ABE TUTORIALS

AREA 2 PROBLEM  2 : SCS CURVE NUMBER ANALYSIS

MARIKINA WATERSHED (500 SQ. KM.) HAD AN AVERAGE CN VALUE OF 65 IN 1970 WITH A STORM DEPTH OF 100 MM. AFTER 30 YEARS, THE SAME STORM OCCURRED BUT THE CN VALUE INCREASED TO 84 DUE TO LAND USE CHANGE. WHAT IS THE PERCENTAGE CHANGE IN THE DEPTH OF RUNOFF?

---

a. 1.33%

b. 80%

c. 41%

d. 133%

Correct Answer: D (approx. 133%)

Step-by-Step Solution:

We use the SCS Runoff Equation where S = (25400 / CN) - 254 and Q = (P - 0.2S)² / (P + 0.8S).

1. Calculate for 1970 (CN = 65):

S1 = (25400 / 65) - 254 = 136.77 mm
Q1 = (100 - 0.2×136.77)² / (100 + 0.8×136.77) = 25.32 mm

2. Calculate for 30 Years Later (CN = 84):

S2 = (25400 / 84) - 254 = 48.38 mm
Q2 = (100 - 0.2×48.38)² / (100 + 0.8×48.38) = 58.98 mm

3. Calculate Percentage Change:

% Change = [(Q2 - Q1) / Q1] × 100
% Change = [(58.98 - 25.32) / 25.32] × 100
% Change = 1.329 × 100 = 132.9%

Final Result: Approximately 133%