Question

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

Answer 1

Answer:

5.625 inches

Step-by-step 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:

$Q = \dfrac{(P-0.2S)^2}{(P+0.8S)}$    for P > 0.2S

Q = 0                     for P < 0.2S

$S = \dfrac{1000}{CN}-10$

where:

curve number CN = 80

$S = \dfrac{1000}{80}-10$

S = 2.5 inches

Since the rainfall (P) is greater than 0.25

Then:

$Q = \dfrac{(P-0.2S)^2}{(P+0.8S)}$  

$Q = \dfrac{(8-0.2(2.5))^2}{(8+0.8(2.5))}$

$Q = \dfrac{(8-0.5)^2}{(8+2)}$

$Q = \dfrac{(7.5)^2}{(10)}$

$Q = \dfrac{(56.25)}{(10)}$

Q = 5.625 inches

Thus, the runoff volume = 5.625 inches