NDR Model: Crit_len_n/p

Hi everyone,

I was wondering if anyone had a good source for determining the critical length to use in the NDR model? (Ideally for NLCD land classes)

Thank you,


Hi @sarahhalperin,

I see the User’s Guide chapter for NDR has a couple of suggestions, but I’ll ping our hydrologists to see if they have any additional recommendations.


Hi @sarahhalperin this is indeed tough information to find. I have approached this in the past by treating the critical length as a calibration parameter. In the absence of any other information, I would start with a value at the mid-point of the range given in the User Guide (that is, 150m). Then vary that value up and down during calibration to find a good fit. Some things to remember if you are working in highly modified landscapes: (1) if you have croplands with tile drainage, then your proportion subsurface n will be very high, and if you set that to 1 then the critical length parameter does not apply as it relates only to n retention in overland flow. (2) Further, when working with urban land use classes where storm/sewer treatment exists, I have set the the critical length to 1 pixel width and the retention to 100%, under the assumption that the wastewater treatment facility will collect and “retain” all nutrients before the flows are released downstream. You can check out the sources and model parameterization for an area in the northeastern US in Griffin et al. 2020 https://doi.org/10.3368/wple.96.4.457.