Quantifying spatio-temporal soil water content using electromagnetic induction
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Date
2017
Authors
Edeh, Judith Amarachukwu
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Publisher
University of the Free State
Abstract
Water scarcity is still a global concern, and the fact that water is not evenly distributed within the soil remains a case study in agriculture. Apparent electrical conductivity (ECₐ) measured with the EM38 devices have been consequently used to distinguished areas of water management in precision agriculture, before irrigation planning. However, to efficiently use EM38 and its newer model “EM38-MK2” required site specific calibration. This involves collecting soil samples for volumetric water content the same time the device is used. Repeated soil sampling over time series have been reportedly stated to be time consuming and destructive. Therefore, this thesis proposed to use DFM capacitance probes that only need to be installed once in the soil to continuously record water content. The study presented three main objectives to: (i) examine the influence of DFM probes, and other possible obstructions including neutron water meter galvanized-steel access tubes and profile pits on ECₐ measurements with the EM38-MK2, (ii) calibrate the EM38-MK2 using DFM probes installed in the field, and (iii) spatially characterize soil water content estimated from multiple EM38-MK2 surveys.
On relative homogenous soils of Kenilworth Experimental Farm and with DFM probes, steel access tubes and profile pits consecutively inserted into the soil, EM38-MK2 was moved towards these interferences, over it and away from it without zeroing the EM-device. Results showed that while trenches had no effect, both DFM and steel tubes influenced ECₐ readings when the EM-device was closer than 1 m to these instruments. This effect was inconsistent with large values that were either negative or positive. After encountering the interferences and without EM zeroing, ECₐ readings were either less stable (only at vertical mode for the DFM) or reduced. Although the instability was statistically significant, the mean ECₐ before and after the probe-interference were not significantly different. The decreases in mean ECₐ values at horizontal mode for DFM and at both modes for steel tubes were all relatively small (< 2 mS m-1). This study concluded that the EM38-MK2 can be used together with DFM probes, but keeping the EM-device at least 1 m away from the probes. On a practical level, there should be no need to re-zero the EM38-MK2 after an encounter with such metal-containing interferences. Rather, re-zeroing is advised after extended use in the field as suggested by other researchers.
On the heterogeneous soils of Paradys Experimental Farm comprising of four diverse soil forms, field calibration of DFM probes and EM38-MK2 were performed under both dry and wet conditions. The calibrated capacitance probes accurately predicted water content that spatially explained on average, up to 96% of the observed water content. The DFM estimated soil water values on individual plots were consistent and were used for site-specific calibration of EM38-MK2. ECₐ-based estimated water content for individual plot models explained on average, 97% and 90% of variation in soil water content, at 0.38 m and 0.75 m depth, respectively. With the general models ECₐ values could predict 74% and 69% of the volumetric soil water content at 0.38 m and 0.75 m, respectively. This was regarded as satisfactory, especially considering the heterogeneity of the soils on the experimental site. Therefore, the models developed in this study, performed well both at individual plot and over spatial scales.
When the general models were applied on spatial scale, ECₐ-based estimated water content was temporally stable. The spatio-temporal soil water maps produced an accurate representation of topographical effects on soil water distribution over the area. Therefore, the proposed use of the DFM capacitance probe method for site specific-calibration of EM38-MK2 was successful and could be adopted for future research.
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Keywords
EMI, EM38-MK2, ECₐ interference, Soil water content, Calibration, Soil heterogeneity, Site-specific calibration, Mapping, Water-supply, Spatial and temporal water characterization, Dissertation (M.Sc.Agric. (Soil, Crop and Climate Sciences))--University of the Free State, 2017