2. Saturated Soils Needed n Water is initially drawn into soil by gravity and capillary forces. n Runoff occurs once the soil is saturated and rainfall exceeds infiltration rates. n How quickly this occurs depends on the soil… NCSU Soil Science R. A. McLaughlin

3. Soil Infiltration Rates Decline Over Time NCSU Soil Science R. A. McLaughlin

4. Raindrops: The Start of Erosion TIME: 0 sec 1/1400 1/150 1/70 From Hillel 1998 Crater formed Soil is displaced Displaced soil is deposited Rain droplet falls on saturated soil. NCSU Soil ScienceR. A. McLaughlin

5. Slope Makes Big Difference TIME: 0 sec 1/1400 1/150 1/70 From Hillel 1998 Soil is displaced Displaced soil is deposited, farther downslope than upslope NCSU Soil Science R. A. McLaughlin

6. Actual Droplet Splash on Saturated Soil NCSU Soil Science R. A. McLaughlin

7. Splash Has Removed Fine Materials, Left Rocks NCSU Soil Science R. A. McLaughlin

8. Surface Sealing and Crusting n Rain droplets break down aggregates, sort the soil. n Smallest particles form layer on surface. n Infiltration rates decline 10X, often after only one storm. n A crust forms as the soil dries. NCSU Soil Science R. A. McLaughlin

9. Soil Erodibility All Soils Are Not Created Equal n Texture or Particle Size Distribution –Silt is the most easily eroded component. –Clay tends to remain bound in the soil structure. Once caught up in runoff, it is very difficult to settle out. –Sand promotes infiltration, reducing runoff volume, and tends to settle quickly. n Organic Matter: increases infiltration so runoff volumes are lower. NCSU Soil Science R. A. McLaughlin

10. Soil Erodibility (cont.) All Soils Are Not Created Equal n Structure –Blocky, platy, or massive are less erodible than granular. n Permeability –Rapid is less erodible than slow! NCSU Soil Science R. A. McLaughlin

11. Creedmoor Sandy Loam Sandy Loam Sandy Clay Loam Clay NCSU Soil Science R. A. McLaughlin

12. Erodibility Changes With Depth NCSU Soil Science R. A. McLaughlin

13. BIOTITE GNEISS Plott Edneyville-Chestnut Tuskasegee Dellwood Tuckasegee GRANITE Wayah Climate, Landscape Position and Parent Material Affect Soil Characteristics NCSU Soil Science R. A. McLaughlin

14. Soil Erodibility (K) Calculation % Silt/Very Fine Sand % OM Structure Permeability % SAND NCSU Soil Science R. A. McLaughlin