Estimating Soil Losses using Universal Soil Loss Equation (USLE)

8/9/2019 Estimating Soil Losses using Universal Soil Loss Equation (USLE)

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Estimating SoilLosses


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Introduction

• Soil losses or relative erosion rates are estimated to assist farmers, nat

resource managers, and governments agencies in evaluating existingmanagement systems or in future planning to minimize soil losses.

• Between 1945 and 1965 a method of estimating losses was developed resulted in the universal field soil loss equation (USLE).

• A revised version of the USLE (RUSLE) has been developed for compuapplications.

• A RUSLE 2 Version with a windows interface is now available and greaincreased the capacities of the RUSLE.(http://www.ars.usda.gov/Research/docs.htm?docid=6038)

July 22, 2012 Footer text here2


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The Universal Soil Loss Equation (USLE)

• Widely accepted method for estimating sediment loss.

• Useful for determining the adequacy of conservation measures in resoplanning and for predicting nonpoint sediment losses in pollution contprograms.

• Has been modified and adapted to different regions of the world and fat urban or highway construction sites (USDA, USEPA)

• The average annual soil level can be estimated from the equation:

=



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Average Annual Soil Level (USLE)

=

Where:

• A=annual erosion rate or soil loss in tons/acre per year (dry weight)

• R= Rainfall and runoff erosivity factor for a geographic location. (Ra

• K= Soil erodibility factor

• LS= the slope steepness and length factor• C=the power management factor

• P= the conservation practice factor



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Rainfall Erosivity R

• Erosion index for a given storm period.

• Product of rainfall energy times maximum 30-minutes rainfall intensity

• Units: 100 ft-tons in/(ac h) = 17 MJ-mm /(ha h)

• How to estimate R for Puerto Rico?

• R depends on rainfall

• ATLAS 14 from NOAA provides Precipitation Frequency-Duration Data for Pu

• NOAA Atlas 14: http://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_pr.html

• USDA (1972) established relationship between TYPE II, 2 yr frequency – 6 hr

rainfall and Annual Rainfall Erosion Index (R )

• Type II distribution rainfall is commonly used for design in Puerto Rico



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Rainfall Erosivity R (Annual Index)

• This is one way to estimate R. Rusle2 could be more accurate !

• Locate the coordinates of your are or study: Lat. 18,4399 Long -66.2903

• Determine the value of the precipitation corresponding to 2 yr- 6 hr rainfa

from Atlas 14

P = 3.11 inches

Assume TYPE II distribution to use:R = 27 P2.2 = 27 X 3.112.2

R = 328


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Soil Erodibility (K)

• Erosion rate for specific soil in fallow condition on a 9% slope having a of 22.1 m

•

Obtained by direct soil loss measurements from fallow plots located inU.S. states .

• Soils that have silt contents tend to be the most erodible.

• The presence of organic matter, stronger subsoil structure, and greatepermeabilities generally decrease erodibility.

• Soil erodibity factors for the 10 most common soils in Puerto Rico are

presented in the next table.


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Soil Erodibility (K)

Sand

Clay

Loam

Very Fine Sand

Silt

1 2 3 4 5 6

Organic Matter %

Soil Factor


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Soil Erodibility (K): NOMOGRAMExaDeterowit

proversanmafinestrumo

AnK=0


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Topographic Factor LS• The topographic factor LS adjusts the predicted erosion rates to give g

erosion rates in longer and steeper slopes and lower erosion rates on sor flatter slopes compared to a USLE “standard” slope of 9% and slope

of 72.6 ft.• The slope length is measured from the point where surface flow origin

(usually the top of the ridge ) to the outlet channel or a point downslopwhere deposition begins.



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Topographic Factor LS


Example:Determine the LS factor for the field

with a slope steepness of 4% and aslope length of 700 ft.

