Hello,

I don't think the SCS method is fully adapted to what you are doing. The SCS is an event-based method, so the S parameter account for the runoff potential of the soil but also the Antecedent Moisture Content (AMC) which varies during the year with precipitations. The SCS's main assumption is the rainfall is continuous in time and uniform in the watershed. So it does not work during periods without rainfall, because evapotranspiration is not computed.If you look closely at the formula, you will see that Initial Abstraction Ia and the retention potential S is fixed. So if you apply the same formula for 12 events, you will remove 12 times the Ia and S from the storm rainfall. On the other side, if you apply it one time on the cumulated rainfall of these two events, you will just remove it one time so these two volumes are differents.

So the SCS method is conceived for storm events and I think you have to choose an other method to compute annual runoff.

Thanks Felix for this answer. Based on your reply, I think the SCS methods is suited for daily runoff simulation. This I think would be similar to event based rainfall.

Hello

No, it is event based model.

Best Wishes

As Earlier mentioned the SCS CN model is an event-based rainfall-runoff model. The NRCS-CN model is popular worldwide for runoff estimation both in gauged and in ungauged watersheds and has been described more attractive due to its convenience, simplicity, stability, good predictability, its authoritative origins, and its responsiveness to grasp the watershed characteristics (Ponce and Hawkins, 1996), Still it has undesirable foundations. For better understanding regarding the application of this method in different biomes I would like to refer the following articles.

The uses and abuses of CN model application

1. http://ascelibrary.org/doi/abs/10.1061/(ASCE)1084-0699(1996)1%3A1(11)

2. http://onlinelibrary.wiley.com/doi/10.1111/j.1752-1688.2005.tb03742.x/abstract

3. http://www.sciencedirect.com/science/article/pii/S157064431500012X

4. http://ascelibrary.org/doi/abs/10.1061/%28ASCE%29IR.1943-4774.0000805

5.  Long-term hydrologic simulation using SCS-CN-based improved soil moisture accounting procedure

Hope these articles will help understanding the use of this model or its alternatives.

Please note that the SCS method is used for rainfall and runoff events. It may also be used for daily calculations, assuming them as events. But it is not appropriate to use this method for annual calculations.

As early  said correctly, the SCS method is valid for storm event. It does not work for monthly or annual time scale.

Most folks who had answered ahead of me addressed your concern very well. The SCS-CN method best works in a daily  time step if you could get daily data. However, if you could not get a daily data, event based approach could as well perform so well. The method does not work so well in a monthly or annually time step.

See the following:

http://ascelibrary.org/doi/abs/10.1061/(ASCE)HE.1943-5584.0001038

All of above are right, scs method was applied for storm event (individual rainfall) and when you use this method besides mentioned Raynaud, must be calculate the amount of rainfall during 5 days before.

All comments are right. It is event based model and there is no possibility to apply it correctly for monthly data. I would like to add one more comment. For modelling in general, you should mind besides others the purpose and the catchment size. SCS-CN method is intended for event modelling of storm rainfall as mentioned by others. Moreover, in small catchment, you can face the problem, that daily data are not sufficient for such type of modelling. In this case, you should be aware of the fact, that you can obtain rough estimate of direct runoff depth/volume but generally nothing else.

The SCS/NRCS curve number model to determine runoff from rainfall is an event model.  The developers used recorded daily rainfall/24 hr  and runoff volumes to get the basic curve numbers.

Thank you all for your answers. Ajmal, thanks for those articles. It is clear from all comments that the SCS curve number is an event-based model. Most probably, the runoff coefficient method is most suited for monthly or annual data.

SCS method is designed based on daily rainfall data not on monthly or yearly rainfall...

SCS method is an event based method. So it only estimate runoff for certain storm duration. It can be less that 24 hr even. But I did not try it for more than one day. It could be possible but for certain limitation I guess. 

SCS CN method is suitable for estimating daily runoff. This daily runoff can be added together to get monthly or annual runoff.

It right that SCS curve no. method is suitable for event basic, but I have one query. If i want to compute Annual runoff from daily rainfall data (IMD data) using SCS CN method, how should i proceed? weather i should consider different events in the year or it can be applied on daily scale and how?

Using daily rainfall data (and considering other parameters) in SCS CN method, we will get the daily runoff. Like that we have to estimate the daily runoff throughout the year (for all the days considering the AMC conditions). By taking the cumulative value of all the daily runoff in that year, we will get the annual runoff for that year.

Thanks R. Rejani..

Thanks Jain and Rejani for that question and answer. Yes indeed I think you can use the SCS curve number to estimate the runoff (or excess rainfall) using daily data and then after you can add up the runoff values for the whole year to get annual runoff. Of course, it is important to take into account the antecedent moisture content (whether dry or wet). 

NO

Hello Seyed, kindly clarify why NO. thanks.

As several  people have correctly pointed out, the CN method is meant for a rainfall event (not for daily rainfall, per se). So it is very important to understand what an event is. A rainfall event is defined as the period between two non-rainfall(dry) periods. A non-rainfall period is defined as a the period with rain less than x (zero or .001 in)  for Y hours(or longer than Y hours). A typical value of Y is 6 hours. So you may have zero rain just short of 6 hours during a rainfall event with that definition.  It would be best to calculate the annual runoff by splitting your annual rainfall into multiple rainfall events that add up to your total annual rainfall.

What is the error associated with runoff estimation from NRCS-CN method in ungauged watershed. e.g a general error percentage.

Can I use SCS method for hourly runoff estimation?

Dear James, it depends on what do you mean by hourly runoff. From the principle of the method, it can be applied at any time from the beginning of rainfall. This means that you can calculate the runoff at any time and get the runoff values hour by hour. However, you should work with the totals from the beginning of rainfall instead of application of the method to each hourly total separately. This would give you wrong results.

Thank you Vaclav. If I understand you correctly, if I have hourly rainfall data for 24 hours, because not every hour has rained, there are some hours in between don’t have rain. So I cannot use this method hOIRLY, correct?

Moreover, even I can use, can I use those curve numbers directly since they are detrieved from daily experiment? Or I need detrieved my own hourly curve number based on a hourly runoff experment in a field?

Well, it depends on how do you define the rainfall event. Usually, one hour free of rain is not considered to interrupt the event and therefore you can use it. The duration of rainless period to be considered as an interuption of rainfall event depends on several factors such as the area of the drainage basin, time of concentration etc. Hence, you should look at your data and assess it from this point of view. Then, you can apply the CN method at each time within your event (using cumulative values of precipitation and getting cumulative values of runoff). The rate of runoff can be then easily calculated as a derivation of the runoff cumulative line (the hourly total will be the difference between the cumulative value at the beginning of the hour and its end).

Got it. Thank you Vaclav for the explaining and suggestion!

How to map annual runoff

SCS - CN method estimates surface runoff depth using daily rainfall magnitude. But annual rainfall will not be accurate to estimate surface runoff depth due to inconsistency of surface and climate characteristics.

CN procedure was developed for daily rainfall. The total amount during the period.