So for the Urban Flood Risk Mitigation model, the estimated runoff is calculated with the Curve Number method. Inside of this formula, the initial abstraction is defined as λ=0.2.
As far as I can tell, this number is probably based on the original SCS-CN method, and has, rightly so, been employed in many studies.
Now, more recent research comes to the conclusion that, also especially for urban environments, which this model is ultimately aiming for, the initial abstraction should be way smaller, around 0.05 as opposed to the original proposed 0.2 value (Woodward et al. (2003) Runoff Curve Number Method: Examination of the Initial Abstraction Ratio)
As the initial abstraction is not a user influenceable input value, I was wondering if there might be any plans for future versions where this more up to date initial abstraction value might be taken into consideration, or where the initial abstraction value might become a user influenceable value depending on local conditions (f.ex. proposed by Krajewski et al. (2020), Variability of the Initial Abstraction Ratio in an Urban and an Agroforested Catchment) in the user interface?
Thanks for this suggestion @Milkiman . This sounds like something we should consider. It would be great to hear from others in the field about this too. @b.janke , do you think it’s a good idea to expose this parameter to users?
I would not see a problem with making the abstraction parameter modifiable by the user, as there will certainly be some users familiar with the use of the Curve Number method for runoff prediction.
If we did expose this parameter, it might be good to provide some additional guidance in the user’s manual. For example, according to the Natural Resources Conservation Services manual describing this method, the curve numbers were derived assuming that initial abstraction equals 20% of the maximum abstraction; any adjustment to initial abstraction requires recalculation of the curve numbers (page 10-4). So, one would need to modify both initial abstraction and the curve numbers (the latter of which are user accessible already). Some guidance exists for modification of the curve numbers for a different initial abstraction (e.g. here).