Padhi, Jyotiprakash and Misra, R. K. (2009) Monitoring spatial variation of soil moisture in crop fields with EM38. In: Irrigation Australia 2009: Irrigation Australia Irrigation and Drainage Conference: Irrigation Today - Meeting the Challenge, 18-21 Oct 2009, Swan Hill, Australia.
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Electromagnetic induction (EMI) technique is currently available as a commercial equipment to measure apparent electrical conductivity (ECa) of soils as a non-contact method by using EM38. A Field experiment with wheat was conducted to investigate the effects of variation in soil water and temperature (i.e. soil and air) on the EM38-measured ECa in vertical and horizontal modes of operation. Additional measurements were made to study the effects of placing EM38 at various heights above the ground (0.1 and 0.4 m) on ECa. We used an EM38 and a neutron probe to measure ECa and profile soil water content, respectively within top 1.33 m of soil in 3 replicated plots of 4 irrigation treatments. Irrigation treatments (T50-T85) were designed to allow soil water depletion down to 50%, 60%, 70% and 85% of the plant available water capacity in soil. During EM38 survey, we also recorded the locations of all measurements with a hand held GPS. Air temperature and soil temperature at 5, 10 and 25 cm depths was recorded for all the 12 plots during EM38 survey. Results indicated spatial variation in the soil water in the field to be detected well with EM38 measurements. Significant effects of soil water was observed on all ECa measured with the EM38 probe. Soil water content within the shallow and deep parts of the crop root zone could be explained by the measured values of EM38 at multiple depths above the ground. The coefficient of determination (R2) for regression models used to describe the relationship between ECa and soil water content was larger for horizontal mode than vertical mode of operation. Both soil and air temperature also had significant effects on measured ECa. Overall, EM38 was found to be quite easy to use and helpful for monitoring spatial variation of soil water content in the field. Similarities in the pattern of spatial variation in soil water and ECa over the entire crop field observed in our study suggests that this technique can be used successfully to determine soil water deficit in clay soils such that precise and variable quantity of irrigation water can be delivered to crops at various parts over the same crop field.
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