What does soil look like as it rains?

This is a quick six second video that covers a 24 hour period. Around the half way mark (mid day) the rain begins and you can see the change in soil color/brightness(?).  Images were taken every 15 minutes.

I used the program "Identify" from the software suite ImageMagick to get a rough idea of the average brightness of each image taken. Values below are from samples taken every hour. Over a 24 hour period this looks like:

The numbers on the left, 10200 thru 11400 are the values provided by identify when running the command: 

identify -format "%[mean]" image.jpg

I am not familiar with the method identify determines the average brightness. When comparing percentage differences of luminosity histogram values in Adobe Photoshop CS2 with those of identify, they were quite close. Though only a couple images were compared

I am guessing the scanner affects this in a number of ways:

1. Scanner surface is very different than soil, may affect water path/flow
2. Light from the scanner, may (seems negligible) provide enough heat to evaporate water quicker?
3. Slight vibrations from the scanner operating may (seems negligible) affect water path/flow

Regardless, it's awesome to see this visually in terms of video and numbers/graphs.