1. Soil erosion by water EROSION Detachment – particle or aggregate loosened from surface Entrainment – initiation of motion by flow TRANSPORT Rill – concentrated overland flow Inter-rill (sheetwash) – diffuse overland flow

2. Detachment processes Raindrop impact Bioturbation Frost action Drying What soil properties govern susceptibility to detachment?

3. Runoff generation Runoff generation depends on antecedent soil moisture conditions, soil infiltration capacity, rainfall intensity. Runoff may be generated either when rainfall intensity exceeds infiltration capacity or when soil is saturated.

4. Entrainment & transport First, generate runoff…

5. Generating runoff Soil erosion by water (usually) requires surface runoff or overland flow

6. Generating runoff How does runoff vary in space and time?

7. Soil Conservation Service “Curve Number” method

9. CN example 4-inch rainfall on pasture in good condition. Soils are in hydrologic group D (slow infiltration rates; compacted or clayey), antecedent moisture conditions are II (near field capacity).

10. CN example 4-inch rainfall on pasture in good condition. Soils are in hydrologic group D (slow infiltration rates; compacted or clayey), antecedent moisture conditions are II (near field capacity).

11. CN example

12. What are possible applications of the CN method for erosion problems? What are drawbacks?

13. Green-Ampt Infiltration

14. Green-Ampt cont’d Infiltration capacity parameters are strongly sensitive to soil type.

15. Q: How does runoff flux vary with position on a hillslope with uniform soil properties?