The vertical movement of water in a soil is called infiltration. The ease or difficulty with which water can pass into and through a soil profile is important so as to avoid detrimental effects such as compaction, surface smearing and other properties that generally lead to structure decline.

To look at the hydraulic properties of the first 20mm of surface soil in the field, ponded single-ring and tension disc permeameters are used (Geering, 1995). The infiltration characteristics are based on three dimensional flow and properties such as sorptivity and hydraulic conductivity can be established from these methods.
 
 

Disc Permeameter
 

The disc permeameter allows the determination of hydraulic conductivity over a range of negative potentials. Water is supplied to the soil through a porous membrane from a water tower. The required potential of this water is set by adjusting the height of water in a second smaller bubble tower. Such things as soil hydraulic conductivity, soil diffusivity, macroscopic capillary lenght and representative pore size can be measured (White. et al, 1992).

Using a ponded disc permeameter , water is supplied to the soil surface via a shallow circular pond at a constant supply pressure. In this method the smaller of the water towers is used to supply an initial pond of water between the plate and soil surface for the start of infiltration. This apparatus can be used to measure hydraulic conductivity under small positive potential between 0 and 10mm.

The advantages of using a disc permeameter is, that they are very easy to set up, operate and transport. Obtaining data from these permeameters is a lot easier than the double ring infiltrometer, although it is a lot more complicated to analyse due to the flow being in three dimensions. When analysing this data absorption and capillary forces, which act in all directions, and the geometry of the water source have to be considered. (White et al, 1992). When using the tension disc permeameter, a good intimate contact between the disc and the soil surface needs to be established. This is often achieved by using a contact material such fine sand as was used at Narabri. A drawback of using such a material is that it will interfere with the measurements especially in the early stages of infiltration giving inaccurate sorptivity values. Another disadvantage when using the ponded infiltrometer is that if there is a large macropore in the site the water tower may not be able to supply water quick enough, also causing inaccurate results.

During infiltration events, the water enters the soil in response to potential gradients of water potential and gravitational potential. The water potential term is governed by the dryness of the soil and the pore structure of the soil. These two factors combine to form a sorptivity factor which is made up of the combined influences of capillary action and adhesive forces to soil solid surfaces. The sorptivity of the soil is often expressed as "S" . The gravity term is a constant for different soils and is due to the impact the pore size, continuity and distribution on the rate of water flow through soil under the influence of gravity This term is known as "A".

The initial water infiltration rate is largely governed by the sorptive forces of the dry soil, this is then replaced once the soil wets up by the gravitational term. Thus equations describing infiltration can be made these include

I = S t^0.5 + A t

The sorptivity values obtained which are presented in Table 2, are quite high. This could be due to too much sand being used when preparing a contact surface for the tension disk. The sorptivity values for the ponded disc were significantly higher than the vales obtained from the tension disc. This is due to the ponded disc having an excess of water at the soil surface at positive pressure. This compares with the tension disk which supplies the water at a negative pressure and thus excluding some pores.
 

 Borehole Permeameter

The hydraulic conductivity is a measure of the ability of a porous medium to transmit water. Hydraulic conductivity under saturated conditions is called the saturated hydraulic conductivity. Methods for measuring this include using the double ring infiltrometer and the Amoozegar permeameter, both of which were used at Narabri. These methods imitate conditions for such things as flood irrigation and/or rain events that leave water ponded on the surface. Surface condition, structure ,swelling properties of the soil, initial moisture content and macropores will influence infiltration.

The principle behind the Amoozegar permeameter is that it provides a constant head of water at a known depth down a hole. By measuring infiltration over time , saturated hydraulic conductivity can be calculated once steady state flow has been reached.

The equipment consists of four constant head tubes, a four litre water reservoir, a one litre flow measuring cylinder, a water dissipating unit, and a base with a three way valve. The four constant head tubes can provide up to -200cm of water pressure. In each constant head tube is another tube referred to as the bubble or air tube which allows the potential to be varied.

The Amoozegar permeameter is a convenient way to measure hydraulic conductivity as it is very easy to transport and set up in the field. With the four constant head tubes , it is possible to maintain a constant head of water up to two meters below the soil surface and therefore measure the hydraulic conductivity anywhere from the soil surface to a depth of two meters. A drawback when using this device is that during augering, smearing of the hole may occur and consequently interfere with infiltration. Smearing at the bottom of the auger hole was overcome at Narabri by applying Araldite and then peeling it off. This created an undisturbed surface and worked quite well.
 
 

Double Ring Infiltrometer
 

The double ring infiltrometer is a way of measuring saturated hydraulic conductivity of the surface layer, and consists of an inner and outer ring inserted into the ground. Each ring is supplied with a constant head of water from a mariotte bottle. Hydraulic conductivity can be estimated for the topsoil when the water flow rate in the inner ring is constant.

Having the two rings, eliminates the problem of overestimating the hydraulic conductivity in the field due to three dimensional flow. The outer ring supplies water which contributes to lateral flow so as the inner ring is contributing to the downward flow.

Water moves from the mariotte bottles into the rings via a tap at the base of the vessells until the height equals that of the base of the bubble tube. When water moves into the soil, reducing the height of ponded water to below that of the bubble tube, more water is fed into the ring.

Some draw-backs of the double ring are that it is very time consuming, requiring trial and error when adjusting the bubble tubes to get the water levels in each ring equal. The practicality of the instrument is reduced by the fact the rings are extremely heavy to move. It also requires a flat undisturbed surface which sometimes is not available. During the experiment it is sometimes necessary to refill the mariotte bottles. To do this, the tap has to be turned off and this disrupts the experiment.