Sensitivity Analysis - Sobol Method gives values above 1 and below 0?

[Christina]

Hi all,

I have been trying to calibrate my model and am running into an issue withthe method. From my understanding, Sobol values of sensitivity should be above zero and below 1, summing for a total of 1. 

For some reason I keep getting values above and below these limits (both in Toolbox v1.0 and v.2.1).

Does anyone know why this is allowed in this program? Can I adjust something to prevent such values?

Thank you,

Christina 

[Celray James]

Normally, this should not happen, but it is not uncommon to see very small negative numbers due to numerical errors and rounding off.

This is further made worse if the number of samples is too small. Usually, when using sobol, you need thousands of parameter-sets for your sample size. so your sample size should be very large, a quick check will tell you that you need at least 1500 samples or more for your run. If you have a slow running model and cannot run all that, then you may want to run using the LH-OAT with at least 400.

This page lists references you may find useful: https://celray.github.io/SWATPlusToolbox/runSensitivityAnalysis/#sobol

With Kind Regards,
Celray James CHAWANDA