Principles:

Known boundary and pressure conditions are used to determine the unrestricted 3-dimensional infiltration capacity, of a soil confined below the single, steel ring. The ability for the soil to transmit water (be permeable) is hence determined through steady state calculations and, this Saturated Hydraulic Conductivity (Ksat), which is universally used as an empirical constant to quantify specific, soil-water flow and infiltration capacity problems. Ksat “is one of the most important factors playing a role in many agronomic, engineering and environmental activities” (Herman et al, 2003) and hence, is a useful term to quantify.

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Methods:

A single steel ring with dimensions (20cm diameter and 17cm tall) is inserted 2cm into the soil surface.

It is with 2L of fresh water by sealing with a plastic sheet, pouring the water over the plastic and pulling the plastic slowly.

The initial height is measured once all the water is released and time initiated.

Further water level (height) measurements are taken at 15-second intervals for the first few minutes and, after 2 minutes another 15 seconds is added to the interval period (subject to intensity of infiltration; figure 2).

This process is continued until all the water has infiltrated or a steady state rate is recognisable.

The single ring infiltrometer was used at all 10 sites within the study area and at the soil pit.

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Filling ring with water and taking initial measurement.

Method for taking continuous height measurements.

Analysis:

Analysis is completed in the JMP5.1 statistical analysis program.

For each individual site, the x,y distribution of time against cumulative infiltration is displayed, so that the point steady state infiltration can be visually determined.

A line is then fitted along the steady state portion of the graph. Within the resulting linear equation, the gradient is equated as the soil-water flux (q).

Using q = Ksat.{(C/G.pi.r)+1} the Ksat (mm/h) of the soil can be determined as all other components are known or estimated.

q = statistically determined (cm/min)

C = wetting front potetenial (estimated for clays at 150 cm)

G = shape factor for infiltration (0.8456)

pi = 3.1415...

r = radius of ring (10cm)

An example is shown in figure 3, with the steady state infiltration represent by the red, fitted line and nearby equation. The gradient and hence, q = 0.0025 (cm/sec) which, is then inserted in the above equation to compute the Ksat, as summarised below in the results section.

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JMP 5.1 output for analysis of C1, linear equation sited (note the exclusion of points not in a steady state of infiltration, the red line represent the steady state function).

Results and Discussion:

A comparison of Ksat values for land use concludes that soil under pasture has a characteristically higher Ksat value as shown be the distribution of values and the summarized data, to the right and below, respectively.

Hence, it is extrapolated that pasture soils have a higher ability to transmit water within the soil profile. This is due to the interaction of a number of soil physical and hydrological factors. Soil under pasture is more stable and less likely to slake and disaggregate and therefore, not restrict water movement. More so, there is also a greater chance of plant roots and macro pores promoting infiltration, because of the pastures greater soil structure.

Distribution of Ksat values as a function of 0.land use.

The soil pit Ksats, although, positioned within a vacant pasture were low primarily due to them being placed on a platform almost 1m within the soil profile. These sites were either picked back and left rough (Pit: site A) or smoothed over (Pit: site B) and hence, had low and differing Ksat results evident in the table below. This introduced the effect of surface condition as the two methods of leveling the soil, resulted in varying Ksat values.

Summary of results for single ring infiltrometer.

The extreme outlier at P1 is distracting, however could simply be put to a large crack in the profile or an incorrect experimental practice, resulting in the rapid infiltration or loss of water.

In conclusion , single ring infiltrometers in conjunction with Darcian flow equations are very important in water management issues; in regards to irrigation, contaminant and solute movement. Flow can inexpensively be quantified and useful parameters (Ksat and flow fluxes) can be deduced, which expand the usefulness of single ring infiltrometers (Wu, 1998).

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Problems:

Single ring infiltrometers measures all dimensions of flow, as represented below and therefore, has limited applications as vertical flow is the critical flow concerning agriculturalists and environmentalists.

Infiltration is influenced by ring size, ring insertion depth and soil type, therefore the process is site specific and limited accordingly (Wu and Osten).

Ksat calculations are highly sensitive to sample size, flow geometry, sample collection procedures and soil physical-hydrological properties such as clay content and mineralogy and therefore have many areas for error during calculation. (Herman et al, 2003).

Conceptual 3-D wetting front.

The falling head technique is limited by 2L volume of water, and therefore sometimes does not indicate a steady state, which is needed for subsequent analysis.

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References:

Herman, S., Mertens, J., Timmerman, A., and Feyen, J. 2003. Comparison of tension infiltrometer, single-ring pressure infiltrometer and soil core Ksat estimates on a sandy loam hill slope. Geophysical Research Abstracts, Vol. 5, 02662, 2003.

Wu, L., and Oster, J. Instrument for Plant and Water Management. Department of Soil and Environmental Sciences, University of California. http://esce.ucr.edu/WU/CETRAIN.html

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