Review: overland water erosion PROCESSES Generate runoff – Saturation excess – Infiltration excess INFLUENCES Hydraulic soil properties – Conductivity – Compaction/use history Ground cover (interception) Antecedent conditions – Existing moisture – Seasonal barriers

Review: overland water erosion, cont’d PROCESS Raindrop impact – Detachment makes particles available to overland flow Overland flow – Sheet/interrill flow – Rill flow Requires overcoming threshold for erosion INFLUENCES Ground cover/canopy Roughness/resistance – Grain roughness – Form roughness, from furrows, veg., etc. Slope length, steepness and shape – Convergent slopes collect water, focus water/soil energy dissipation

Review: overland water erosion, cont’d PROCESS Runoff entrainment – Grains (cohesionless) – Aggregates (cohesive) Threshold behavior Flow depth, Re smaller than in rivers: different dynamics INFLUENCES Soil texture, structure – Relative exposure Cohesion agents – Clay minerals – Roots/hyphae Flow depth, slope, velocity – Depth often increases downslope

Review: overland water erosion, cont’d PROCESS Transport capacity – Bedload – Suspended load – Dissolved load? INFLUENCES Flow depth, slope, velocity – Critical shear stress – Critical stream power – Critical unit stream power Capacity not necessarily the same as supply – A flow may carry less than its transport capacity, but not more.

Review: overland water erosion, cont’d PROCESS Deposition/storage – Bedload threshold – Suspended load Deposition by settling, may lag changes in fluid motion INFLUENCES Flow depth, slope, velocity – Critical shear stress – Critical stream power – Critical unit stream power Along-slope changes in slope, velocity, can promote deposition – Increased roughness, footslope

Modeling soil erosion EMPIRICAL *USLE – Easy for end-user – Possibly accurate predictions – Broadly used, promoted – Not spatially-explicit – Limitations to time and space scale PHYSICAL/THEORETICAL WEPP, LISEM, MIKE-SHE – Not as intuitive – Difficult to constrain, capture complexity – Research tool – Spatially-explicit, event or long-term basis – Physically sound