Question

A 1 hour - 1 cm unit hydrograph of catchment is in the form of a triangle of base width 20 hr and peak discharge $75m^3/s$ occurring after 7 hr from start, then the area of catchment is____ $k{m}^2$.

• A. 270

Answer 1

The correct option is A 270
The unit hydrograph can be shown as


Total runoff = Area under the hydrograph

$=\frac{1}{2}\times 75\times 20\times 3600$

$=2.7\times {10}^6{m}^3$

Let the catchment area = A $k{m}^2$

Total runoff from the catchment area A $k{m}^2$ from a precipitation of 1 cm/h for 1 hr
$=(A\times {10}^6)\times \frac{1}{100}=A\times {10}^4{m}^3$

Since runoff calculated in (i) and (ii) are equal
$A\times {10}^4=2.7\times {10}^6$
$A=270k{m}^2$