Design & testing of experimental measuring equipment.
The majority of irrigated agriculture in Australia is located on fine grained cracking soils. While their high nutrient content and water holding capacity support high yielding crops and profitable farming, the formation of shrinkage cracks during drying intensifies the adverse effects of suboptimal irrigation scheduling. Preferential flow through soil cracks can rapidly move irrigation water into deep parts of the soil profile and can quickly transport solutes and agrochemicals through the unsaturated zone.
In addition, preferential drying and wetting along crack faces increases the soil moisture variability within a soil profile. The commonly used point measurement techniques to measure soil moisture (Neutrone Probes, Capacitance Probes) are hence inadequate to capture the spatially highly variable soil moisture in cracking soils.
To allow the most appropriate irrigation management of cracking soils, a better understanding of soil crack dynamics and the resulting influence on the soil moisture distribution are essential.
To do so, the CWI team aims at providing tools and methods to non-destructively monitor
Research fellow Anna Greve is working with Professor Ian Acworth and Assoc/Professor Bryce Kelly to develop new methods and equipment.
3D electrical resistivity tomography between 4 vertical boreholes allows three dimensional monitoring of moisture changes in the undisturbed soil between the resistivity probes.
Borehole resistivity probes have been designed and constructed and were tested in laboratory weighing lysimeters. Once tested in the laboratory the equipment has successfully been used in the field during the growing seasons 2007/08 and 2008/09 in irrigated Sorghum and Cotton fields.
Results show that small scale 3D tomography allows monitoring of preferential flow paths and 3D soil moisture changes during irrigation and drying periods.
Furthermore the electrode arrangement used to carry out 3D resistivity tomography allows the collection of square array measurements with can be used to calculate the Anisotropy Index (see below).
To date, an understanding of crack dynamics has been hampered by the lack of techniques to observe or monitor crack dynamics below the soil surface.
This study introduces a new technique for the detection of subsurface cracks that relates the development of soil cracks to changes in the electrical anisotropy of the soil. Here electrical anisotropy is defined as the ratio of the apparent resistivity measured with the alpha and the beta square array.
The electrode array proposed for the collection of 3D electrical resistivity tomography (above) also allows anisotropy measurements at regular depth intervals throughout a soil profile.
Results of numerical modelling, laboratory and field tests show that the anisotropy index is an excellent tool to monitor dynamics of subsurface cracks and to measure the depth of crack extension.
Deep drainage due to suboptimal irrigation scheduling can result in increased soil moisture storage in the unsaturated zone. As the estimation of the water content above the zone of permanent saturation is notoriously difficult these changes are rarely detected. To map and to quantify soil moisture changes due to irrigation, detailed 2D resistivity surveys in conjunction with soil sampling have been carried out.
Results show that 2D resistivity surveys together with soil sampling are effective in quantifying changes in soil moisture storage due to crop irrigation. Quantifying the water stored in the unsaturated zone will improve estimates of deep drainage and will increase our understanding of residence time in the unsaturated zone.
Research fellow Anna Greve is working with Jenny Foley and Mark Silburn from the Queensland Department of Environment and Resource Management to quantify these soil moisture changes.
Journal publications
Greve, A.K., Andersen, M.S., Acworth, R.I. (in press) Investigations of soil cracking and preferential flow in a weighing lysimeter filled with cracking clay soil. Journal of Hydrology. Available online: 10.1016/j.jhydrol.2010.03.007
Greve, A.K., Acworth, R.I., Kelly B.F.J. (in press) Detection of soil cracks by vertical anisotropy profiles of apparent electrical resistivity. Geophysics.
Kelly B.F.J., Acworth R.I. and Greve A.K. (submitted July 09) Monitoring Deep Drainage Beneath Irrigated Cotton using Two-Dimensional Surface Electrical Resistivity Tomography, Journal of Applied Geophysics.
Greve, A. K., Acworth, R. I., (submitted January 10) 3D cross-borehole resistivity tomography to monitor small scale soil moisture changes in the root zone. Journal of Hydrology.
Patents
Acworth, R.I., Greve, A.K., 2009. System, apparatus and method for measuring soil
moisture content. International Patent Publication Number WO 2009/117784
A1.
Conference Papers
Foley J., Silburn M., Greve A.K., 2010. Resistivity imaging across native vegetation and irrigated vertosols of the Condamine catchment a snapshot of changing regolith water storage, 19th World Congress of Soil Science, 1-6 Aug. 2010, Brisbane, Australia. Published on CD ROM.
Greve, A.K., Acworth, R.I., 2009. Monitoring soil moisture changes and deep drainage pathways, 37th IAH Congress 6-10 Sep. 2009, Hyderabad, India.
Greve, A.K., Acworth, R.I., 2009. Electrical resistivity methods to monitor soil moisture changes and deep drainage, Cotton Catchment Communities CRC Science Forum, 17-19 Aug. 2009, Narrabri, Australia.
Greve, A.K., Acworth, R.I., Monitoring depth of soil cracking win irrigated fields based on
directional dependence of electrical current flow, XXXVI IAH Congress 2008 Toyama.
Greve, A.K., Acworth, R.I., Kelly, B.F.J., 2008. Monitoring soil moisture changes with 3D resistivity tomography, AGU, 89(23), Western Pacific AGU meeting, Cairns 28 Jul. - 1 Aug. 2008, Abstract H43A-04.
Greve, A.K., Acworth, R.I., 2007. Dynamics of soil cracking & borehole resistivity tomography, Cotton Catchment Communities CRC Science Forum, 8-9 Aug. 2007, Narrabri, Australia.
