Just to add to the discussion:
another artist has suggested that the difference is due solely to the change in the specular characteristics of the stones. The dry stones appear brighter because you are looking at the combined diffuse reflection and the scattered specular reflection, whereas...
A couple of pictures to get started:
I'm not a physicist, but a professional artist, so an overly technical explanation may not have any meaning (especially if it's maths!). However I am very interested in how light interacts with matter and I am puzzled as to what is going on here...