Sediment retention - model output values too high

Hello all,
I've been working on trying to get my units straight for the sediment retention model for some time. I am working in watersheds in West Maui, which have extremely steep sloping hillsides and a very large ridge to reef rainfall gradient. My watersheds are small (300 - 2000 ha).
Here are some details about my input files. I have been working in 2.5.4 x64 Standalone, but plan on upgrading to 2.5.6 soon.
DEM - in meters
Erosivity - in MJ*mm/ha/yr/h --> I converted this from the NRCS contour maps (by multiplying by 17.3), and my values range from 1000~7800.
Erodibility - between 0 and 1
Biophysical - Using 0 to 1 for usle_c (eg. 0.002); 0-100 for sedret_eff (eg. 10)
I am not yet doing valuation, so still have the original dummy values there.
Just looking at the RUSLE2 outputs, I expect to have about 20 tons/ha/yr, but my values for everything are *very* high - about 1000 for usle_mn, and then 50000 (5e4) for sed_export (or about 600 tons/ha/yr).
I am off by two orders of magnitude.
Does anything look off to anyone? I really can't see my error.
Thanks,
Kim
I've been working on trying to get my units straight for the sediment retention model for some time. I am working in watersheds in West Maui, which have extremely steep sloping hillsides and a very large ridge to reef rainfall gradient. My watersheds are small (300 - 2000 ha).
Here are some details about my input files. I have been working in 2.5.4 x64 Standalone, but plan on upgrading to 2.5.6 soon.
DEM - in meters
Erosivity - in MJ*mm/ha/yr/h --> I converted this from the NRCS contour maps (by multiplying by 17.3), and my values range from 1000~7800.
Erodibility - between 0 and 1
Biophysical - Using 0 to 1 for usle_c (eg. 0.002); 0-100 for sedret_eff (eg. 10)
I am not yet doing valuation, so still have the original dummy values there.
Just looking at the RUSLE2 outputs, I expect to have about 20 tons/ha/yr, but my values for everything are *very* high - about 1000 for usle_mn, and then 50000 (5e4) for sed_export (or about 600 tons/ha/yr).
I am off by two orders of magnitude.
Does anything look off to anyone? I really can't see my error.
Thanks,
Kim
Comments
My K values are in the English system. To convert to the metric, you need to multiply by 0.1317. *Or*, don't convert the R from the English system to begin with (the x17.02 mentioned in the Handbook).
This is a convenient conversion sheet:
http://www.techtransfer.osmre.gov/nttmainsite/Library/hbmanual/rusle/ah703apa.pdf
In the meanwhile when you upgrade from 2.5.4, you can try our cutting edge nightly build if you're brave: http://ncp-dev.stanford.edu/~dataportal/nightly-build/InVEST_dev79_2_5_6 [1029bd49a77a]_x86_Setup.exe That has a fixed version of our flow algorithm and addresses those overestimation of retention issues.
You're right, both the K and R values should be in SI units (so K is in roughly in the [0-0.1] range).