Multi-temporal EM38 for Monitoring Soil Moisture Dynamics

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Soil water is a major driver of EM38 response (apparent electrical conductivity- ECa).  As moisture content changes and/or water redistributes within the soil profile the EM38 response will vary.  Experimental work shows that there can be a direct correlation between soil volumetric water content and the ECa from EM38 in soils with uniform profiles. However, the EM38 is also responding to variations in the levels of dissolved ions, charged clay particles, soil structure and organic matter in the soil.  Water carries the ions in the soil and the more water in the soil the higher the potential electrical conductivity of the soil.  Water may therefore be the dominant driver of EM38 response in the soil. The EM38 as a possible tool for broad scale mapping of soil moisture variation on farming land is a possibility, however more work needs to be done to perfect the methodology.
The use of multi-temporal EM38 surveys to investigate soil water dynamics is a relatively new area of investigation. The thinking behind multitemporal surveys is they will help to ommit the background error introduced by non-uniformity in soil textural properties with these factors not varying in the shorter term.  As rain-fed soil moisture permeates progressively deeper into the soil profile, the depth response function of the EM38 sensor coupled with depth-related ion content will provide a characteristic time-signature that may yield spatial water infiltration maps. One approach is to create a paddock specific template that holds all of that background variation information and then after a EM survey the template is applied and the moisture change easily extracted. However multi-temporal surveying, especially when involving relatively small changes in eCa, relies heavily on accurate instrument setup to ensure that any variations between surveys is soil based and not artificial instrument drift.

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Multi-temporal EM38 Surveys