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Author: Adam Pirie |
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Hydraulic conductivity refers to the ability of a porous medium to transmit a liquid, when the matrix of the media is saturated the rate of conductivity is known as the saturated hydraulic conductivity. One device used to measure this conductivity is the constant head borehole permeameter or amoozemeter. The amoozemeter has been designed to measure the saturated hydraulic conductivity (Ksat) of the vadose (root zone) from the surface to a depth of two metres. |
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· The amoozemeter allows a constant head of water to be placed in the vadose zone by the auguring of a thin hole to the depth of interest in the soil and then placing a water dissipation device down the hole. · Water can then leave the dissipation device provided the tension created by the air’s path through the head tubes can be overcome. Each head tube can be filled with up to 50cm of water, thus being able to provide up to 50cm of tension on water leaving the dissipation device. Through the joining of tubes greater levels of tension can be placed on the water trying to move out of the dissipation device. · Tubes 2, 3 and 4 have a fixed bubble tubes so provide -50cm of tension and by varying the water level in tube 1 any required tension (up to -2m) can be produced. The amount of water stored in the head tubes should be the depth of the hole – the depth of head required to be maintained in the hole. · A flow-measuring cylinder is also incorporated in the amoozemeter allowing infiltration to be measured at a variety of time intervals until a constant rate of infiltration is measured, once this is achieved the experiment is stopped and Ksat can be calculated. The set up as described is shown below. |
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(A. Amoozegar) |
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Four sample pegs were chosen at the Narrabri experiment site, two each from the cultivated (C2, C4) and pasture sites (P2, P5). Peg location At these points holes were then augured (~1m) and the infiltration rate measured. The measured infiltration rates were then entered into JMP and plotted, when infiltration had become constant a line was fitted and this was taken to be the steady state infiltration rate as shown below.
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From this rate the saturated hydraulic conductivity was then calculated using the Glover solution. |
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The Glover solution is simple and requires only a single measurement of steady state rate water flow (A. Amoozegar) |
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Adapted from "Laboratory and Field Soil Physics-Notes and Exercises" |
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As can be seen from the above results there appears to be a relationship between hydraulic conductivity and management practice, it can also be seen that the conductivity of the soil is moderate - slow for all pegs. |
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The calculated Ksat values for each site were then entered into JMP and analysed for statistical significance using a one-way analysis of variance as well as a means comparison through the Student's t test.
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Although there appears to be a relationship between hydraulic conductivity and management practice, statistically this was found to be untrue. It is highly possible that with a larger data set that significant results would be found. However it is questionable the effect that cultivation would have had on subsoil structure and hence infiltration rates due to it's distance from tillage implements, though the effects of compaction cannot be discounted. |
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The major benefit of this technique is that it allows the measurement of subsurface conductivity and hence the calculation of Ksat without having to engage in the digging of a pit. |
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Problems however can be encountered through the smearing of hole sidewalls as auguring is being completed. Pores that have been smoothed over are unable to conduct water and significantly reduce the infiltration rate; this is a problem in clayey soils such as the one at the study site. Perhaps this can account for differences in infiltration rates between the two sites. The smearing of walls can be overcome through the application of resin to a mop-head which is then stick down the hole until dry and then removed (McBratney, pers. comm). Capillary rise was also observed to be occurring in some of the holes, this would be best easier to account for with modelling if the borehole was lined and water was only able to exit through the bottom of the hole. |
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Amoozegar, A. (1992) Compact Constant Head Permeameter: A convenient Device for Measuring Hydraulic Conductivity. Soil Science Society of America. Advances In Measurement of Soil Physical Properties: Bringing Theory into Practice. McBratney, A. Minasny, B. (2004) Laboratory and Soil Physics. Faculty of Agriculture, Food and Natural Resources. University of Sydney. |
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