Proportion of Colors in a Given HSL Range

That's a really interesting project.

So a few thoughts. One, this really depends on how you are going to present the information. Side by side imagery, etc. But I don't think presenting it as a percentage of the total area will be accurate enough because I imagine you won't be keeping a tripod out in the exact positioning, zoom, etc.

Idea #1

So from a digital forensics approach, you would want to place some markers on the ground that are hard edged geometrically. Think small disks that stake into the ground. You'll use these so you can align your pictures each time for scale and position. They sell survey markers on amazon for this reason. It looks like a nail with a really large head that is colored. Don't pick green.

If you get the pictures taken over say a few weeks, and align them all (check into Photoshop auto align layers)
Align and distribute layers in Adobe Photoshop

With those layers stacked you can use Blend Modes like Difference, Exclusion, etc, to see the variance in how the surface has changed. It'll be a little chaotic because all of the grass will have moved, but you'll see a difference in the size of the central divot which is the focal point. Using a grid, you could calculate how much smaller the area is becoming.

Idea #2

Using your photograph, again framing from a similar angle, perspective, scale, etc. Use Select> Color Range to sample the colors of your grass and build a selection of anything that HUE. Load that selection by pressing OK and on a new layer fill it with a solid green. Now inverse the selection (Select > Inverse) and fill that selection with an opposing value like bright red.

Although the geometry is abstract, you could treat this as a heat map for comparison/analysis and show each time how the red area is reducing week over week. If you draw a boundary line around each one, you could also calculate how much you reduce or increase that shape to the nearest alignment of the next image and say "the original geometry has been reduced by 15% since week 1".

Hope that helps.