Home Forums RORB – general use Initial Loss Sensitivity

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    Grant Speldewinde

    Good morning,

    I am using the FIT routine to estimate IL-CL values for a gauged catchment. I have created a catchment file and multiple storm files (some attached) but when fitting the calculated hydrograph to the actual, I note the IL value below a certain threshold does not change the hydrograph or CL value.

    For example, for the storm dated 1987 07 28, the hydrograph and CL value (9.6mm/hr) are the same for any IL value less than 41mm.
    Limits at different values are also noted in the other two storm files (1981 05 29 and 1991 06 24).

    I suspect there is an error in the catchment or storm file and I’m seeking some guidance on the reason.

    Please note I am an engineering student using so please excuse the novice question.

    Thank you in advance.


    Benson Liu

    Hi Grant,

    I can’t view any of the files you have uploaded as I think the websites security has blocked it but there are a few things to consider that could be causing this issue.

    How the initial/continuing loss model works is that the initial loss is applied first and wipes out the rainfall until the initial loss is satisfied, continuing loss then kicks in after that. So what you could be seeing is that even if you lower the IL, the CL is still wiping out the rainfall and so the hydrograph doesn’t change but this is dependent on the rainfall depths and temporal pattern.

    A second thing to note, while losses are catchment and event specific and can vary quite a bit between these things, a CL value of 9.6mm/hr I would consider extremely high, almost unreasonable. Again, this is catchment and event specific so it could be possible but I think you would need a good justification for applying such a high CL. This feeds into another point about the fit routine. The fit routine is generally no longer used as computers now have enough power to run RORB very quickly. The design routine is recommended as this allows you to specify the losses, gives you greater control of the parameters and allows you to see the incremental changes with changing parameters.

    Kind regards,


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