Core Ideas We developed an apparatus with automatic registration of flow (Q) and pressure (ΔP). A curvilinear Q–ΔP relation was found for seven of eight soil samples tested. Ignoring non‐linear pressure loss gave permeability errors up to 65% at ΔP=100 Pa. We suggest an index for pore tortuosity based on the non‐linear Q–ΔP relation. Air permeability affects a range of soil functions and is useful in the quantification of soil pore characteristics. Measurements of air flow used to quantify air permeability are mostly performed at a fixed pressure difference, assuming a linear relation between flow and pressure. However, evidence exists that nonlinear pressure losses may occur even at low pressure gradients. We constructed an apparatus that allows automatic measurement of air flow at a range of pressures. The new methodology was applied to eight soil samples deriving from a loamy, Stagnic luvisol. Three artificial cores were also tested: a solid cylinder of plastic with drilled, vertical holes; a cylinder of autoclaved aerated concrete (AAC), and an AAC cylinder with drilled holes. The historical Forchheimer approach, including a polynomial regression of flow–pressure data, was applied to derive the true Darcian flow based on the coefficient to the linear part of the relation. Flow‐pressure data appeared to be curvilinear for all test specimens, except for one of the soil samples. The results showed up to 65% errors in estimates of air permeability if the nonlinear pressure losses were ignored when applying a pressure difference as low as 100 Pa. Our results strongly suggest use of the Forchheimer approach based on measurements of flow and pressure difference at a range of air pressures. We suggest an index for soil pore tortuosity, which appears to reflect the pore characteristics of the artificial samples tested. More studies are needed to evaluate the applicability of the index for soil samples.
<p>The near-saturated hydraulic conductivity is an important parameter in relation to the analysis of heterogeneous transport in the soil macropore system. To a high degree, leaching of phosphorus out of the root zone takes place in the macropores either in a dissolved form or as phosphorus bound to colloids. In this work, a newly constructed and improved drip infiltrometer (DIM) is presented being able to measure the unsaturated hydraulic conductivity in the near-saturated range (i.e. in the range of matric potentials between -0.1 and 3 -kPa) on undisturbed soil columns (20 cm by 20 cm). The DIM is a modified version of the classical multistep system establishing gravity flow at decreasing flow rates. The procedure is that the soil column is placed on top of a ceramic plate. Five tensiometers measure the change in the matric potential a different flow rates applied by a drip-irrigation device mounted on the top of the column. By applying a certain inflow at the top and suction at the bottom of the sample, a steady state flow is established based on tensiometer readings showing a constant gradient along the soil sample. This allows the determination of the near-saturated hydraulic conductivity by applying Darcy&#8217;s equation. Compared to an earlier version of the infiltrometer, the instrument has been improved in several ways. This involves a high level of automation of the computer program controlling the analysis making it possible to setup a number of settings and constrains in order to optimize the analysis. Examples are given for newly developed pedotransfer functions predicting the saturated and near-saturated hydraulic conductivity. Results were used to model water transport in the vadose zone spatially distributed over Denmark using variation in the hydraulic properties as well as spatially distributed metrological data. Models results ended up with a map pointing out risk areas of macropore transport in relation to the leaching of phosphorus.</p>
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