Use Double Mass curve if you know it and convert all the data in a single unit first.

There are several methods available.

1) Flood routing techniques like muskingum method

2) regression analysis

3) Artificial Neural Networks

4) Time series analysis methods such Auto Regressive models

5) Flood frequency studies

Please, clarify your problem, because it is not clear your question now. Inflow is measured in volume units (gallons, cubic meters etc.). per time unit (including month and year too). Runoff can be presented with the same units or with layer units (inches, mm). Would you like to estimate probability characteristics only or you would like to find suitable runoff model, which will give you comparable measured results of monthly and annual inflows in some points or?

Victor Popovych thanks for guidance I have Inflow data in Million Gallons and I need Runoff in Millimeter how i can do this conversion? If you know any suitable literature kindly guide me

Muhammad, 

As Victor pointed out, I am not quite sure what exactly you're  looking for. If you are looking for appropriate method to estimate runoff, I have attached a comprehensive summary of the various methods available at various scales. However, if this is entirely a conversion issue as you pointed out in your clarification then all you need to do is to divide by the area over which the inflow occurred in order to obtain the Runoff in millimeters. 

Technical Report A Comprehensive Literature Review for Soil Covers and the Ro...

Thank you, now your question is clear enough.

To recalculate volume discharge into layer units it is necessary to have basin area upstream to your inflow point(s).

I hope you have data on upstream area (in square miles or kilometers or hectares), because usually such data available from local organizations. If you have not such data you can use some GIS software (ArcGIS, MapWindow, QGIS etc.) or, for example, SWAT agrohydrological model  to create map of the your upstream basin territory, using digital elevation model data of this area.

 If you have data on upstream area (in hectares - F, for example) you can use next relations.

 1. I do not know which units you wrote about US or UK “gallons”, so below are formulas how to recalculate different units to litres.

 A US capacity measure (for liquid) equal to 4 quarts or 3.785 liters. Note also there are different measures of US dry gallons and UK gallons.

 L =  US gal lqd / 0.26417

 The imperial (UK) gallon is officially defined as 4.54609 litres.

L = UK gal / 0.21997

 2. Runoff in layer units (per certain period of time, month or year, for example)

         R = V/F, taking into account units (area (F) in hectares and volume in liters (L)

        R= 10 * 0.001 * L / F, mm

Here,

         10 – coefficient to transfer cubic meter per hectare to millimeters,

0.01     – transferring liters to cubic meters,

L – inflow in litres,

F – upstream basin area in hectares ( 1 hectare = 10000 square meters or 0.01 square kilometer).

I hope you will take into account that your inflow data presented at million gallons.

Thank you, now your question is clear enough.

To recalculate volume discharge into layer units it is necessary to have basin area upstream to your inflow point(s).

I hope you have data on upstream area (in square miles or kilometers or hectares), because usually such data available from local organizations. If you have not such data you can use some GIS software (ArcGIS, MapWindow, QGIS etc.) or, for example, SWAT agrohydrological model  to create map of the your upstream basin territory, using digital elevation model data of this area.

 If you have data on upstream area (in hectares - F, for example) you can use next relations.

 1. I do not know which units you wrote about US or UK “gallons”, so below are formulas how to recalculate different units to litres.

 A US capacity measure (for liquid) equal to 4 quarts or 3.785 liters. Note also there are different measures of US dry gallons and UK gallons.

 L =  US gal lqd / 0.26417

 The imperial (UK) gallon is officially defined as 4.54609 litres.

L = UK gal / 0.21997

 2. Runoff in layer units (per certain period of time, month or year, for example)

         R = V/F, taking into account units (area (F) in hectares and volume in liters (L)

        R= 10 * 0.001 * L / F, mm

Here,

         10 – coefficient to transfer cubic meter per hectare to millimeters,

0.01     – transferring liters to cubic meters,

L – inflow in litres,

F – upstream basin area in hectares ( 1 hectare = 10000 square meters or 0.01 square kilometer).

I hope you will take into account that your inflow data presented at million gallons.

Victor Popovych What about this conversion?

I have Inflow in Million gallon I will convert it into Acre-feet 

Runoff in feet = Inflow in Acre-feet / Catchment area in Acres

Runoff  in millimeter = Runoff in feet*304.8

I have done conversion by using this formula excel sheet is attached kindly guide me is this valid conversion.  

Victor Popovych What about this conversion?

I have Inflow in Million gallon I will convert it into Acre-feet 

Runoff in feet = Inflow in Acre-feet / Catchment area in Acres

Runoff  in millimeter = Runoff in feet*304.8

I have done conversion by using this formula excel sheet is attached kindly guide me is this valid conversion.  

Dear Muhammad, sorry I did mistake yesterday (put a wrong coefficient) at formula for runoff calculation in millimeters and Volume measured in cubic meters with Area measured in hectares.

10 cubic meters per hectare is equal 1 millimeter of layer, so first coefficient in last formula has to be 0.1 instead 10.

To avoid such mistake, which I did, it is recommended to use the same units for calculation, for example, cubic meters for volume and square meters for area. In this case layer will be received in meters too.

                         Rm = Vm / Fm,  [m]

Here,

Vm – inflow, cubic meters;

Fm – upstream basin area in square meters.

After that you can calculate layer in necessary units (1 m = 1000 mm).

I hope you have time to correct my mistake.

If you don’t have basin area figure I can send you link to downloading necessary software to calculate it.

Please, find your excel sheet with different variants of calculations.

You did s mistake on second page when transfered data from MG to AFT/

Victor Popovych I am much thankful for your guidance. Stay blessed.

You can use either time series analysis or Artificial neural networks with wavelet and SVM techniques