Answer:LS=0.9


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Cover Management Factor “C”

• Ratio of soil loss from an area with specified cover to that from the sambut under bare soil conditions

• Includes the effects of vegetative cover, crop sequence, productivity lelength of growing season, tillage practices, residue management, BMPsediment and erosion control at construction sites

• Values range from 1 for bare soil down to 0.003 for well-established placover

• Factors C must be adjusted during construction because ground surfac

conditions differ from original cover• Conditions could refer to

• Before grading• During grading• During Construction• After Construction



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-

+

E r o s i o n

100

75

50

25

0

C o v e r i n g %

vegetation and crop factor


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Fifield J


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Fifield J


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• Original land was 60% rangeland with brushes and weeds. Small canopycover.

• Was cleared using bulldozers with scrappers up and down the hill

• Determine the relative erosion increase associated with this land use:• Before: Grass and brushes: C = 0.082

• During clearing: C = 1.3

• Change in land increases soil loss by 94% !!


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Erosion Control Practice P (Management practice fa

• Ratio of soil lost with a given surface condition to soil loss from a hill wplowing is perpendicular to the contours

• There are practices besides vegetation management that can be emplcontrol erosion.

• Erosion control practices includes contouring, strip cropping, terracingstabilized waterways.

• When no erosion control practices are used the P factor is 1.0.

• Values range from 0.80 for contouring on steep slopes (18 to 24%) to 0terracing on gentle sl0pes



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Erosion Control Practice P

-

+

E r o s i o n

Terracing*

Strip cropping

Contour

None* Must contain the 2-yr Without overflowing, ot


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Fifield J


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• Contouring: conducting field operations , such as tillage, planting, andharvesting, approximately on the contour.



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• Strip cropping is the practice of growing alternate strips of different cthe same field.

• For controlling water erosion , the strips are on the contour.



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• TerracingA common erosion control practice is construct terraces in eroding slopes. Terraces rethe slope length and reduce runoff.



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RUSLE, RUSLE1 and RUSLE2• Is the revised RUSLE has the same formula as USLE, but has several

improvements in determining factors.

• New and revised isoerodent maps

• Time-varying approach for soil erodibility factor

• Subfactor approach for evaluating the cover-management factor

• New equation to reflect slope length and steepness; and new conservapractice values (Renard, et al., 1997).\

•

Is empirical and estimates average annual soil loss.• It is not meant to calculate soil loss due to specific storm events

• Only computes sediment from hill slopes profiles and at the outlet of tchannels

• RUSLE2 Program: http://www.ars.usda.gov/Research/docs.htm?docid


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Downstream Sediment Yields

• It is the net amount of sediment leaving a dischapoint

• The USLE method estimates gross sheet and grierosion, but not account for sediment depositedroute to the place of measurement or for gully ochannel erosion downstream.

•

Methods to calculate sediment yield:•SDR (sediment delivery ratio)

•Modified soil loss equation (MUSLE)



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Sediment Yield: Sediment delivery ration (SDR

• SDR = is the ratio of the sediment delivered at a given location to the gross efrom the drainage area above that location

• Factor affecting the SDR: • Size of drainage area

• Topography and channel density

• Relief and length of watershed slopes

• Precipitation

•

Runoff • Has major limitation due to limited data availability


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Single event Sediment Yields ( MUSLE)

• MUSLE (Williams and Bernt, 1976) calculate sediment yield of a basin result of a specific event.

= Ψ()0.56

• Where Y is the single storm sediment yield in (tonnes or tons)

• Q is the peak flow (cubic feet/second)

• V is the volume of runoff (cubic meters or acre –feet)

• Ψ = 89.6 for SI unit and 95 for English units

• The other terms are the standard USLE factors


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Single event Sediment Yields ( MUSLE)

• Estimate runoff volume using TR-55 from NRCS (1986)

•

This method uses the SCS Curve Number• Calculate peak discharge by using

• Rational method: Q = C i A• SCS CN method

• Where Y is the single storm sediment yield in (tonnes or tons)

•Q is the peak flow (cubic feet/second)

• V is the volume of runoff (cubic meters or acre –feet)

• Ψ = 89.6 for SI unit and 95 for English units

• The other terms are the standard USLE factors


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USLE vs MUSLE

• Use of MUSLE requires knowing the watershed parameters


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Review

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Estimating Soil Losses using Universal Soil Loss Equation (USLE)