SDR sensitivity analysis -- unexpected result for C factor
This question is probably best suited for Perrine Hamel, my favorite NatCap hydrologist --
I've completed a "One at a Time" sensitivity analysis of the various SDR input parameters for the total export values and the results turned out as expected with the exception of the C factor.
Note: the DEM and advanced calibration constants, ICo and Kb, were not included in the analysis.
I quickly noticed that adjusting the K, P, and R inputs would produce linear increases and decreases, which is logical and kind of a "no duh" given the equation for the USLE component of the total export.
Upon first glance, it appeared that the C factor simply resulted in a near-linear results (e.g. +15% C = +20% export, or -15% C = -20% export). I figured this was a function of the C factor and its role in the IC value and eventually the SDR component.
However, when the watershed results were aggregated by sub-basins I found that a 15% decrease in C led to a few non-linear outcomes.
The sub-basins are 7 unequal sized reservoir catchments along the river course within the larger study basin. The model used 92 total HUC12 catchments as the watershed input, and the sub-basins ranged from 2 to about 50 HUCs. So vastly different sub-basin catchment areas.
Out of the 16 C factor sensitivity results (+/- and including the full basin) 12 of them had the nearly-linear +/- 20% outcome, with only 4 of the sub-basins producing non-linear results. Also, all of the non-linear results were from a 15% decrease in C. Specifically they were -4%, -16%, -35%, and -44%. All of the other outcomes, both for the increase and decrease of C, were between 20 and 22%.
I double checked the inputs for the sensitivity runs and even inspected the SDR intermediate raster output and didn't see anything that would explain the 4 non-linear results. At least nothing that jumped out to me. I even inspected the C factor summary stats for each sub-basin for clues and correlations but I didn't find anything that might explain these results.
Ultimately, my question is: are my C factor sensitivity results out of the ordinary? Is there something in the full SDR model function that would explain them? Or is it just a coincidence that 12 of the 16 C factor sensitivity results fall into that near-linear range?
I've attached a spreadsheet of all the sensitivity results needed to follow the content of this thread including the baseline scenario and the +/- C factor outcomes.
Many thanks to Perrine or whoever else can provide some feedback!
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