Where are the roots? Can we deduce the edge of a tree's root zone without seeing it?

In the Australian environment water shortage is a continual threat to many areas of agriculture within both the cropping and grazing sectors. From this perspective trees within the landscape are often viewed as having a major negative effect, significantly depleting the available surface moisture for crops and pasture, and thus reducing yield potential. This has led to extensive areas of cleared land within the agricultural landscape. However, whilst this effect of trees is 'common knowledge' it has been relatively scientifically unexplored. Techniques to measure tree water use by individual stems have been well developed. However, these techniques are often expensive, complicated and time consuming, and do not address the changes in soil moisture content around trees i.e. how far out their influence (root zone) extends. A potentially quick and uncomplicated way to address this question is using the tool EM38.