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.
Field and laboratory determination of saturated hydraulic conductivity.
Low water volume needed.
Fast results on most soils.
Accounts for three dimensional nature of infiltration from a circular source.
Made of polycarbonate and acrylic materials.
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.
References: Dr. Wooding, R.A. Steady infiltration from a shallow circular pond. 1968. Water Resour. Res. 4: 1259-1273
Inside diameter of water tower
Height of water tower
Volume of water tower
Inside diameter of priming tower
Inside diameter of soil ring
Inside diameter of optional soil column
Height of optional soil column