Description The ponded disc permeameter has a circular base that is placed on the area of the ring. Water then flows out of the water towers and is controlled by the steel ring inserted 5mm in the soil. The smaller water tower supplies the initial head of water to the surface of the soil. Once this tower has drained and the infiltration pond of 5mm has been reached; water will start to flow out of the larger tower. Principle The ponded disc permeameter is a field apparatus that is used to measure the hydraulic conductivity (steady state infiltration rate) of a saturated soil (Ksat) under a small positive potential between 0 and 10mm. When used in the field, water is applied to the soil at a constant pressure through a circular pond. The large water towers of the disc permeameter and a metal ring (imbedded in the soil surface) control the water at a constant depth of 5mm. It is the change in height of the large water tower that is measured at time intervals.
Methods The ponded disc permeameter is quite easy to set up in the field. At the site chosen for a measurement, a large steel ring is inserted in to the soil at a depth of 5mm. Vegetation or rocky material on the surface of the ring area is cleared. The ponded disc permeameter is fitted with water, which is measured to a depth where it will create an infiltration pond of 5mm. To do this the apparatus is placed in a bucket of water so that both towers can fill with water. Once the experiment is ready to start the circular base of the ponded disc permeameter is placed on the area of the ring. Water that flows out of the water towers is controlled by the steel ring inserted 5mm in the soil. The smaller water tower supplies the initial head of water to the surface of the soil. Once this tower has drained and the infiltration pond of 5mm has been reached; water will start to flow out of the larger tower. The height of water in this tower is recorded at specific time intervals which is used in the calculation of the rate of infiltration.
	The important process of filling the ponded disc with water

Analysis of data

The ponded disc permeameter measures the steady state infiltration rate taking into account the effect of the sorptivity and gravitational flow of the soil. Sorptivity is a combination of the influences capillary action and adhesive forces have on the soil solid surfaces (White et al, 1992). The sorptivity of the soil greatly depends on the moisture content of the soil, that is if the soil is dry then the sorptivity is high. The gravitational flow is a constant value, which is affected by the size, continuity and distribution of the soil pores.
The short and long-term movement of the water through the soil affects the steady state infiltration rate.
The equation below is used to determine the short term cumulative infiltration due to capillary forces:

                                               I = S t0.5                                                (1)

Where I is the cumulative infiltration in cm, k is the gradient cm min-0.5 (taken from the line of best fit of the cumulative infiltration vs. time), and t is time in min. The value calculated for I therefore gives the sorptivity (S) of the soil.

The long term infiltration rate or steady state Infiltration Rate (A) is calculated from this equation:

                                                  I = A 2 t                                                  (2)

The wooding equation is then used to find the steady state infiltration rate (saturated hydraulic conductivity) of the soil without the effects of sorptivity.

                                                            (3)

Where: k0 = saturated hydraulic conductivity
A = steady state rate
S = sorptivity
r = 10cm
= final moisture content
= initial moisture content

Results

The ponded disc permeameter was measured at 4 of the 10 sites at Narrabri, 2 were located on the cultivated soil and the other 2 were measured on pasture soil. At each of the 4 sites specific time measurements and heights were recorded off the ponded disc permeameter. Once back in Sydney these results were analysed using the statistical program JMP. The results of the analysis are as follows:

The analysis of the data showed that overall the pasture field soil is dryer than the soil in the cultivated field. This conclusion was taken from the sorptivity results, as the pasture soil has a higher sorptivity reading than the cultivated field.While the low saturated hydraulic conductivity (k0) measurements for the pasture soil suggest the faster infiltration rate is due to the presence of cracks. There is a large difference between P3 and P5 saturated hydraulic conductivity measurements; the cracks in P3 are most probably larger than the cracks in P5. This suggests the cracks in P5 are closing or are more closed than the ones in P3. Cracks that are present in the soil affect the rate at which water infiltrates the soil; therefore due to the time fact the double ring experiment would produce better results for the pasture soil. The experiment would have to be conducted over a 3 day period to eliminated the time taken for water to fill the cracks. The soil in the cultivated field was much wetter than the soil in the pasture field. The sorptivity and the saturated hydraulic conductivity results are much lower, therefore the the infiltration rate is slower and at a steadier rate.

Problems with method

The advantage of using the ponded disc permeameter is, that it is very easy to set up, operate and transport. However the disadvantages of the ponded disc permeameter include:

• human error when taking measurements of height and time

• human error in setting up of experiment

• the presence of cracks and macropores does not give a good representation of the infiltration rate of the surrounding soil, water can be lost quickly through the soil.

• leakage of water into the soil outside the ring area may cause incorrect results. As the water in the towers will run out before the steady state infiltration has been reached.

• Inaccurate sorptivity readings may be a result of field conditions. If the soil is wet before measurement the high clay content could cause swelling therefore changing the sorptivity value.

References

Hillel, D. (1998). Environmental Soil Physics. Acedemic press, San Diego USA.

Perroux, K.M., and White, I. (1988). Designs for disc permeameters. Soil Sci. Soc. Am. J. 52, 1205-1215.

White. I, et al, 1992. Measurement of Surface-Soil Hydraulic properties. Advances in Measurement of Soil Physical Properties: Bringing Theory into Practice. Soil Science Society Of America Special Publication No.30. SSSA, Madison, Wisconsin.