Falling-head Double Ring Infiltrometer
	Home Principle Description of apparatus Analysis of Data Results Problems with method References

Principle of how it works:

The double ring infiltrometer is a way of measuring saturated hydraulic conductivity of the surface layer, and consists of an inner and outer ring inserted into the ground. Each ring is supplied with a constant head of water either manually or from mariotte bottles. Hydraulic conductivity can be estimated for the soil when the water flow rate in the inner ring is at a steady state.

It works by directing water onto a known surface area due to the parameters of the inner ring. The rate of infiltration is determined by the amount of water that infiltrates into the soils per surface area, per unit of time. Infiltration can be measured by either a single or double ring infiltrometer, with preference usually lying with the double ring because the outer ring helps in reducing the error that may result from lateral flow in the soil.

The outer ring virtually turns a 3D single ringed system of looking at infiltration into a one dimensional model by allowing water in the centre ring to flow nearly exclusively straight down. This allows much easier calculation by taking out the need to account for lateral flow. This schematic diagram shows the vertical flow that is generated form the inner ring which is allowed by the outer ring accounting for all the lateral flow. Source: (www.sdec-france.com).

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Description of apparatus/method:

The arrangement of the rings in the ground after being knocked in, applying the water to the centre ring, then finally measuring infiltration at certain time intervals.

Before the double rings can be placed in position the ground cover must be removed without disturbing the soil surface. Once this is done the rings can be set in position and knocked into the ground about 10cm, or until the rings are set firmly in the ground.

Water can be supplied inside the rings either by a marriotte bottle assembly, to keep a constant head or manually. When filling up the rings the outer ring is filled first so that the soil profile around the inner ring would be wet and only vertical flow would occur when the inner ring is filled later. When filling the inner ring water is poured in on a plastic sheet, then once the desired amount of water is in the ring on the sheet, the sheet is removed quickly. The level of the water is recorded at this point as the start of infiltration.

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Analysis of Data:

Data is gained by a drop in water height, giving an infiltration of water over time. These data points are plotted in a graph of infiltration versus time. Theoretically once the soil becomes saturated a steady state infiltration rate will be reached, which is seen on a graph as a section of data points infiltrating showing linearality. Once this linear section is identified other outlying data points can be excluded from the analysis and a line of best fit can be fitted to the applicable data. The gradient of this fitted line gives the steady state infiltration rate for that particular soil. The steady state infiltartion rate can then be multiplied 1.45, to obtain the hydraulic conductivity (Ksat), of that soil. This is a measure of how conductive a soil is at saturation, which is a measure of permeability.

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Results:

Graphs show the infiltration rate over time at the pasture sites.

At pasture site 1, shows a steady state infiltration rate being reached at around 10 minutes. The infiltration rate before this is fast and the steady state infiltration rate is uniform, giving good results. At pasture site 4, also has a fast initial infiltation rate, which does not settle down to give a uniform steady state infiltration rate. The pasture site had a high presence of cracks in the soil, which manipulates the infiltration rate, until the profile is totally saturated and equilibrated. unfortunately this point was not reached at site 4 as the experiment was not run for long enough, or until a unifrom steady state infiltration rate was reached.

Graphs show the infiltration rate over time at the cultivated sites.

For the cultivated sites the graphs show a high initial infiltration rate which settles to a slow steady infiltration rate. This slow steady state infiltration rate is due the cultivated sites being clear of vegetation, and being cultivated their structure easily swells when wetted up and disperses to impede infiltration. The experiment should have been run for longer to obtain more data points at the end of this experiment, and to assure that the steady state infiltration rate had actually been reached.

Steady state infiltration rate in the pasture is over estimated and influenced by the cracks that were present in the surface. A steady state infiltration rate also was not reached as the measurable head ran out before it was reached. The infiltration rate of the cultivated soil is steady to start with, and reaches a very slow steady state infiltration rate. This is due to the surface soil being covered over by the topsoil slacking and dispersing, impeading infiltration.

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Problems with method:

Some draw-backs of the double ring are that it is very time consuming, requiring frequent attention, either by recording measurements or by maintaining equilibrium in the height between the rings. The practicality of the instrument is reduced by the fact the rings are extremely heavy to move. It also requires a flat undisturbed surface which sometimes is not available. The infiltration rate varies with different soil types, which can effect the accuracy of the results. The soil type that the above results were obtained from is a Vertosol. These soils are known for their shrink/swell properties, which result in cracks when dried and can lead to misleading results. The double ring method is not reliable on soils with vertic properties. (McKenzie et al., 2002).

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References:

http://www.sdec-france.com/us/doubleanneaux.html

McKenzie N, Coughlan K and Cresswell H. (2002) Soil Physical Measurement and Interpretation for Land Evaluation. (CSIRO Publishing: Melbourne).

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