Lab 13 - Predicting Discharge and Soil Erosion

• Estimating Runoff Depth using the Curve Number method – Land use or cover type– Hydrologic condition– Soil type

• Estimating Peak Runoff– A function of the Time of Concentration, Tc– The faster the runoff time => the bigger the peak

• Estimating Erosion Losses using the Universal Soil Loss Equation– Precipitation– Topography– Soils– Land use or land cover– Treatment or conservation practices

Example calculation:

• 100 ha Piedmont watershed• Residual soils (impermeable B/C horizons)• 4-6% slopes• 150 mm rainfall event

• What is total runoff (mm) and peak runoff rate (L/sec)?

Curve Number Method

• Hydrologic group: C• CN: 70• Total (direct runoff) = 70 mm

Time of concentration:Depends on watershed area, CN, and slope

• Time of concentration = 1.5 hr

Unit peak discharge = 1.2 L/sec/mm/ha

Peak Discharge• Qp = Up · A · Q

– Qp is peak runoff (L/s)– Up is unit peak (L/s/ha/mm)

• a function of Tc• Tc is the time of concentration, or the time for water to runoff

of the watershed• the faster the runoff time, the bigger the peak

– A is watershed area (ha)– Q is runoff depth (mm)

• From the curve number method

1.2 L/s/mm/ha x 100 ha x 70 mm = 8400 L/s

Universal Soil Loss Equation• A = R · K · LS · C · P

– A is soil loss in tons per acre per year– R is rainfall erosivity factor– K is soil erodibility factor– LS is length-slope (topographic) factor– C is the land use or land cover factor– P is the treatment or conservation practices factor

Example Calculation:

• Cecil sandy loam, 3% humus• Clarke Co. GA• 5% slope grade, 200’ long• Initial land use: undisturbed forest,

– 95% leaf litter, 95% canopy

Estimate soil loss (t/a/yr)

Rainfall Erosivity Factor

R = 275K = 0.215LS = 0.758

P Factor: Use only when CONTOUR TILLAGE is specifiedTillage parallel to contour lines: P=0.5All other cases: P=1

C = 0.0005P = 1

A = 275 x 0.215 x 0.758 x 0.0005 x 1 = 0.022 t/a/y