Basin lag times can be calculated by the CN Lag Method; which is based on the hydraulic length of the sub-basin, CN value, and basin slope. Some paper says,The CN Lag Method was developed for sub-basins less than 2,000 acres (approximately 8 km2 ). It is such a small area. My question is, is it feasible to calculate basin lag times by CN- method for larger areas? if it is, then up to what extent?
Well,
In an area of homogenous sediments and of low relief ratio, it is feasible to calculate basin lag time, but in heterogeneous sediment with high relief ration, you can not used CN- method for large area to determine lag time in small scale area.
advice to search about same condition for your application.
Sincerely
"The NRCS lag method was developed from observations of agricultural watersheds where overland flow paths are poorly defined and channel flow is absent." In addition, this method is using average slope, CN,and a regression formula for calculating the length from the draining area. having said that, the area is a limiting factor because as the size increase your averaged estimation and regression formula well less be representative of real condition. I suggest if you have a gaged watershed with similar size nearby use the formula, compare it with the recorded data, make some corrections and then apply the corrected method to your watershed.
hope this helps.
Amin Kiaghadi
Dear sir,
As I am working with a project area of over 2000 square km. I had sub-basin size of maximum 434 sq.Km and minimum of 24 sq.Km. I am using Passini model to calculate time of concentration and 0.6*time of concentration take as basin lag. Some papers says, Passini method is good for calculation of lagtime in big watersheds and there are other model like Bransby-Williams too .May I use both NRCS and Passini considering the size of the watershed? And for the watershed I am working on has limited observed discharge data and has no automated gauge.
I would say use both methods and compare the results. If they are similar you are good to go. Passini model also have an alpha that need to be determined. The other option is to delineate your sub-watersheds even more if you have the flow-lines and then apply each method.
Like all empirical formula, the CN method for calculating lag time should only be used within the parameter range used to derive it. Having said that, when compared to other methods for calculating lag time (all of them empirical), the CN method has a better physical basis - in that it is based on the time of travel of water along various flow paths. So, there appears to be no compelling reason why it could not be used for larger basins. I found that the CN method is very reliable and used it for basins as large as 20 km2.
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