First, divide the discharge values (m3/s) by the area of your watershed.

Then you get the flow values in m/s. Finally, multiplying the values by time in second give you what you want as follows:

Since the precipitatin unit is in mm/day, you could get mm/day for discharge by

Q (mm/day) = (Q(m^3/s) *1000*24*3600)/(Area (m^2))

thank you. What do I do next?

Hi Keerthi,

Firstly, you should transform the recorded streamflow data to daily numbers by doing this ( Current data * 3600*24), now you have daily streamflow numbers. Afterward, there are bunch of empirical formulas you can apply them to calculate run-off , but to do it precisely you should simulate the run-off using lump or distributed models considering what kind of basin you work on . Second scenario is that you have already computed run-off data and you need to compare it with discharge right in the concentration point of target basin, so change them into daily numbers no matter what their units are.

Thanks

Hi, Keerthi

From your post, I presume you want to compare the runoff data you have to the discharge data you have. The conversions have been well covered by Naderi, so I won't repeat them here.

As I'm sure you know, your larger question--is the runoff I've measured or modeled correct compared to the rainfall that fell on the basin--can be difficult to determine. How much runoff you get from a particular drainage basin--for a particular storm, over the course of a sampling period, or even annually--is dependent upon a variety of independent and dependent watershed variables. Generally speaking, however, your calculated or observed runoff should be approximately anywhere from 10% to 30% of your total available precipitation. Gordon and others (2004) give a good description of the parameters in a general ecological context. An out-of-print book I like is "Water in Environmental Planning" by Thomas Dunne and Luna B. Leopold. They also exhaustively discuss the factors that influence watershed runoff characteristics. Good luck!

Reference:

Gordon, ND, TA McMahon, BL Finlayson, CJ Gippel, and RJ Nathan. 2004. Stream Hydrology: an introduction for ecologists. Published by John Wiley & Sons, Ltd. pp. 49-73