I want to know the uits of the SDR model 's output.Usle’s unit is the t/pixel/year or t/ha/year, i see the user guide is tons/pixel/year,my map is 30m*30m,if it is right ,does that mean i need to divide the result by 0.09?
Hi @Monika -
Yes, the USLE unit is tons/pixel/year, I see that the model description says that USLE is tons/ha/year, and the original equation does produce those units, but our methods convert that to tons/pixel/year. I’ll add a note about that to the User Guide.
So yes, if you want the results to be in tons/ha/year, you’ll need to covert using your pixel size.
~ Stacie
thank you Stacie,if i convet unit ,this data will be very big .I want to know as a novice, want to take the liberty of asking how the data out of the model should be compared with the actual, there is only one hierarchical data map in China, if it is the site data comparison is not required sediment transport.
If your pixel size is 30x30m, to convert 900m2 pixel to hectares would be multiplying by 0.09, since 900m2 is a small fraction of a hectare, which is 10,000m2.
~ Stacie
If you multiply by 0.09 the result doesn’t make sense. These are very low values.
How small are the per pixel values? There might be something wrong with the inputs or calculations.
~ Stacie
Hi @Monika -
I want to know as a novice, want to take the liberty of asking how the data out of the model should be compared with the actual, there is only one hierarchical data map in China, if it is the site data comparison is not required sediment transport.
It is best if you have observed data for at least one point in the watershed you are modeling. The User Guide provides a very brief outline of the process of calibration. You can also search this forum for other discussions related to calibration. This recent discussion is for NDR, but much of it also applies to sediment.
~ Stacie
An average of 5.47 t/pixel/year when I multiply by 0.09 the average drops to 0.48 t/ha/year.
Hi @MariaRita and @Monika -
Apologies, but I gave you incorrect advice before. You are correct that the per pixel value should be divided by 0.09 to get values per hectare. I got a sanity check from @Lisa who spelled this out:
If 1 pixel = 900 m2 for example, then the conversion from tons/pixel to tons/ha would be:
tons/pixel x pixel/900 m2 x 10000 m2/ ha
or
the per pixel value x (10000/900)
The number will get bigger when the pixel is smaller than a hectare.
It’s just confusing looking at a pixel, and seeing values per hectare, which then look very large. Converting to values per hectare makes more intuitive sense when summing to the watershed, instead of when looking at the per pixel raster values.
~ Stacie
This is also my most confused point. Referring to the parameters of many literatures, the value calculated is really very large, so that I cannot calculate according to the existing soil erosion standards in China. According to the t/pixel classification, the slight erosion should be 0.45, but the value calculated by me is two orders of magnitude greater than Qi, so I cannot carry out subsequent calibration and verification. Do you have any good suggestions? I would appreciate it！@swolny，It suddenly occurred to me that if the unit of usle is t/pixel, how about the unit of erosion factor R and the unit of K? I don’t know if there is a problem in the calculation here, because my value is based on ha！Looking forward to your prompt reply！
Hi @Monika -
Are the modeling results two orders of magnitude higher on every pixel? If not, is there a pattern to where they are excessively high? If you search this forum, you’ll find previous posts about the SDR model over-estimating erosion, particularly in areas that have very steep slopes. The USLE equation was not designed for steep slopes, so we have added adjustments to the model to compensate for this, but perhaps it’s still too high in the circumstances in your watershed.
Another thing to verify are the units of your model inputs. The User Guide lists the units that must be provided for each input. In particular, the values of K are often given in US units, not metric, and the model requires metric. US units are generally on the order of 0.3, metric units on the order of 0.03.
~ Stacie
Thank you for your reply@swolny. I have confirmed that the value of K is indeed around 0.03, and the unit of R is MJ·mm/(ha·h·yr), and the unit of K is (t·hm2·h)/(MJ·hm2·mm). If these two units are correct, I will have to adjust the drainage area, which is indeed a mountainous hilly area
It may be worth looking through the files in the Workspace intermediate_outputs folder to see where the large numbers begin to appear. The intermediate outputs correspond with many of the equation steps in the User Guide, so they can show which equation (and related set of inputs) is causing the high values.
~ Stacie