Recent PARG Graduates
Dr Bilal Hossain (PhD)
Project: EM38 for measuring and mapping soil moisture in cracking clay soil (2005-2008).
Funding Source : Cooperative Research Centre for Irrigation futures and University of New England.
Summary:
Soil moisture content in the crop root zone varies both spatially and temporally. Moisture in this zone is critical to plant development and health, so understanding the variability and dynamics of moisture distribution in this zone is crucial for optimal crop management. The EM38 electromagnetic induction sensor, a tool for measuring apparent electrical conductivity in soils, can be used to infer a range of soil properties, including soil moisture. The ability to configure an EM38 for on-the-go sensing and mapping of soil apparent electrical conductivity (ECa), if related to key soil properties such as moisture content, means high resolution soil maps can be produced that may significantly aid in the management of agricultural fields. This thesis describes an investigation of the EM38 to quantify soil moisture variability in a paddock of homogenous Black Vertosol soil (cracking clay).
The research project comprised three main components: (i) an assessment of two ‘conventional’ soil moisture probes; (ii) an investigation of the relationship between soil moisture, and its vertical distribution, and EM38-dervived ECa readings; and (iii) a brief investigation of the potential of a mobile on-the-go EM38 survey to assess temporal and spatial patterns of paddock-scale soil moisture. Data from a soil moisture neutron probe (SMNP) and a Diviner 2000 capacitance probe were compared against volumetric soil moisture content (VMC) determined via soil core samples and both wet and dry pit-based samples. Pit-based samples proved to be more reliable for soil moisture determination when compared to core-sampling. The Diviner 2000 was shown to provide useful soil moisture information to a depth of only above 0.6 m (R2 = 0.81 - 0.92), but the SMNP was successful at all depths down to 1.2 m (R2= 0.94 - 0.97).
A model linking VMC to EM38 derived ECa was developed and subsequently verified. Comparison of the model predictions with multi-height EM38 measurements showed that the EM38 depth response function was not perturbed by the depth profile of soil moisture content and the subsequent model showed that ECa for both dipole orientations explained 99% of the variation observed in qv averaged to a depth of 1.2 m. The calibration results show that the on-ground EM38 measurement could measure qv with an accuracy of ~±0.007 m3/m3 in either mode. However, the horizontal mode of EM38 was found to be better for predicting VMC. The forward-propagation models of Rhoades & Corwin (1981) and Slavich (1990) were refined and tested for their ability to directly predict the vertical profile of soil moisture content. The Slavich model, incorporating both vertical and horizontal dipole configurations, was found to produce the best predictions with an error of approximately 10%. A complex inversion process involving Thikonov regularization was also tested and was found to consistently under-predict qv by approximately 50 – 75%. The capture and comparison of multiple on-the-go EM38 surveys showed that soil moisture was the primary driver of temporal variation in the EM38 derived ECa at this site. Maps of derived qv values were correlated to site topography and the inclusion of multi-temporal EM38 survey data gave the most accurate representation of topographic effects (drainage).