Sensitivity of EM38 in determining soil water distribution in an irrigated wheat field

Geonics EM38 is a portable, non-invasive equipment that induces an electrical current in the soil for rapid measurement of apparent electrical conductivity (ECa) in the field. We used an EM38 in a wheat field to evaluate the effects of systematic variation in soil water content (within three replicate plots of four irrigation treatments) and seasonal variation in soil temperature on ECa. The effective depth of sensing of EM38 could be varied by using it in both vertical and horizontal dipole modes and by placing it at various heights above the ground. Accumulated water within various soil depths was measured with a neutron probe throughout the season. Values of ECa over the season for a soil depth related linearly or nonlinearly with soil water within that depth with a coefficient of determination (R2) of 0.70–0.81. ECa values were also influenced by variation in soil temperature within 5–25 cm depth (range 10.1–29.3 °C) and air temperature (range 14.2–34.0 °C), but to a smaller extent than soil water. The overall relationship between ECa, soil water and soil temperature improved considerably when multiple regression was used (R2 = 0.82–0.98). Good correspondence between maps of ECa and soil water content over the experimental field suggests that ECa maps could be useful in determining spatial distribution of soil water within crop fields so that variable rather than uniform quantity of irrigation water can be applied to improve water use efficiency.