CALCULATION OF DIRECT RUN-OFF FROM STORM HYDROGRAPH

• From storm hydrograph measure ordinates at fixed time intervals.
• These ordinates represent discharge or flow (Q).
• Also measure the ordinates of base flow, at the same time interval as adopted earlier.
• By subtracting ordinates of base flow from corresponding ordinates of total discharge Find out the net ordinates which represent direct run-off in that time interval.
• Add up all the ordinates of direct run-off which is nothing but total volume of direct run-off.
• By dividing total volume of direct run-off by area of the basin, direct run-off available from the catchment is obtained.
• The method can very well be understod from the following illustrative example.

Direct run-off in depth of water = Total

$= \frac{Total volume of direct .run-off }{Area of the basin}$

$= \left [ \frac{\Sigma E\times t\times 60\times 60}{A\times ^{}10} \right ]\times 100$

$= \frac{0.36 \Sigma R\times t}{A}cm$

where

• t = time interval in hours between successive ordinates
• A = Area of catchment in km2
• Q = Discharge in cumecs.
• ΣR = Sum of discharge ordinates in cumecs or direct run-off is cumecs. In above example t = 2 hrs. and let A is 40 km2.