NREM 461 Dr. Greg Bruland

Transcription

1 USLE & Other Models NREM 461 Dr. Greg Bruland 1

2 I. Universal Soil Loss Equation (USLE) A. In math terms Erosion = ƒ[(erositivity)(erodibility)] y)] EROSIVITY ERODIBILITY RAINFALL PHYSICAL CHARACTERISTICS MANAGEMENT ENERGY CROP MGMT LAND MGMT 2

3 B. USLE developed 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

4 USLE Term English Units Metric Units A: avg. annual tons/acre-year Mg/hectare-year soil loss R: rainfall 100s of ft-tons t rainfall/ac-yr (MJ mm)/(ha h yr) erosivity range: range: K: soil erodibility tons soil/100 ft tons rainfall (Mg ha h)/(ha MJ mm) range: range: LS: slope length & dimensionless dimensionless gradient factor range: range: C: cover-mgmt factor P: supportingpractice factor dimensionless range: dimensionless range: dimensionless range: dimensionless range: 4

5 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

6 Troeh et al. (2004) 6 6

7 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

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9 K factors tabulated for soil series in County Soil Surveys 9

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11 Slope Length & Steepness Factor (LS) Ratio of soil loss per unit area of plot with slope X, compared to what would be lost from a fallow 72.6-ft-long plot with 9% slope (can be <1 or >1) A = R x K x LS x C x P 11

12 LS can be determined from lookup tables or from the empirical equation: LS = (x/22.13) n ( s s 2 ) Where x = s = n = empirical parameter that should be varied based on slope steepness 12

13 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

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15 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

16 4. Notes about USLE a. USLE is an empirical equation based on measurements e e rather than theory b. Designed for Eastern U.S. needs to be reparameterized c. Provides annual estimatesd. Interdependence among variables & nonlinear relationships 18

17 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 026x 0.26 x 060x 0.60 x 020x 0.20 x 10= t/a-y 19

18 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

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20 K factor nomograph: K = ƒ(5 soil properties) (Troeh et al. 2004) 22

21 Multi-segmented slopes (2-5) 23

22 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. Crop canopy d. Surface roughness e. Soil moisture 25

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28 4. Data gathered to develop local databases for C factors 5. P factor includes data from 32

29 II. Other erosion models 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

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31 2. AGNPS: Agricultural Nonpoint Source Pollution Model a. developed by USDA ARS to estimate runoff water quality from AG watersheds b. cell-based, distributed-parameter, event-driven model c. requires > input parameters d. e. integrated with Arcview GIS interface & includes RUSLE subroutines 36

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34 3. EPIC: Erosion Productivity Impact Calculator a. designed to assess effect of erosion on productivity b. computes erosion from a single point on the landscape c. 39

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36 B. Process-based models 1. WEPP: Water Erosion Prediction Project a. process-based, distributed ib t d parameter, continuous simulation, erosion prediction model b. 1 st 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 D i = delivery rate of particles detached by interrill erosion D f = rate of detachment/deposition by rill flow 41

37 D i = interrill erosion rate K i = interrill erodibility I = Ce = effect of plant canopy Ge = effect of ground cover Rs = spacing of rills w = width of rills D i = K i I 2 CeGe(Rs/w) 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

38 D f = D c (1-Qs/Tc) D f = rate of detachment of soil particles by rill flow D c = detachment capacity Qs = sediment load in the flow Tc = sediment load at transport capacity Dc = Kr(τ - τ c ) Kr = τ = flow sheer stress acting on soil τ c = critical flow sheer stress for detachment to occur Tc = k t τ 3/2 k t = is a transport coefficient τ = hydraulic shear acting on the soil 43

39 d. When tested for 4,000 storm events across 9 experimental stations ti 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: 44

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42 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, roughness that change w/ time c. d. 49

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44 (Morgan 2005) 51