Falling Head Lined Borehole
Author: Anna Houston
 
 

Description

 

Principle

The falling head lined borehole method is used to determine the saturated hydraulic conductivity (Ksat) of soil. This method will detemine the sub-surface conductivity. Using the lined borehole allows for a 1-D study of the steady state infiltration rate whereby lateral flow is hindered by the lining apparatus. The Ksat can be calculated once the steady state has been determined. This method of determining the Ksat is based on the Philip-Dunne Permeameter which is a relatively new approach in the field. The lined borehole is simple to use and inexpensive in the field.

 

The Ksat is a measurement of the ability of a soil to transmit water, this ability is effected by the variabilty of pore size, connectivity and texture of the soil. It is also one of the most difficult parameters to determine in the field due to the relative uncertainty and variabilty (Munoz-Carpena et al, 2002). Therefore when studying a field it is nessesary to obtain many points of data to to reach a reliable estimation of Ksat.

Measuring Ksat has become very important agriculturly and environmentally. Understanding how the soil transmitts water through the profile will aid irrigators on how much water to apply before water is lost past the root zone. This will decrease water use, being cost effective for the user and a positive outcome for the environment. Knowledge of Ksat is required when using chemicals on land or waste depots so the chemicals do not contaminate the groundwater.

 

 

 

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Measuring the water depth in the lined borehole

 

 

 

 

 

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Description

The lined borehole method consists of a borehole dug with an augar to a desired depth. A circular tube with known radius (r) is then inserted to the base of the borehole in the unsaturated borehole. There should be no space between the borehole and the tube lining, to decrease the effect of lateral flow at the base, the tube should be tightly secured into the soil. Water is then added to a depth (D) and at time (t=0). The rate of flow into the soil is determined by measruring the change of depth in the tube with time. The depth is then recoreded at certain times over the 5 days. The recording of the depth at Narrabri was by a ruler placed into the borehole.

The lined borehole method was preformed in the cultivated area for three sites - C1, C3 and C5

 

 

 

 

 

 

 

Diagram of the falling head lined borehole

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Analysis of Data

The JMP 5.1 statistical package was used for the analysis of the falling head lined borehole. The data was entered to obtain cumualtive time in hours and change in water height (cm). A "Fit x by y" bivariate model was used to determine the steady state infiltration rate. This was obtained by ommiting the data not apart of the steady state infiltraion rate.

An example on site C1. The points were excluded shown in red so a line could be fitted to obtain the steady state infiltration rate whereby the dracian flux can be obtained(q). The darcian flux is obtained from the gradiant of the line, in this example it is 0.174.

 

 

Once the darcian flux had been found the linerized equation is used to calculated Ksat

 

 

Where q = the darcian flux

.......... C = wetting front potential . . . . . . .. . (150 for clay soils)

...........G = shape factor (0.919)

...........r = the radius (3cm)

 

 

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Results

 

Site
q (cm/h)
Ksat (cm/h)
C1
0.174
0.009
C3
0.07
0.004
C5
0.06
0.003

 

The sub-surface saturated hydraluic conductivity of the cultivated area is extremely slow an average of 0.005 (cm/h) was obtained. The steady state infiltration rate was obtained after 20 hours for the three experiments indicating the soil in the subsurface is possibly compacted reducing the hydraulic conductivity. In Vertosols the hydraulic conductivity is moderate to slow, the clay content is possibly greater down the profile, decreasing the hydraulic conductivity. The subsoil in most cases nearly always have a lower hydraulic conductivity, due to the lower rates of biological activity therefor reducing the soil structure (NLWRA, 2001).

The results in C3 indicate recording error, as the infiltrartion rates during two of the recording times were equal which we would not expect. This would possibly be due to the use of a ruler to record the water level.

This slow rate of Ksat would be a problem in the field if there was heavy rainfall as flooding would occur and possibly cause waterlogging. However if the field is used for irrigation this would be ideal as the slow rate of permeabilty would be ideal as the plants would be able to use optimum amount of water, without excessive deep drainage.

 

Steady state infiltraion graphs

Cultivated site 1

Cultivated site 3

Cultivated site 5

 

 

 

 

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Issues

The main issues encounted with this method is the smearing of the borehole with the auger, therefore the infiltration of the water would be hindred as the natural connectivity of the pores would be altered. When placing the lining in the bore spaces betweeen the bore and the lining may occur especially at the base causing lateral flow, this can be ommited if the lining is pushed hard in to the soil. Although this method is relatively cost effective and easy to use the accuracy of measuring the water depth with a ruler can certainly be improved. This was evident in cultivated site 3 where the measurements contained some outliers.

 

 

 

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References

 

Amoozegar, A. Wilson, G. W Methods for measuring hydraulic conductivity and drainage porosity. Agricultural drainage. American Society of Agronomy,Madison, USA: 1999. 1149-1205. over 4 pp.

Munoz-Carpena, R., Regalado, C.M., Alvarez-Benedi, J., Bartoli, F., 2002. Field evaluation of the new Philip-Dunne permeameter for measuring saturated hydraulic conductivity.Soil Science 167:9-24

National Land and Water Resource audit, 2001 http://audit.ea.gov.au/ANRA/agriculture/docs/national/Agriculture_ASRIS_shc.html

Vanderlinden, K., Gabriels, D., Giraldez, J.V., 1998 Evaluation of infiltration measurements under olive trees in Cordoba. Soil and Tillage Research 48:303-315

 

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