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Laryea, K. B.; Elrick, D. E.; Robin, M. J. L.
doi: 10.2136/sssaj1982.03615995004600040001xpmid: N/A
The analysis of hydrodynamic dispersion of solutes during one‐dimensional absorption (horizontal flow) of solution into soil columns is extended to solutes which interact with the soil particle surfaces. The exchange process involved is assumed to be instantaneous and is described by a nonlinear adsorption isotherm. The relevant equations are solved for the apparent longitudinal dispersion coefficient using a computer program written in system/360 CSMP.
Topp, G. C.; Davis, J. L.; Annan, A. P.
doi: 10.2136/sssaj1982.03615995004600040002xpmid: N/A
The time‐domain reflectometry (TDR) technique has been demonstrated to be a viable method for determining soil water content in uniformly wet soils. The TDR response in nonuniformly wet soils was examined herein both theroetically and experimentally. An extensive set of experiments was carried out in a controlled laboratory test facility which permitted synthesis of steep water content gradients and wetting fronts. The water content measured by the TDR technique has been found to be the same as the average water content to within 0.01 cm3cm−3 in extremely nonuniform conditions. This result is quite surprising at first but is readily explained by the nature of the dielectric constant and water content relationship. The TDR technique has also been found useful in detecting and monitoring the progression of wetting front advance through a soil.
Topp, G. C.; Davis, J. L.; Annan, A. P.
doi: 10.2136/sssaj1982.03615995004600040003xpmid: N/A
Time‐domain reflectometry (TDR) applied to parallel transmission lines in soil was used to measure the soil water contents during infiltration, drainage, evaporation, and rising water table conditions in a 1.05‐m column of silt loam soil in the laboratory. Three types of installation and several types of transmission lines were evaluated. Comparisons of volumetric water content from gravimetric samples and from TDR showed differences were always <0.03 cm3cm−3. Trends of the TDR values with time showed that these differences likely resulted from soil density variations. Neither the installation method nor the design of the rods of the transmission line affected the water content values, provided there was good electrical contact with the soil. Transmission lines with discontinuities gave an effective and efficient measure of the soil water‐content profile.
Bradford, J. M.; Grossman, R. B.
doi: 10.2136/sssaj1982.03615995004600040004xpmid: N/A
This study explored the use of the Swedish fall‐cone device for insitu measurements of shear strength near the surface of soils. A laboratory fall‐cone device was adapted for field use by replacing the original base and lengthening the cone stems. In‐situ strengths were measured on selected soils in southwestern Iowa and southeastern Nebraska. A small increase in water potential resulted in a large decrease in shear strength. A heavier cone gave a lower strength value than a lighter cone. The near‐surface strength of Sharpsburg soils in winter wheat fields was measured throughout the year. The strength immediately after fall planting was 10 to 20 kPa at about −3 kPa water potential. In the spring of 1980, largely because of consolidation from rainfall in the immediate post‐thaw period, the strength was 30 to 50 kPa. In contrast, the spring of 1981 was dry and consolidation did not occur; consequently, there was no increase in strength from that immediately after planting.
Boast, C. W.; Robertson, T. M.
doi: 10.2136/sssaj1982.03615995004600040005xpmid: N/A
A new (“micro‐lysimeter”) method for estimating evaporation from soil consists of pushing a thin‐walled cylinder 76 mm in diameter into field soil, removing the soil‐filled cylinder from the field, closing the bottom to make it water‐tight, determining the mass of the micro‐lysimeter, replacing it in the field with its top surface even with the surrounding soil, leaving it exposed to environmental conditions for a period of time (typically 1 d), and redetermining its mass. Evaporation loss from the micro‐lysimeter is the difference between the two masses.
Clothier, B. E.; Scotter, D. R.
doi: 10.2136/sssaj1982.03615995004600040006xpmid: N/A
Existing theories of Raats, Parlange, and Warrick describing constant‐flux absorption and infiltration from hemispherical cavities are reviewed. A simple theory for absorption is derived, using the flux‐concentration relation of Philip. Three‐dimensional infiltration experiments are described that were used to test these theories. The results showed that infiltration at 360 ml/h into a 4‐mm‐radius hemispherical cavity in a repacked fine sand was described quite well by the simple absorption theory for the first 100 min. Beyond this time, at this flux, the effects of gravity became evident, as expected from a comparison of the matric and gravitational potential gradients. The nonlinear, steady‐state infiltration theory of Raats gave reasonable estimates of the water content profiles behind the wet front. The linearised (constant diffusivity) theory of Warrick was of no value in this context. The success of the Raats theory adds some credence to the use in trickle irrigation design of the Wooding's theory which describes infiltration from a circular pond on the basis of the same assumptions.
Parlange, J.‐Y.; Starr, J. L.; Barry, D. A.; Braddock, R. D.
doi: 10.2136/sssaj1982.03615995004600040007xpmid: N/A
The transport of a solute in a soil column is considered for zero‐order kinetics. The visible displacement of the solute is affected by dispersion. The dispersion coefficient enters both the transport equation and the boundary condition. It is shown that the latter is the most important effect and a simple equation is proposed to describe solute transport, which takes into account the influence of dispersion in the boundary condition, but not in the transport equation. Validity and limitations of this equation are discussed in some detail by comparison with the complex but exact solution for zero‐order kinetics.
doi: 10.2136/sssaj1982.03615995004600040008xpmid: N/A
The root surface as an absorption mechanism is briefly reviewed and various absorption models are discussed, e.g., the Koshland kinetic and Michaelis‐Menten models.
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