Soils 205- General Soils

Lecture 15- Soil erosion

I.  Erosion is a natural process that involves the forces of both water and wind.  What we are concerned about is accelerated erosion.

a.  Water erosion

The process of water erosion includes two steps:

1)

2)

    i.  Types of water erosion

        1. Splash erosion

        2.  sheet erosion

        3.  Rill erosion

4.  Gully erosion- The process has reached the gully erosion stage when the channels have developed into deep incisions in the landscape.  At this point, major landmoving is needed to restore the land.

Note:  These types of erosion differ only by the degree of severity.  Sheet erosion may develop into rill erosion as water concentrates into channels.  Deep rills may become gullies.

II.  Universal soil loss equation (USLE)-

A = RKSLCP

A =

R=

K =

S =

L =

C =

P =

We will discuss how each of these factors influence the soil lost from a given area over time:

1) Rainfall factor = R

Depends on:

    -

    -

    -

R factor map

2)  Soil erodibility factor- K

Depends on:

- susceptibility of soil materials to be transported.  Transport is a function of particle size and aggregate or structure stability.

-

Most erosive soils = nonaggregated, intermediate textures (loams, silt loams)

- K is less than 1, usually ranging from near 0 to about 0.6.  This means that it decreases the rainfall factor.

Coarse textured soil with high infiltration rates have erodibility indexes of less than 0.2, while easily eroded soils with low infiltration rates have a K factor of 0.3 or higher.

3) Slope gradient- S

4)  Length of slope- L

These two topographic factors are considered together.

S- the steeper the slope, the greater the velocity of water flowing down it and the greater the erosiveness of the water.

L- related to how far the water flows without obstruction.  The greater this distance, the greater the volume of water passing over the lower end of the field and the greater the erosiveness.

-The value of the topographic factors (LS) varies enough to be both above and below 1.  When the value is about 1, these factors serve to increase the rainfall factor.  For example:

For a 2% slope of 15m length, LS = 0.16

For a 12% slope of 90m length, LS = 3.13

Topographic factors

5)  Cover and management factor- C

-This factor refers to the amount and type of vegetation on the land as well as how the land is managed (i.e. conventional tillage, min. tillage, or no till).  Vegetation on the surface protects the soil from raindrops and reduces the amount of detachment and the velocity of water flow across the soil.

C is less than 1.

6)  Erosion control practices- P

-

-

Examples of practices that reduce P

- grassed waterways

-contour farming

-terraces

    b.  Wind erosion- The same two process (detachment and transport) are active with wind erosion, but occur in different ways.

Detachment-

Transport- The method of transport depends upon the size of the particle.

      suspension-

      soil creep-

      saltation

        i.  Wind erosion equation

        E = f(ICKLV)

        E = erosion- amount per unit area

        I =

        I is dependent on the texture and aggregation of the soil

        C =

        For wind erosion, this deals mainly with the frequency and the strength of the prevailing winds

        K =

        The rougher the surface of the soil, the less wind erosion will occur

        L =

        The thinner the width of bare soil, the less erosion will occur.  Rows of plants are often grown perpendicular to the prevailing wind direction to reduce the effective width of the field.

        V =