USLE & Other Models NREM 461 Dr. Greg Bruland
1 I. Universal Soil Loss Equation (USLE)
A. In math terms Erosion = ƒ[(Erositivity)( Erodibilit y)]
EROSIVITY ERODIBILITY
RAINFALL PHYSICAL MANAGEMENT CHARACTERISTICS
ENERGY CROP LAND MGMT MGMT
2 B. USLE developpyed by scientists at ARS , SCS, Purdue Univ. under leadership of Walter Wishmeier
1. 1st took form in
2. Equation published by Wishmeier & Smith in
3. Developed to predict long-term average annual soil loss from erosion on uniform cultivated fields in eastern U.S.
How universal is it?
Dr. W. Wischmeier
3 USLE Term English Units Metric Units A: avg. annual tons/acre-year Mg/hectare-year soil loss R: raifllinfall 100s o f ft-tons rain fa ll/ac-yr (MJ mm)/(h a h yr ) erosivity range: 0-2000 range: 0-700
K: soil erodibility tons soil/100 ft tons rainfall ((gMg ha h )()/(ha MJ mm ) range: 0.01-0.7 range: 0.001-0.09
LS: slope length & dimensionless dimensionless gradient factor range: range:
C: cover-mgmt dimensionless dimensionless factor range: range:
P: supporting- dimensionless dimensionless practice factor range: range:
4 Components of USLE
Rainfall Erosivity Factor (R)
A = R x K x LS x C x P
R estimated from maximum 30 minute rainstorm intensity values displayed on map in Troeh pg. 140
5 Troeh et al. (2004)
6 6 Soil Erodibility Factor (K)
Rate of soil loss on a standard plot 72. 6 ft (22 m) long with 9% slope
A = R x K x LS x C x P
7 8 K factors tabulated for soil ser ies in Coun ty Soil Surveys
9 10 Slope Length & Steepness Factor (LS) Ra tio o f so il loss per un it area o f p lo t w ith slope X, compared to what would be lost from a f all ow 72. 6-ft-lltith9%long plot with 9% slope (can be <1 or >1)
A = R x K x LS x C x P
11 LS can be determined from lookup tables or from the empirical equation:
LS = (x/22.13)n (0.065 + 0.045s + 0.0065s2)
Where x = s = n = empirical parameter that should be varied based on slope steepness
12 Cover-Management Factor (C)
Ratio of soil loss under specific cover conditions compared to fallow
A = R x K x LS x C x P
13 14 Supporting-Practice (P) Factor
The fractional amount of erosion that occurs when “special practices,” i.e. contour cultivation, contour strip cropping, & terracing are used compared to erosion that would occur w/o them A = R x K x LS x C x P
16 4. Notes about USLE
a. USLE is an empirical equation based on measueasueetsatetarements rather than teoytheory
b. Designed for Eastern U.S. needs to be reparameterized
c. Provides annual estimates-
d. Interdeppgendence among variables & nonlinear relationships
18 Example USLE Calculations
With conventional tillage: A= 170 x 0.26 x 1.62 x 0.20 x 1.0 = 14.3 t/a-y With contour cultivation: A= 170 x 0.26 x 1.62 x 0.20 x 0.61 = 8.7 t/a-y With conservation tillage & contouring: A = 170 x 0.26 x 1.62 x 0.11 x 0.61 = 4.8 t/a-y With conventional tillage & terracing: A = 170 x 0260.26 x 0600.60 x 0200.20 x 101.0 = 5. 3 t/a-y
19 C.MUSLE (Modified USLE 1978)
1. R: expanded to cover western U.S. including HI (20- 450), but not AK
2. K: erodibility nomograph developed based on clay, silt, sand, OM , structure , & permeability
3. LS: adapted to handle multi-segmented slopes
4. C: expanded to 6 crop stage periods, C values provided for
5. P: not changed for contour cult, & contour strip,
20 21 K factor nomograph: K = ƒ(5 soil properties)
(Troeh et al. 2004)
22 Multi-segmented slopes (2-5)
23 D. RUSLE (Revised USLE 1992)
1. Improved mapping of R values in lower 48 & Hawaii
2. K & C allowed to vary seasonally by climatic data
3. C becomes a continuous function w/ 5 subfactors a. Prior land use b. Surface cover c. Cropppy canopy d. Surface roughness e. Soil moisture
25 26 27 28 29 31 4. Data gathered to develop local databases for C factors
5. P factor includes data from
32 II. Oth er erosi on mod el s
A. Empirical Models
1. RUSLE2: computerized extension of RUSLE
a. includes much more detailed information about slope, veg, residues, P factors, etc.
b. used to compare erosion under
34 35 2. AGNPS: Agricultural Nonpoint Source Pollution Model
a. developed by USDA ARS to estimate runoff wat er qualit y f rom AG wat ersh ed s
b. cell-based, distributed-parameter, event-driven model
c. requires > input parameters
d.
e. integrated with Arcview GIS interface & includes RUSLE subroutines 36 37 38 3. EPIC: Erosion Productivity Impact Calculator
a. designed to assess effect of erosion on ppyroductivity
b. computes erosion from a single point on the landscape
c.
39 40 B. Process-based models
1. WEPP: Water Erosion Prediction Project
a. process-bdditibtdbased, distributed parame ter, continuous simulation, erosion prediction model
b. 1st model for erosion prediction in the U.S. not based on USLE c. Based on equation:
Qs = sediment load per unit width per unit time x = distance downslope
Di = delivery rate of particles detached by interrill erosion Df = rate of detachment/deposition by rill flow 41 2 Di = KiI CeGe(Rs/w)
Di = interrill erosion rate Ki = interrill erodibility I = Ce = effect of plant canopy Ge = effect of ground cover Rs = spacing of rills w = width of rills
Ce = 1 – Fe-0.34PH
Fe = fraction of soil protected by canopy PH =
Ge = e-2.5gi gi = fraction of interrill surface covered by vegetation or residue
42 Df = Dc(1-Qs/Tc)
Df = rate of detachment of soil particles by rill flow Dc = detachment capacity Qs = sediment load in the flow Tc = sediment load at transport capacity
Dc = Kr(τ - τc)
Kr = τ = flow sheer stress acting on soil
τc = critical flow sheer stress for detachment to occur
3/2 Tc = ktτ kt = is a transport coefficient τ = hydraulic shear acting on the soil
43 d. When tested for 4,000 storm events across 9 exper imen ta l s ta tions in the U .S ., WEPP model gave predictions of mean annual soil loss at the plot scale of similar accuracy to those of USLE & RUSLE (Zhang et al. 1996)
e. Web version: http://milford.nserl.purdue.edu/wepp/weppV1.html
44 45 46 2. European Soil Erosion Model (EUROSEM)
a. Funded by EU, developed by scientists in late 80s & early 90s
b. Modular structured, process-based model that incorporates terms of erodibility,,g roughness that change w/ time
c.
d.
49 50 (Morgan 2005)
51