This company claims that they can determine accurate irrigation patterns on a terrain just by using 3 sensors (https://www.youtube.com/watch?t=80&v=U48XTXXi_MA).
They don't talk about the actual sensors or how they work, but there are not many variables you can measure besides chemical composition of soil and impedance, so I assume the use the 3 sensors to measure impedance between them (here you can see the actual sensor https://youtu.be/AXUlxaatpYY?t=16s).
As you can see in the picture, they deliver soil humidity patterns that seems very specific:
The picture not only shows how much water there is on each sensor but it also builds a map distribution of the water on the soil. Theoretical speaking, how do think they do it?