Question

Using the soil-cover-complex (SCS) method, determine the total volume of runoff, in acre-ft, to be expected from a 50-yr, 24-hr rainfall event occurring at the southwest corner of Wyoming. Surveys indicate the soil is a clay loam, the land has been used as rangeland, recent conditions have been wet, and the land has been heavily grazed for a long time. The drainage area is 200 mi^2.

Answer 1

Soil is a clay loam. Hence, it belongs to Hydrologic soil group D

Land has been used as rangeland, recent conditions have been wet, and the land has been heavily grazed for a long time. Hence, curve number = 80

Curve number adjustment for wet condition, CN = 80 * 1.14 = 91.2

Potential maximum soil moisture retention, S = (1000/CN) - 10 = (1000/91.2) - 10 = 0.965 in

Assume, 50-yr, 24-hr rainfall = 1.2 in

Runoff, Q =

Q = = 0.514 in

Drainage area = 200 = 200 * 640 = 128000 acres

Total runoff volume = Drainage area * runoff = 128000 * (0.514/12) = 5482.67 acre-ft