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 = 275 K = LS = 0.758

P Factor: Use only when CONTOUR TILLAGE is specified Tillage parallel to contour lines: P=0.5 All other cases: P=1 C = P = 1 A = 275 x x x x 1 = t/a/y