by length, L, and radius, a, of borehole discharge zone) were used to generate perturbation-free borehole test data for use in the USBR and BP analyses. The numerically generated test data consisted of steady borehole discharge rate, QS, versus steady ponded head, H, for the constant head borehole tests, and ponded head, Ht, versus time, t, for the falling head borehole tests. The constant head USBR estimates of Kfs were generally accurate (≤25% error) when H/a≥10, α*≥12m−1, a≥10cm and L/a≥10, but could overestimate or underestimate by more than an order of magnitude when the above parameters were outside those ranges. The falling head USBR analysis appeared to always overestimate or underestimate Kfs (>25% error), and the overestimates could be more than an order of magnitude when early-time Ht vs. t data were used. The constant head and falling head BP analyses, on the other hand, provided accurate Kfs estimates (≤26% error) for all test scenarios. The BP analyses were therefore recommended for all tested borehole configurations and porous medium characteristics, while the USBR analyses were recommended only for constant head tests when H/a≥10, L/a≥10, α*≥12m−1, and a≥10cm. [Copyright &y& Elsevier]

Abstract: This study investigates the use of 3D soil pore characteristics (volume, surface area, and tortuosity) for prediction of saturated hydraulic conductivity (K s). The pore characteristics were determined by cluster labeling of X-ray CT images and random-walk simulations of 3D pore space. The flow characteristics of the four soil cores were measured in the laboratory pre- and post-CT imaging. The predicted values of K s were arrived at through the use of Darcy''s equation and a modified Poiseuille equation. For comparison the 2D pore characteristics were determined from individual slice of X-ray CT imagery, and fed into three established methods of predicting K s. The predicted K s by the proposed method exhibited a positive correlation to both pre- and post-imaging measured K s, and attained a greater correlation than the 2D K s prediction. [Copyright &y& Elsevier]

Abstract: Although agricultural land management is known to affect near-surface soil physical quality (SPQ), the characteristics of these affects are poorly understood, and diagnostic SPQ indicators are not well-developed. The objective of this study was to measure a suite of potential SPQ indicators using intact soil cores and grab samples collected from the 0–10cm depth of a clay loam soil with the treatments: (i) virgin soil (VS); (ii) long-term continuous bluegrass sod (BG); (iii) long-term maize (Zea mays L.)—soybean (Glycine max (L.) Merr.) rotation under no-tillage (NT); (iv) long-term maize–soybean rotation under mouldboard plough tillage (MP); (v) short-term (1–4 years) NT after long-term MP; (vi) short-term MP after long-term BG; (vii) short-term MP after long-term NT. Organic carbon content, dry bulk density, air capacity, relative water capacity and saturated hydraulic conductivity appeared to be useful SPQ indicators because they were sensitive to land management, and proposed optimum or critical values are available in the literature. Soil macroporosity was also sensitive to land management, but optimum or critical values for this parameter are not yet established. Soil matrix porosity and plant-available water capacity did not respond substantially or consistently to changes in land management, and were thus not useful as SPQ indicators in this study. Converting long-term BG to MP caused overall SPQ to decline to levels similar to long-term MP within 3–4 years. Converting long-term NT to MP or vice versa caused only minor changes in overall SPQ. With respect to the measured SPQ indicators and their optimum or critical values, both VS and BG produced “good” overall SPQ in the near-surface soil, while long-term maize–soybean rotation under NT and MP produced equally “poor” SPQ. [Copyright &y& Elsevier]