Description A tension disc permeameter measures the unsaturated hydraulic conductivity. It consists of two water towers over a circular base and water flows out through a nylon mesh. The smaller tower creates the tension that is applied to the soil. The amount of water that infiltrates is measured using the attached ruler. These measurements are then used to determine sorptivity, gravitational flow and unsaturated hydraulic conductivity. Principle The tension disc permeameter was created by the CSIRO. This apparatus is used to measure the unsaturated hydraulic conductivity (Kus) and the sorptivity of a soil. These variables are measured by putting the soil under a negative tension, forcing the soil to “suck” the water from the water chamber. This apparatus is used agriculturally and environmentally to quantify the effects of macropores and preferential flow paths (Dane and Topp, 2002). A clearer understanding of these properties helps land managers to apply the water balance when analysing irrigation quantities (McKenzie et al, 2002).

Brief description of method Before the site is to be analysed an area needs to be cleared, i.e. any plants and debris in the area need to be removed. A 20cm ring is placed on the cleared area and filled with fine sand (few millimetres in thickness). The sand is then leveled and the ring removed, creating a flat surface for the tension disc permeameter to be placed on. The sand creates a good contact for the permeameter and the soil below. The water tower is filled and the bubble tower is filled to 2cm. The height of the water in the bubble tower creates a negative tension between the water tower and the soil. This causes the pores 2cm or greater to be excluded from the water flow. After the apparatus is placed on the levelled sand, timing begins when the bubbling begins in the bubble tower. Measurements are taken every 15seconds for the first minute, every 30seconds the next five minutes and every five minutes until a constant reading is obtained. At the site tension disc permeamter measurements were taken at P2, P4, C2 and C4.

Analysis of Data

When analysing the data obtained the effects of both sorptivity and gravitational flow must be calculated. Sorptivity is a combination of capillary action and adhesive forces on the soil solid surfaces and is dependent on the moisture content of the soil. Gravitational flow is defined by the gravity constant, which is different for each soil. The gravity constant is influenced by the pore size, continuity and distribution of the rate of water flow through soil under the influence of gravity. The sorptive forces govern the initial infiltration rates, i.e. when the soil is dry, and after the soil has wetted up the gravitational forces dominate. Phillip's equation is used to combine these parameters to analyse infiltration:

I = S t0.5 + A t

Where I is the cumulative infiltration, S is the sorptivity (mm/h 0.5), t is time (hrs) and A is the steady state infiltration rate. A sorptivity value can be estimated from the tension disc permeameter measurements by plotting the cumulative infiltration by the square root of time, the sorptivity is then the gradient of this line.

Once these values were estimated the hydraulic conductivity was then calculated. The hydraulic conductivity is the same as the steady state infiltration rate, this can be found using the wooding equation. This equation finds the infiltration of the soil without the effects of sorptivity.

The wooding equation

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

Results

4 of the 10 sites at Narrabri were analysed using the tension disc permeameter. 2 of the sites were in the cultivated area and 2 were in the uncultivated area. The specific time measurements and the heights from the tension disc permeamter were recorded at each of the sites. Once back in Sydney the results were analysed using a computer program called JMP. The results of the analysis are shown in the table below.

The results from the tension disc permeameter trials at Narrabri, NSW.

From the above results an obvious difference between the cultivated and uncultivated sites can be seen. The initial saturated hydraulic conductivity of the uncultivated sites is significantly lower than that of the cultivated sites (0.686 and 0.04). However due to human error and changing of apparatus may have caused an error in the measurements thus resulting in a lower K0 for C4. Plant roots promote the movement of water through the soil profile by creating pathways and through the uptake of water. Therefore the initial saturated hydraulic conductivity of the uncultivated sites is low and for the cultivated sites is higher. Another reason for the higher K0 for the cultivated sites is that the soil structure has been damaged by tillage practices.

This is important for agricultural and environmental management as this experiment displays the difference between the cultivated and uncultivated soils. Therefore it is important to understand that the removal of plants affects the movement of water and for the heavy clays of the region this could cause waterlogging if mismanaged. However managers are able to calculate the initial saturated hydraulic conductivity of their soil to determine how wet or dry and then they can use this to determine if irrigation is needed.

Problems with method

The levelled sand may impede with the early stages of the infiltration rate leading to inaccurate sorptivity values. Since the soil being tested is clayey swelling may occur during the measurements, this could be avoided by wetting the soil before testing. The temperature of water determines its viscosity and therefore could determine the speed at which the water is infiltrated into the soil.

References

McKenzie N.,Coughlan K., Creswell K., 2002, Soil Physical Measurement and Interpretation for Land Use, CSIRO publishing.

Dane, J.H. and Topp, G.C. (Co-editors), 2002, Methods of Soil Analysis: Part 4 Physical Methods, Soil Science Society of America, pp 981-983.