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. Firstly, the dry stones have mostly diffuse reflection and a small amount of specular reflection which is blurred by the roughness of the stones (you can see a faint sky reflection on them). The wet stones obviously have a much greater specular component and you can see things being reflected quite clearly. However the diffuse component appears to be much darker - this is especially clear in the second photo where the small remaining wet patches are much darker than the surrounding areas. I believed that the reason the stones get darker is because the layer of water is reflecting a greater proportion of the light directly (specular reflection) so there would simply be less light for the diffuse material to reflect, hence it appears darker. However this may be too simplistic, for instance maybe the layer of water is changing the refractive index of the surface thereby somehow contributing to this effect. Any answers or help with this problem would be greatly appreciated. I did try a search but came up with nothing.