Lecture 23

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Chapter 15

Soil Erosion

 

Soil erosion has been our nation's most devastating environmental disaster. Soil in quantities of incomprehensible magnitude has been lost from the land where its forfeited productivity could have contributed to our national wealth. Instead, this soil has become sediment, plugging channels and raising riverbeds to levels that threaten human habitation along flood-prone rivers. In dryer climates this sediment becomes airborne dust, fouling the air-intake mechanisms of machinery and men. This lecture explores the nature of soil erosion, and methods of quantifying and controlling erosion.

 

1. Nature and extent of soil erosion. Natural or geologic erosion preceded farming. It has always occurred whether man was present or not. However, man and his farming practices have greatly accelerated erosion. In the corn belt, we presently lose about 1 pound of topsoil for every 0.67 pound of corn harvested. By some estimates about one half of America's topsoil was lost in the first 150 years of our nation's existence. The most dramatic series of events, those portrayed in the 1930s book and movie Grapes of Wrath, are collectively called the "dust bowl". Soil erosion is a two-fold problem:

loss of topsoil, often exposing the less productive B horizon
sediments that alter and elevate the course of rivers or pollute the air with dust

Soil erosion has two causes, wind and water. Wind erosion is most important in drier regions where it can proceed at staggering rates on dry, poorly vegetated soil. Water erosion is more important in wetter regions, including most of America's best farmland. In the United States soil erosion averages 5.6 ton/acre annually. Of this 3.1 tons/acre is caused by water, 2.5 tons/acre by wind.

 

2. Water erosion. Falling raindrops have considerable momentum as they hit the ground. These raindrops detach soil particles from peds. Raindrops falling at a rate of 30 ft/sec can move particles 150 cm laterally. Removing plant cover accelerates this detachment. Fine sands and silts are most easily detached. Detached particles are susceptible to transport. Sheet erosion is non-turbulent flow across the soil surface. Channelized flow is more destructive. Channelized, turbulent flow forms rills and gullies. Sheet and rill erosion is still rampant in the United States. The Universal Soil Loss Equation (USLE) was developed to predict sheet and rill erosion.

 

3. Predicting water erosion. The USLE developed in 1965 is still used by the USDA's Natural Resource Conservation Service (NRCS). The USLE has the following form:

A = R x K x LS x C x P

where,

A = predicted actual annual erosion in tons/acre

R = rainfall intensity factor, such as 550 in New Orleans, 35 in Salt Lake City

K = erodibility based on texture and structure (see Table 15-1 in the textbook)

LS = length-slope both length of slope and degree of slope are important. Of the two, degree of slope is the more important. For example:

1000 ft field with 0.5% slope: LS = 0.15
1000 ft field with 20% slope: LS = 12.9

C = cover and management These are complex local values based on crop, tillage, and timing. Some examples include:

continuous fallow C = 1.0
permanent grass pasture C = 0.003
typical row crop C ~ 0.4

P = practice such as contouring or strip cropping

 

To use the USLE, one finds through actual measurements and tabulated data the appropriate value for each factor, then multiples the five factors together to find "A", the predicted actual annual erosion.

Other models in use include:

RUSLE — Revised Universal Soil Loss Equation; more precise than USLE
WEPP — Water Erosion Prediction Project

 

4. How much erosion is too much? Theoretically, we could afford to lose soil at the rate at which soil forms. The USDA default position is that soil forms at the rate of 1 inch per 30 years. This assumption may be nearly true for some soils, but many soils form much more slowly than this. Nevertheless, soil erosion evaluation is generally based on the assumption that 1 inch of soil loss per 30 years is acceptable.

1 acre-inch of soil weighs about 167 tons. If 167 tons form in 30 years, the annual soil formation rate is 5.6 ton/year. 5 tons erosion per acre per year is generally considered tolerable.

So, the question arises, how can farms stay within the tolerable level of erosion? Farmers cannot change the R factor or the K factor. Management hinges of the LS factor, the C factor, and the P factor. Most attention is given to the C factor because changing the C factor can dramatically change the predicted "A" value, often with only minimal cost to the farmer (see Table 15-3 in the textbook).

 

5. Wind Erosion. In the presence of wind, soil moves by suspension (particles <0.05 mm are suspended in moving air), saltation (particles between 0.05 and 0.5 mm in diameter move in short hops along the soil), and creep (particles between 0.5 and 1.0 mm move along the soil without leaving the surface).

Wind erosion is negligible east of the Mississippi but can be extreme in arid western regions. Nevada farmland averages 22 ton of erosion per acre annually. Dryland soils have less vegetation, less clay, less moisture and are therefore readily eroded. To make matters worse, eroded soils are more coarse and therefore more draughty. This downward spiral in arid soil quality is called desertification.

Factors affecting soil erosion by wind are assessed by the Wind Erosion Equation (WEQ). Factors in the WEQ are:

soil erodibility
surface roughness
climate
length of field
vegetative cover

The vegetative cover factor is the most manageable factor (see Figure 15-17 in the textbook) and is the factor usually addressed by farmers attempting to minimize wind erosion.

 

6. Soil Conservation. In actuality we only know of four powerful, practical tools for soil conservation. Those tools are: conservation tillage (to leave more residue on soil), pesticides (to minimize need for tillage), nutrient management (to enhance vegetative growth), and vegetative buffers (to protect surface waters from sediments).

 

Students are encouraged to look up the following vocabulary words in the textbook glossary or elsewhere and to browse the following web sites.



Vocabulary

rill
gully
suspension
saltation
creep
geologic erosion
conservation tillage



Web sites

Applications of the USLE.
URL: pasture.ecn.purdue.edu/~aggrass/agnps/usle.html-

Home page for the RUSLE.
URL: www.itc.nrcs.usda.gov/focs/RUSLE/

Home page for the Soil and Water Conservation Society.
URL: www.swcs.org

 

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