What makes the The Universal Soil Loss Equation Go ?

What makes the What makes the The Universal Soil Loss EquationThe Universal Soil Loss Equation

Go ?Go ?

Universal Soil Loss EquationUniversal Soil Loss Equation

Erosion = f (climate, soil, topography, landuse) A = R K LS C P

A = average annual erosion in field sized areas R = rainfall-runoff (erosivity) factor K = soil (erodibility) factor LS = topographic factors (L re slope length

S re slope gradient) C = crop/crop management factor P = soil conservation practice factor

P.I.A. Kinnell Univesity of Canberra


Erosion = f (climate, soil, topography, landuse) A = R K LLS CC PP

C, PC, P & LL are the main factors modified by

land management

Erosion has units of weight per unit area (t/ha)

- the weight is an average value over that area but that dose NOT mean that erosion is uniform over that area.P.I.A. Kinnell Univesity of

Canberra


Erosion = f (climate, soil, topography, landuse) A = R K LS C P

The Revised USLE (RUSLE):1997

An update of the USLE to take account of new information gained since the 1960s and 70s

USLE/RUSLE used widely in the worldUSLE/RUSLE used widely in the world


Based on Erosion from PlotsBased on Erosion from PlotsKey issue =Key issue = Unit PlotUnit Plot 22 m long22 m long 9% slope gradient9% slope gradient Bare fallow (no vegetation), cultivationBare fallow (no vegetation), cultivation

up and down slopeup and down slope

L = S = C = P = 1.0L = S = C = P = 1.0


A1 = R K=10 t/ha

AC =A1 ( L S C P )

AC =10 (1.22 x 0.57 x 0.16 x 1.0) = 1.1 t/ha

L, S, C and P are all ratios with respect to unit plot conditions

The model operates in two stages - predicts A1 then AC

Unit Plot

22m long

9% slope

Wheat Plot

33m long

6% slope


R: rainfall-runoff factorR: rainfall-runoff factor

N N = number of events in Y

years

Re Re = Event erosivity factor

e=1

R = ————

Y Y = number of years


Event ErosivityEvent Erosivity

RRee = E I = E I30 30 E = Event Energy I30 = max 30 min intensity


K: soil erodibility factorK: soil erodibility factor

NN N= number of events

AAe.1e.1 Ae.1 = loss for C=P=LS=1

e=1e=1 (5+ years of data)

K = K = —————— ——————

NN C=1 : bare fallow

(EI(EI3030))ee L=1 : 22.13 m

e=1e=1 S= 1 : 9% slope

P=1 : cult up/down slope


K: soil erodibility factorK: soil erodibility factor

K from field experiments: Time - 5 years or more Expense - setup of plots (equipment and labour)

- maintenance (equipment and labour) - resources tied up in data collection

Predict K from soil properties - less time and expense


K from soil characteristicsK from soil characteristics

K = 2.77 M1.14 (10-7) (12-OM) + 4.28 (10-3)(SS-2) + 3.29(10-3) (PP-3)

Developed by Wischmeier el al (1971) for soils where silt + very fine sand is 70% and less K in SI units M (% silt + % very fine sand) (100 - % clay) - soil texture OM % organic matter SS soil structure code (USDA Soil Survey Manual) PP profile permeability class (USDA Soil Survey Manual)

Other equations exist for other soils (Volcanic) and using other properties


Seasonal variation in KSeasonal variation in K

In RUSLE, K can be considered to vary during In RUSLE, K can be considered to vary during year in association with soil moistureyear in association with soil moisture

In USA wet in spring >>> dry during summerIn USA wet in spring >>> dry during summercausing K to fall spring >>> summercausing K to fall spring >>> summer

Not necessarily appropriate in all geographic locationsNot necessarily appropriate in all geographic locations


L: slope length factorL: slope length factorL = ( / 22.13) m USLE:USLE:

m=0.6 slope >10% m=0.6 slope >10% m=0.2 slope <1% m=0.2 slope <1%

RUSLE:RUSLE:m = m = / (1+ / (1+) ) = ratio rill to interrill erosion = ratio rill to interrill erosion

depends on soil and slope %depends on soil and slope %

is the projected horizontal distance travelled by runoff before deposition or a channel occurs


Erosion for non-uniform slopesErosion for non-uniform slopes

How is it used to calculate erosion for non uniform How is it used to calculate erosion for non uniform slopes ?slopes ?

