Question

) A watershed experiences a rainfall of 8 inches. What is the runoff volume when the curve number is 80

Answer 1

5.625 inches

Explanation:

Given that:

Total Rainfall in inches (P) = 8 inches

The runoff volume (in inches) Q = ???

The curve number CN = 80

Recall that: The runoff volume can be calculated by using the formula:

`Q = ((P-0.2S)^2)/((P+0.8S))` for P > 0.2S

Q = 0 for P < 0.2S

`S = (1000)/(CN)-10`

where:

curve number CN = 80

`S = (1000)/(80)-10`

S = 2.5 inches

Since the rainfall (P) is greater than 0.25

Then:

`Q = ((P-0.2S)^2)/((P+0.8S))`

`Q = ((8-0.2(2.5))^2)/((8+0.8(2.5)))`

`Q = ((8-0.5)^2)/((8+2))`

`Q = ((7.5)^2)/((10))`

`Q = ((56.25))/((10))`

Q = 5.625 inches

Thus, the runoff volume = 5.625 inches