The Hood Infiltrometer (Punzelmeter)

Watch Dr. Punzel in action

The hood infiltrometer or Hauben-infiltrometer or sometimes called the Punzelmeter is invented by Dr. Jurgen Punzel (left), measures the unsaturated hydraulic conductivity (K) of the soil in the field. The infiltration is done by first placing a hemispherical hood filled with water directly on the soil surface. The hood is connected to a Mariotte bottle, which controls the suction of the water on the top of the soil. The negative pressure controlled by the Mariotte bottle compensates the depth of water ponding and hence, water can be supplied at different pressure heads. The experimental hood infiltrometer setup allows the measurement of hydraulic properties from saturation up to the air entry value of the soil. The effective pressure head on the soil surface can be measured by an U-tube manometer with a precision of ± 1 mm (Punzel and Schwärzel, 2004).

Steady-state infiltration rates at hydraulic pressures between 0 hPA (cm water) and the air entry pressure of the soil system (up to 16 hPa) can be measured (Buczko et al, 2006b). The sequence of applied pressure heads can have an effect on the estimated hydraulic conductivity values (Bagarello et al., 2000).

The pressure head and the bubble point can be measured directly via an U-pipe manometer and the stand pipe of the hood. Vertical infiltration is initially governed by capillarity or sorptivity of water into the soil matrix, containing both vertical and horizontal components. Later-time infiltration becomes gravity driven and linear with time as soil capillarity forces are reduced, indicating that infiltration is at steady state.

Although the hood infiltrometer is different in its design to the tension and ponded disc permeameters its environmental and agricultural implications are still very much the same. The measurements taken by the hood infiltrometer are converted to a measurement of the soils' unsaturated hydraulic conductivity. Measurements taken between air entry point and saturation are able to give an indication of the infiltration characteristics at the soil surface.

The infiltration is done by placing a circular shaped hood (base diameter of 16 cm) filled  with water directly on the soil surface. The circular contact line between the hood rim and the soil is sealed with medium textured sand. Compared with the tension disc infiltrometer, the hood infiltrometer requires little preparation on the soil surface and no contact materials (Buczko et al., 2006a). The pressure head in the water filled hood is regulated by a Mariotte water supply. The effective pressure head at the soil surface can be adjusted between zero and any negative pressure up to the bubble point of the soil. Although no preparation of the soil surface is required, unlike the tension and ponded disc infiltrometers  vegetation should be cut down to about 5mm high and a level site should be chosen.

The hood infiltrometer is set up as per the diagram below.

1) Preparation: The bubble tower is then filled to mark "B" by pulling out the air pipe and by use of a funnel. The infiltration Resevoir (5) is then filled to mark "I" with valves V1, V2 and V3 remaining shut. The next step is filling the U- tube manometer (8) to the zero mark. The hood (2) is connected to the infiltrometer through the connecting tube and the U-tube manometer through the hose (9), and connected to the air escape hose (10). The outer ring of the hood is initially placed onto the ground and pressed into the soil a few millimeters and the hood is centred in the outer ring. The infiltrometer is brought into a vertical position and the connecting hose is bent into place (11). Finally the U-tube manometer is installed.

2) Filling the hood: The next step involves filling the hood. The submergence depth "T" is set about 2cm higher than the infiltration chamber height "Hk" to create a hydraulic pressure head of 0 beneath the soil surface. The "V1" valve is slowly opened to fill the connecting hose (11) and overflow chamber (3). Air is withdrawn from the hood using a syringe at hose (9) until the water table in the standpipe (4) is approximately mid- scale. The gap between the outer ring and hood is filled with fine sand and sprinkled with a wash bottle. Valve "V2"is opened to expel air from the hood and may be further assisted by opening "V3" if required whilst maintaining the negative pressure under the hood at a desired value (the water pressure 'Us' must always exceed the water scale at the stand pipe (4)). When the water table has reached the mark on the hood "V2" is closed, awaiting the bubble of the mariotte water supply system.

3)  Measurement: The effective water tension on the soil surface may be chosen via the depth of the submergence "T" by changing the depth of the air pipe in the bubbling tower. Deeper insertion leads to higher T values and increased negative tensions (-h= Us-Hs). Initially a water tension of 0 is selected which is achieved by pulling the air pipe slowly until bubbling is initiated in the bubbling tower. When bubbling occurs it is necessary to adjust the pipe so that the calculated tension (-h) is equal to 0. The drop in water level in the marriote bottle is recorded over time. When infiltration has reach a steady state rate the water tension is decreased to more negative values by inserting the pipe into the bubbling tower and decreasing T. Once again the aim is to reach steady state infiltration noting change in water level in the marriote supply, decreasing the tension until bubble point. The effective bubble point is determined by shutting "V1" inducing a pressure rise on the U-tube manometer. This can then be used to calculate the maximum US (BP= Us_max-Hs).

Site

Supply potential (mm)

Steady-state rate (mm/s)

K (cm/day)

C1

0

0.088

360

-20

0.072

298

C4

0

0.188

D1

0

0.152

1287

-15

0.074

622

D2

0

0.053

361

-25

0.024

162

-31

0.018

119

D4

0

0.073

516

Frequently applied procedures for  analysing field measured infiltration rates are based on steady state conditions. Apparent, steady infiltration rates measured in the field frequently overestimate true steady rates. Studies have also found that this methodology is not appropriate in water repellant soils (Buczko et al., 2006b). Infiltration has to be started by ponding (applying a positive head), then adjusted to a negative pressure. The device is also complex to setup and difficult to use.

References

Bagarello, V., Iovino,M.,Tusa, G. (2000). Factors affecting measurement of the near-saturated soil hydraulic conductivity. Soil Science Society of America Journal, 64,1203–1210.

Buczko,U., Bens, O., Hüttl, R.F.  (2006a).Tillage Effects on Hydraulic Properties and Macroporosity in Silty and Sandy Soils. Soil Science Society of America Journal, 70, 1166-1173. http://soil.scijournals.org/cgi/content/full/70/6/1998

Buczko,U., Bens, O., Hüttl, R.F.  (2006b). Water infiltration and hydrophobicity in forest soils of a pine–beech transformation chronosequence
Journal of Hydrology, 331, (3-4),  383-395.

Clothier, B.E., White, I., 1981. Measurement of sorptivity and soil water diffusivity in the field. Soil Science Society of America Journal 45, 241-245.

Dirksen, C., 1975. Determination of soil water diffusivity by sorptivity measurements. Soil Science Society of America Proceedings 39, 22-27.

Dixon, R.M., 1975. Design and use of closed-top infiltrometers. Soil Science Society of America Proceedings 39, 755-763.

Punzel, J., Schwärzel, K., (2004). Hood infiltrometer – a device for undisturbed field measurements of saturated and near saturated conductivity. SuperSoil 2004: 3rd Australian New Zealand Soils Conference, 5 – 9 December 2004, University of Sydney, Australia. http://www.regional.org.au/au/asssi/supersoil2004/s15/poster/1409_punzelj.htm

Topp, G.C., Zebchuk, W.D. (1985). A closed adjustable head infiltrometer. Canadian Agricultural Engineering, 27, 99–104.

Wooding, R.A. (1968). Steady infiltration from a shallow circular pond. Water Resources Research, 4, 1259–1273.