Basically if your outlet station records water level on stream per time, and you have stage (level) relationship to discharge in cubic meters per second, calculate the flow by time unit, and add it up for each month, and your total will be in cubic meters for each month.  Your watershed will have a size in square kilometers.  Each sq kilometer is 1 million sq meters.  Divide the monthly cubic meters by the watershed area in square meters, and you will get meters of water yield averaged over the watershed.  You may prefer to report that in cm or mm, and can make the conversion to compare with rain data from your station.  Most of this can easily be done with an excel spreadsheet, and there are various ways to report results on a daily, monthly or annual basis. 

If this is not what you were wanting, perhaps I misunderstood and you may clarify. 

You can try using SCS- Curve NUmber method to calculate the runoff volume. You could use the discharge values you are collecting at the outlet to calibrate the CN runoff coefficients. Some of the observed values could be used in validation.

Hi Dears

can you guide more clear. I cant understand you.

I have one hydrometric station in the outlet and I have just monthly discharge data there. In addition I have monthly rainfall for eight stations of into and outside but near in the study watershed. By this data how I can map the discharge for entire the watershed.

Thank you a lot in advance.

You can use SWAT software.

Dear Zeinab,

As said by William, if your outlet station records water level on stream per time, then he has explained very nicely how to do it.

But if you have only rainfall data from one station, then question arises if your area is small or not. If your catchment area is very small, it can be assumed that the single station rainfall is equal to rainfall over whole catchment, then you can use any rainfall-runoff calibrated model to find the monthly streamflow. They will use mainly SCS-CN method or Green-ampt-mein-Larsen method for that, as used in SWAT. (But SWAT needs other meteorological data too, apart from rainfall. You can go for a simple rainfall-runoff model.)

But, if your catchment area is large, it is not wise to compute streamflow from only one station. You may have to follow a synthetic approach.

I hope it helps.

All the best !

Dear Zeniab

Please check the attached file may be useful.

Dear Zeinab,

If your aim is to calculate (model) monthly discharge based on monthly rainfall, you probably first want to estimate the catchment averaged rainfall. If you have the coordinates of the stations and boundaries of the catchment (e.g. based on a digital elevation model), you can apply a simple spatial averaging method like Thiessen polygons to obtain the catchment averaged rainfall. In this method, rainfall measurements at individual gauges are first weighted by the fractions of the catchment area represented by the gauges (Thiessen polygons) and then summed. The catchment area is divided into Thiessen polygons by lines that are equidistant between pairs of adjacent stations.

Subsequently, the monthly catchment averaged rainfall can be transformed to monthly catchment runoff using a simple conceptual model (can be even done in Excel) or for instance empirical methods like the rational method or SCS-CN method. Be aware that most of these empirical methods mainly estimate the direct runoff and not the total runoff (including baseflow). A simple model in Excel might consist of two reservoirs (buckets); one for fast runoff and one for slow runoff. Discharge behaviour of the slow reservoir is typically linear and discharge behaviour of the fast reservoir non-linear. Such a model would require 4-5 parameters to calibrate using the observed discharge data. For any model it is important to separate the data series in a calibration and validation period.

Good luck,

Martijn

Dear Dr. Martijn

Thanks but i didnt have storm data for using SCS_CN. I have just monthly discharge data.

That is indeed what I understood: you have monthly discharge data and (after averaging) monthly catchment averaged rainfall data. It would be nice if you can also find at least some temperature data to estimate evaporation so that you can make a simple bucket model in Excel. The SCS-CN method is indeed not appropriate if you don't have (high) flow data at daily or hourly resolutions. Moreover it is an event-based method and therefore less suitable for monthly time scales.

Thank you. I have annual evaporation map for my study watershed. it is good?

and i found monthly ETP amount for the study watershed.

Ok, then it should be possible to make annual and monthly water balances (P-Q-ET = change in storage) and subsequently set up a simple model for discharge calculation.

If you do not have river gauge data but do have rainfall data you can do a crude estimate using the rainfall data and an estimate of percentage runoff based on rational or SCS (NRCS) methodologies and watershed area.. That is, Q = (P - ET) x C x A, where P=Precipitation amount, Et = evapotranspiration, C = the percentage of rainfall that becomes runoff., and A = watershed area. Depending on the soil type, vegetation and topography (slope) and an estimate of the depressional storage abstraction; in rural watersheds C will range between 0.5 - 0,95 (50 - 95%). I hope that helps - Bill Mac 

You can apply simple empirical equation; Q=PCA. Attached my paper will be useful for your case.

If you have daily streamflows from that station you could use a hydrograph-separation program to identify the recharge or baseflow components. You can find several good programs at: 

http://water.usgs.gov/software/lists/groundwater#flow-based

You can calculate the runoff or discharge using Rational or empirical method where rainfall and the characteristics of cacthment area as input parameters. Hope help you.

 helo,

I think everyone has kept there view and might be correct.

now if you have calculated ET

then you can go for water balance equation for watershed.

rainfall and runoff should be in the same scale,you have to convert discharge in terms of depth ,

else I agree with others have given their opinion .

nice

thanks

Use HEC HMS ! a widely used, popular watershed numerical model.

Hello Dr. Marlekian

Thanks. Please introduce them and i ask you to attach some paper about them. 

Best

The one station in the outlet will help you :

example (Q*1000)/A result in l/s/km2 *2.628e+6 (number of seconds in one month)

Q- average discharge at outlet

A-area of the watershed (km2)

I am agree with Mr. Ivan Radevski

Dear Zeinab,

It is ungaged catchment characterisation. You can use simple conceptual rainfall runoff models, or use distributed catchment scale models ( eg. SWAT) with grid based weather and landscape attributes from remote sensing.

If flow prediction is your topic of interest, I would advice you to try rainfall runoff models that uses rainfall, and potential evapotranspiration data.

Get the temperature, Rainfall and potential evapotranspiration data from remote sensing; or employ Hargreaves method to estimate potential evapotranspiration from rainfall and temperature data and easily use conceptual rainfall runoff models like SimHyd, SMAR, Sacramento, AWBM and Tank for ungauged or gauged catchment flow ssmulation.

Sincerely,

Gebiaw Ayele

Dear Gebiaw

Thank you for your message and useful explanation. It is appreciated if send me some related paper.

Bests

Hi Zeinab,

Attached is a list of few papers for conceptual RR and distributed models.

- http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.1.7130&rep=rep1&type=pdf

- Article Streamflow and Sediment Yield Prediction for Watershed Prior...

Dear Gebiaw

Hello

Thank you. Good paper you sent.

The link doesn't work. Please you also check it . If it is possible sent me again.

With Best Regards