AGEN 2363 Chpt. 4 notes.  Water erosion and sedimentation.


I. Three types of soil erosion by water:  

            1) sheet, accounting for most of the losses,

            2) rill, small rills that can be plowed out,

            3) gully, larger rills and canyons that can’t be tilled out.


II.  Problems of soil erosion:

   A. Loss of crop productivity:

            - nutrients and soil organic matter wash away,

            - structure, permeability, and tilth decline,

                        (define soil tilth)

            - exposed sub-soils that may be acidic, clayey, alkaline, or rocky.

            - decrease rooting depth to bedrock or restrictive layers in sub-soil.

   B. Accelerated sedimentation of ditches, drainageways, ponds and lakes:

            - increases flooding, cost of maintenance, eutrophication,

            - alters the existing natural ecosystem,

            - may carry pesticides, excess soil nutrients,

   C. Engineering/structural damage:

            - roads, buildings, canals, etc are damaged from erosion,

            - heavy fines for excessive erosion in some cases. 


III.  Process of soil erosion by water.

   A. Two-step energy process:

            1) detachment,  2) transport.

   B. Detachment:

            1. rainfall characteristics: 1) intensity, 2) duration, 3) raindrop size.  Due to friction,

                larger raindrops have higher terminal velocities and thus, due to their greater mass,

                have proportionally much greater energy than smaller raindrops. 

            2. Wind velocity can sometimes increase raindrop impact velocity.

            3. Return Period Storm (handout): the 30 min. intensity and rainfall amount in

                24 hrs that will occur once every 5, 10, 50, or 100 years for a given region.

            4. Effect of surface residue and cover: dramatically absorbs energy of falling

                raindrops, thus reducing detachment force. 

            5. Runoff.  Flowing water can detach soil particles.

                   laminar flow – level flow lines, less energy.

                   turbulent flow – non-level, circular flow lines, greater energy.

            6. Storm bands – crescent-shaped bands of very high intensity rainfall and wind

                    that occur in intense rain storms and hurricanes.  Can cause considerable damage.








   C. Runoff.

            1.  Runoff  =  rainfall amount  –   infiltration rate  –  micro-relief ponding

            2. Factors affecting runoff:

                        - % slope, slope length

                        - rainfall intensity

                        - infiltration rates:      ‘surface seals’ – define, factors affecting

                        - surface roughness

            3. Percolation = soils capacity to move infiltrated water down/through soil

                        (distinguish between infiltration and percolation and factors affecting)

            4. Initial soil moisture, especially in the upper few cm.


   D. Gravity effects:  landslides, mudslides, slips, slumps, and creep.


IV.  Soil factors affecting erodibility and erosion.

            1. texture –

            2. structure –

            3. organic matter (humus) –

            4. aggregation –

            5. cation exchange capacity –

            6. type of clay –

            7. cementing agents: soil microbe excretions, Ca2+, Mg2+

            8. structure destroying agents:  excess Na+

            9. cropping systems

            10. surface seal formation (related to all of above!)




            1. Reduce raindrop impact energy!

            2. Reduce runoff volume and velocity!

            3. Increase soil resistance to detachment!


Everything done in soil and water conservation keeps these three principles in mind, especially during times of the year when soils are most venerable to damage, like from October to May!