L applies to uniform L applies to uniform slopesslopes



Uniform slope Uniform slope gradient – gradient –

different cropsdifferent crops

Non-uniform slope Non-uniform slope gradient – gradient –

same or different same or different cropscrops



Calculate L for Calculate L for =( =(/22.13)/22.13)mm (L (Lslopeslope))

Calculate L for Calculate L for 1 1 =(=(11/22.13)/22.13)mm (L (L11))

Multiply LMultiply Lslopeslope by by subtract L subtract L11 by by 1 1 (X (X))

Divide X by Divide X by 2 2 = = L for lower segmentL for lower segment

Can only calculate L for lengths starting at the top of

the hillslope







ReverseReverse of calculating the average for whole slope:

(L1 x 11) + (LL22 x 22) Lslope = ————————————————————————————————————————————————








Lslope x = (L1 x 11) + (LL22 x 22)





Multiply LMultiply Lslopeslope by by subtract Lsubtract L11 by by 11 (X (X))



Lslope x - (L1 x 11) = (LL22 x 22)








(Lslope x - (L1 x 11) ) / 22 = LL22


Erosion for non-uniform slopesErosion for non-uniform slopesLLslopeslope = ( = ( /22.13) /22.13)mm where where = distance to bottom of segment = distance to bottom of segment

(Lslope x - (L1 x 11) ) / 22 = LLsegseg

L for a segment increases downslope and so does erosion

Segment L for m = 0.5

segment seg. length dist to bottom L for slope L for seg

1 50 50 1.63 1.63

2 50 100 2.13 2.62

3 50 150 2.60 3.56

4 50 200 3.01 4.21

5 50 250 3.36 4.78



Calculate L for Calculate L for =( =(/22.13)/22.13)mm (L (Lallall))


Multiply LMultiply Lallall by by subtract L subtract L11 by by 1 1 (X (X))


Calculation method the same as for uniform slope gradient because m is determined only the gradient of the 2nd segment

Seg 1 has different slope



Crops are Crops are irrelevantirrelevant to calculation of L to calculation of Lsegseg

But are But are relevantrelevant in the calculation of segment and hillslope in the calculation of segment and hillslope erosionerosion

AA11 = R K L = R K L11 S S11 C C11 P P11

AA22 = R K L = R K L22 S S22 C C22 P P22

(A1 x 11) + (A2 x 22) Aslope = ——————————————————————————


Potential & Real ErosionPotential & Real Erosion For a hillslope

(A1 x 1) + (A2 x 2) Aslope = ——————————————————————

Only valid if no deposition in lower segment RUSLE 2 does deals with deposition using transport capacity (TC) concept A1 = 5 t/ha A2 = 1t/ha both segs are 1ha in area TC2 = 4t Seg 1 produces 5t. 4t passes through to the bottom of seg 2. 1t deposited in seg 2 and no erosion occurs in seg 2. Hillslope has lost 4t of soil because of the control by seg 2.


Potential & Real ErosionPotential & Real Erosion

The USLE predicts The USLE predicts potential erosion potential erosion

Deposition Deposition will result in real erosion differing will result in real erosion differing from what USLE predictsfrom what USLE predicts

The ratio of Real Erosion to Predicted ErosionThe ratio of Real Erosion to Predicted Erosionis the is the Delivery RatioDelivery Ratio


RUSLE 2RUSLE 2Wheat on Wheat on

18m at 10%,18m at 10%, 18m at 6%, 18m at 6%, 9m at 2% 9m at 2%

Slope deliverySlope delivery

3.8 T/A3.8 T/A

Soil lossSoil loss

7.7 T/A7.7 T/A

Delivery RatioDelivery Ratio

0.490.49


Sediment Delivery RatioSediment Delivery Ratio

Varies with catchment size Varies with catchment size

But large variation about the SDR - size relationship But large variation about the SDR - size relationship depending on catchment characteristicsdepending on catchment characteristics

In case of SDR from RUSLE 2 data,In case of SDR from RUSLE 2 data,SDRSDR = = modelledmodelled erosion to erosion to modelledmodelled sediment delivery sediment delivery based on a based on a sediment transport modelsediment transport model


