Automated Ring Infiltrometer:
The Automated single ring infiltrometer, or Wooding infiltrometer, is used to determine the infiltration rate at saturation, and/or the saturated hydraulic conductivity of field soils, as well as of soils in laboratory columns.
With the Wooding infiltrometer, water at atmospheric pressure is allowed to infiltrate into soil. Water is held inside a 15 cm diameter soil ring 5 cm high, pushed less than 0.5 cm deep into the soil. The infiltrometer is placed on top of the ring, and used to maintain a head of water of 1 cm inside the soil ring. As water infiltrates the soil, the water level in the infiltrometer tower decreases. This decrease in water level with time is recorded, either with a pressure transducer connected to a data logger or manually. From these data the volume of water entering the soil in a given time is calculated. This information is then used to compute the saturated hydraulic conductivity of the soil. The infiltrometer can be used in the field as well as on top of soil columns.
This instrument is also valuable for demonstration purposes, and for teaching of water movement in soils in a laboratory setting.
For these purposes a 15 cm diameter by 30 cm long soil column can be used. The use of tensiometers and water content sensors placed at different depths below soil surface, would greatly add to the experiments.
Inside diameter of water tower 7.6 cm
Height of water tower 78 cm
Volume of water tower 3800 ml
Inside diameter of priming tower 2.5 cm
Inside diameter of soil ring 15 cm
Inside diameter of optional soil column 15 cm
Height of optional soil column 30cm
The automated single ring infiltrometer can also be used in the laboratory in conjunction with 15 cm inside diameter soil columns. No other soil ring is necessary. The infiltrometer is simply set on top of a 15 cm plastic or steel cylinder, which is filled with soil as shown in the figure. The cylinder is filled with soil till 3.5 cm from the top. When ready, open the clamp on the priming tower, and water starts to fill the space below the infiltrometer inside the soil ring, and then enters the soil. The rate of infiltration of water into the soil is highest just after starting the infiltration and declines over time. After some time the infiltration rate changes little, and appears constant. The “final” rate of infiltration then equals the saturated hydraulic conductivity of the soil in the column.
To measure the advance of the wetting front (which can also be observed and measured directly if clear plastic is used for the soil cylinder), tensiometers (available from SMS) and other moisture sensing devices can be added through the wall of the soil cylinder.
The output from the pressure transducer on top of the infiltrometer, along with the output from the various moisture sensing devices (if applicable) can be recorded with a Campbell Scientific datalogger for later transfer to a computer.
For soil columns having an inside diameter of 15 cm, water flow is one-dimensional. However, because the cross sectional area of the water tower (45.4 cm2) is smaller than the cross sectional area of the soil column (176.7 cm2) a 1 cm drop in the water level in the water tower causes a 0.257 cm depth of infiltration. Thus to calculate the ”final” rate of infiltration (which is assumed to equal the saturated hydraulic conductivity) one has to multiply the decrease in the water level in the water tower over a given time interval by 0.257.
In the field, water flow out of the soil ring is three dimensional, and thus additional information is needed to calculate the hydraulic conductivity. We use the solution Wooding developed for 3-D infiltration from a circular ponded area, and further assume that initially cumulative infiltration is proportional to the square root of time. Initial and final water contents of the soil in the ring also need to be measured. The infiltration data and water content data are then entered into Dr. Wooding’s equation to get the saturated hydraulic conductivity of the soil.
The method itself and the data analyses are straightforward. However, in actual practice the method greatly benefits from continuous data collection with a datalogger. Suitable data loggers are available from Soil Measurement Systems or Campbell Scientific, Inc.
Dr. Wooding, R.A. Steady infiltration from a shallow circular pond. 1968. Water Resour. Res. 4: 1259-1273
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