For "small" catchment, the "Rational method" gives satisfying results.

May you give more information about you case, it will help to provide you with a relevant information.

Regards. 

I would first search for any streamflow and rainfall stations in extended areas within same climactic and physiographic area as examples of the rainfall-runoff response, even though small, steep catchments may be more responsive than gauged watersheds or basins.  If there are none, and you have have access to soils and other data to help evaluate, you may consider one or more approaches to help compare results and hoping for some consistency.  If you have some stream gauging and rainfall stations, you might try to validate one or more methods against the measurements taken, and/or make adjustments if outputs are consistently or predictably off.  

Hi Si mokrane

i'm doing a research about suitable rainfall-runoff methods for small catchment. For that, i want to know the research based information related with small catchment.

thanks   William F. Hansen

For example the rational method. It is fully documented in any text book on Sanitary Engineering. 

SCS-CN method of the soil conservation services (USA) is simple and predictable method for calculating runoff depth.It is followed in many countries including India.Data required includes storm rainfall depths.Through  Land use information we assign curve number ranging between 0 to 100. The method is well documented. For steep watersheds the land use could be of group D, having high runoff potential. Initial abstraction and antecedent moisture condition is also taken into consideration.

Yes its right that for smaller catchments Rational Model can work sizeable well ...but since here the catchment is not only small , but very steep too; hence to the my way of thinking Rational method may mislead , as on steep slopes the physics is entirely different . The transformation of rainfall into runoff need not be of Hortoriian way...the concept could be better as of ' partial runoff producing area' kind.  so if we like to be adhered to simpler options, perhaps ' Time Area based rainfall runoff model could offer a better result...as the time of concentration on steep catchments is excessively small but having bigger effects on time to peak and also the peak discharge rate..which in physicall sense most important as they influnce the soil  erosion  and other managerial issues of small watershed..

 Comments please ..so let me boost my knowledge too on this important question...

In my opinion, Curve Number Method is a very good option to estimate runoff of a rainfall event. You can estimate runoff for the entire rainfall event or for time periods. Then you can use rational method to estimate peak flow with constant intensity-rainfall intervals  in the first case, or by means of the SCS unit hydrograph and periods with different rainfall intensity.

See: Part 630 Hydrology. National Engineering Handbook. Chapter 10 Estimation of Direct Runoff from Storm Rainfall. USDA NRCS 2004 and other relates in Natonal Engineering HAndbook.

It is very useful to calculate peak flows with different methods to see if the calculus are somehow reasonable. You can compare the differences to see if they are significant or they tend to a similar order of magnitud. Also you should study the control cross section of your catchment to look for clues or signals about the levels of ordinary and extraordinary peak flows. This is useful to know if the results of the different methods are aproximated to reality.

Ratonal method normally overestimates peakflows for different return periods, so you have to consider if results are admisible or very high.  In hydrology results obtained with hydrologic methods often varies +- 20%. If you have peakflow data from a limnygraph station in the watershed or in a very similar one close to yours, the statistical analisys and comparing the characteristics of soils, slopes and vegetation cover of the catchments you can extrapolate good results.

Rational method has been used around the world during decades in very steep catchments to design gavion or concrete check dams. It works reasonably well.

dr M L Gaur

what is the  Time Area based rainfall runoff model that u recommended??

and what are the problem come up with rainfall measurements? it is about 10km^2 steeep catchment area.

Rational method is recommended for smaller catchments than 75 km2

Hi Hashitha Susantha Bandara,

I think you should use more than one method and compare the results.

SCS method that use the curve number (CN) was developed in the US for small catchments where the main use of the soil is agricultural.

You could also try the rational method. It has satisfactory results for small catchments (area < 25 km2).

There are softwares you can use too. Try HEC-HMS (http://www.hec.usace.army.mil/software/hec-hms/).

I hope this helps!

thanks for all

We don't use the rational method on any catchment above 0.8 km2. 

In addition to the Rational method the other methods available are SCS-CN and time-area based method like Clarke's model are most useful for small catchments. The case referred here corresponds to a hilly catchment with steeper slope and of smaller size. The  slope of the runoff conductor system i.e overland and main channel  dominate the runoff process as compared to other geomorphologic and land use parameters. The time of concentration Tc that accounts for the remotest water droplet to reach the outlet has been used in Clarke's model. The accuracy of  Clarke's model depends on how accurately Tc is computed and various literature are available for determination of Tc. So in my opinion Clarke's time-area method is most suitable for such studies.

In rational method Q=CIA, C being the runoff coefficient and it shows signs of non-stationarity, the method may lead to either underestimation of overestimation of the peak runoff rate. 

I would like  to advise Harshita to refer the following papers if those are of some help to him and a paper is attached regarding Tc .

DOI: 10.1061/(ASCE)HE.1943-5584.0000796

DOI: 10.1061/(ASCE)HE.1943-5584.0000828

DOI: 10.1061/(ASCE)HE.1943-5584.0000380

Hope for feedback from all the participants.

There are several methods presented in the US Army Corps of Engineers HEC-HMS (Hydrologic Modeling System) program and documentation. With many of the procedures, you will need local condition calibration. 

You will find many methods to estimate time of concentration or time to peak. Most will not consider very steep slopes. If your slopes are so steep that supercritical flows are likely to occur, most time of concentration or time to peak functions cannot account for this condition. Some very steep slope adjustment equations were initial developed in Denver, Colorado, USA and applied to steep slope criteria in Albuquerque, NM,USA.  These equations are based on cascading flows that may occur in natural steep slope channels.

thanks for all..

all advises are very useful for me

see also the attached file

You can try the good old unit hydrograph - but make sure that rain data has a high resolution.

You can use SCS method that using the curve number (CN) was developed in the US for small catchments. Or Simply apply the rational method Q=CIA.