S: slope factorS: slope factor

USLE:USLE:S = 65.4 sinS = 65.4 sin22 + 4.56 sin + 4.56 sin + 0.0654 + 0.0654 angle to horizontal angle to horizontal

RUSLE:RUSLE:S = 10 sin S = 10 sin + 0.03 slopes <9% + 0.03 slopes <9%S= 16.8 sin S= 16.8 sin - 0.50 slopes - 0.50 slopes 9% 9%

USLE S overpredicts erosion at high slope gradientsUSLE S overpredicts erosion at high slope gradients


C: crop & management factorC: crop & management factor

NN

A Ae.Ce.C

e=1e=1

C = C = —————— ——————

N N

A Ae.1e.1

e=1e=1

Ae.C = event loss with crop

Ae.1 = event loss for bare fallow

NN

A Ae.Ce.C

e=1e=1

C = C = —————— ——————

NN

K K (EI (EI3030))ee

e=1e=1

C varies geographicallyC varies geographically



Australia: New South Wales has 12 Climate

Zones

C for WheatC for WheatZone C Zone C 5 0.20 5 0.20 6 0.14 6 0.14 7 0.15 7 0.15 8 0.15 8 0.15 9 0.15 9 0.15 10 0.16 10 0.16 11 0.29 11 0.29 12 0.14 12 0.14


C varies geographicallyC varies geographically C for WheatC for Wheat

Zone C Zone C 5 0.20 5 0.20 6 0.14 6 0.14 7 0.15 7 0.15 8 0.15 8 0.15 9 0.15 9 0.15 10 0.16 10 0.16 11 0.29 11 0.29 12 0.14 12 0.14


C varies geographicallyC varies geographically C for WheatC for Wheat

Zone C Zone C 5 0.20 5 0.20 6 0.14 6 0.14 7 0.15 7 0.15 8 0.15 8 0.15 9 0.15 9 0.15 10 0.16 10 0.16 11 0.29 11 0.29 12 0.14 12 0.14

1.8x



Zone 11 has grater Zone 11 has grater proportion of R proportion of R during cultivation during cultivation periodperiod

Zone 11 not good Zone 11 not good for growing wheat for growing wheat - - less coverless cover


Calculating CCalculating C C can be calculated by weighting the short term C can be calculated by weighting the short term

value of C (soil loss ratio) by the proportion of R value of C (soil loss ratio) by the proportion of R in the periodin the period

CCii R Rii C = C = _______________ _______________ = = C Cii (R (Rii/R) /R) R R CCii = C during period i R = C during period i Ri i = R during period i= R during period i

Normally 2 week periodsNormally 2 week periods


Calculating CCalculating C

The soil loss ratio may, in turn, be The soil loss ratio may, in turn, be calculated from subfactors accounting for calculated from subfactors accounting for prior land use, crop cover, surface (ground) prior land use, crop cover, surface (ground) cover, surface roughness cover, surface roughness

Crop cover factor includes consideration of Crop cover factor includes consideration of plant structure and heightplant structure and height


P: support practice factorP: support practice factor

Accounts for impact of conservation practiceAccounts for impact of conservation practice eg. cultivation across slope vs up/down slopeeg. cultivation across slope vs up/down slope

P=1.0 for cultivation up/downP=1.0 for cultivation up/downP=0.5 for cultivation acrossP=0.5 for cultivation across

Support practicesSupport practices**Across slope - P varies with ridge height, furrow gradeAcross slope - P varies with ridge height, furrow grade* Strip Cropping, Buffer strips, Filter strips, Subsurface drains * Strip Cropping, Buffer strips, Filter strips, Subsurface drains


Universal Soil Loss EquationUniversal Soil Loss Equation Erosion = f (climate, soil, topography, landuse)

A = R K LS C P

Uses/Misuses

Designed for looking at average annual erosion in field sized areas

Help make management decisions

Not for predicting erosion by individual events or seasonal or year by year variations in erosion


Peter KinnellPeter Kinnell

University of CanberraUniversity of Canberra

Canberra ACT 2601Canberra ACT 2601

AustraliaAustralia

[email protected]@canberra.edu.au

What makes the The Universal Soil Loss Equation Go ?

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