I am using the Surface Volume tool (3D Analyst/Functional Surface) in ArcGIS 10.1 to calculate the volume between a reference plane and two water table surfaces (year 1 and year 2) that I interpolated from monitoring data (i.e., water table elevation points).
I specified that the tool should calculate the volume ABOVE the plane (up to the surface). My x,y, and z units are all in meters, so Z-factor = 1. I used the same reference plane height for both surfaces (it was set to a height that would not intersect either surface).
I am interested in calculating the proportional (percent) change in surface volume between years, not in the raw volume numbers per se. After running the tool, I did the following to calculate percent change: ((Year 2 vol - Year 1 vol)/ Year 1 vol)*100
I ran the Surface Volume tool multiple times using the same two surfaces (year 1 and year 2), but changed the height of the reference plane (the plane never intersected either surface). I expected the percent change to remain the same, regardless of the reference plane height. However, the percent change in volume between the two years changes each time I alter the reference plane height.
Here is an example of my data:
Percent change in volume between year 1 and 2:
-0.31% (ref plane at 725 m) -2.10% (ref plane at 760 m) -5.46% (ref plane at 763.7 m)
As I raise the plane, it gets closer to my surfaces, and the raw volume numbers decline.
Why does the percent change differ as I change the reference plane height?
Perhaps it is related to the overall downhill trend in my water table data.