‘m’ values

[STPwPIh]

Hi,

We are working on a RORB model for a catchment in Christchurch NZ. Our preliminary calibration requires a value of m=0.6. Can we use this value for our design runs or should we be looking to get it closer to 0.8?

Thanks,
Mel

[DavidStephens]

Hi Mel

Thanks for your query – great to hear that RORB is being used in NZ!

For Australian conditions an m value of 0.8 is usually recommended, however this can be changed where there is sufficient evidence to warrant it (such as calibration to gauged streamflow hydrographs). Section 2.2.1 of the RORB Manual suggests that 0.6 is the lower bound of likely m values.

The other thing to consider is interaction between the m and kc values. In Australia there are various regional equations which can be used to check that a calibrated kc value is within a suitable range. The most common of these is the paper titled “A Simple Method for Estimating RORB Model Parameters for Ungauged Rural Catchments” by Mark Pearse, Phillip Jordan and Yvette Collins (http://search.informit.com.au/documentSummary;dn=316886237612094;res=IELENG). This suggests that the value of kc should be between about 0.6 and 2.1 times the value of the average flow distance (dav, which is summarised for you in the kc selection dialogue box) for the catchment. This might be a useful check, although it is based on Australian data only.

Assuming that you are confident in the results of the model calibration and that the kc value is within a reasonable range, it should be OK to proceed to design runs using an m value of 0.8.

Hope this helps.

• This reply was modified 7 years ago by DavidStephens.

[STPwPIh]

Hi,

Thanks for your quick response. Can you please confirm that we should be using m=0.6 for our design runs based on adequate calibration?

Cheers,
Mel

[DavidStephens]

Hi Mel

Sorry for the confusion – the last sentence in my previous post should have read: “Assuming that you are confident in the results of the model calibration and that the kc value is within a reasonable range, it should be OK to proceed to design runs using an m value of 0.6.”

• This reply was modified 7 years ago by DavidStephens.

[STPwPIh]

Thanks again.

[Phillip Jordan]

Hi Mel and David,
I agree with David’s answer – m = 0.6 is within the physically plausible range of values but the reason for sticking with m = 0.8 is that many RORB models have been set up and calibrated with this value and there is a large body of literature providing regional estimates of the kc parameter that assume m = 0.8.

You may also want to refer to the excellent discussion on this issue in Section 5.4.4 of Book 8 of Australian Rainfall and Runoff, which (despite the title of the Book of ARR) discusses selection of m across the full range of flood events. Section 5.4.5 of Book 8 of Australian Rainfall and Runoff then goes on to discuss considerations for selection of m for modelling rare and very rare flood events.
Hope this helps,
Phillip.

[roloko3965]

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