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Soil Conservation Systems for Effective Land Management

Explore principles of soil physics, erosion control, soil mechanics, and slope stabilization. Learn about soil classes, particle sizes, soil water content, and potential, with practical calculations and examples.

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Soil Conservation Systems for Effective Land Management

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  1. I.B Soil Conservation Systems Rabi H. Mohtar Professor, Environmental and Natural Resources Engineering Executive Director, Qatar Environment and Energy Research Institute mohtar@purdue.edu or rmohtar@qf.org.qa August 2012

  2. Materials To Be Covered • Principles of Soil Physics (Mohtar) • Sediment Transport (Mohtar) • Erosion Control (Mohtar) • Soil Mechanics (Khawlie) • Slope Stabilization(Khawlie) This will provide you with an overall review and not necessarily makes you an expert! I.B Mohtar

  3. Sources • Environmental Soil Physics; Hillel; 1998 Hillel (1998) • Essentials of Soil Mechanics & Foundations, 7th ed.; McCarthy; 2007; McCarthy (2007) • Soil and Water Conservation Engineering • 4th ed. Schwab, Fangmeier, Elliott, Frevert: Schwab et al (1993) • 5th ed. Fangmeier, Elliott, Workman, Huffman, Schwab: Fangmeier et al (2006) • Design Hydrology & Sedimentology for Small Catchments; Haan, Barfield, Hayes: Haan et al (1994) • USLE/RUSLE: USDA Agricultural Handbook No. 537 (1978) • Cuenca, R. H. 1989. Irrigation System Design - An Engineering Approach. Prentice-Hall, Inc., Englewood Cliffs, NJ. 552 pp. Cuenca (1989). • Ward, Elliot 1995 (Environmental Hydrology, Lewis Publishers). • http://cobweb.ecn.purdue.edu/~abe325/: Mohtar soil and water resources conservation course. I.B Mohtar

  4. Soil Physics & Mechanics • Soil classes and particle size distributions • Basics of soil water • Water Content • Water Potential • Water Flow • Soil strength & mechanics I.B Mohtar

  5. Soil classes & particle sizes Hillel (1998) page 61 I.B Mohtar

  6. Soil Classes & Particle Sizes - 2 ISSS classification is easiest • Sand 0.02-2.0mm (20-2000μ) • Silt 0.002-0.02mm (2-20μ) • Clay <0.002mm (<2μ) I.B Mohtar

  7. Soil classes & particle sizes – 3 Soil Textural Triangle Example 1: Find the soil texture for this soil: • 50% sand, • 20% silt I.B Mohtar Hillel (1998) page 64

  8. Soil Classes & particle sizes – 4 Particle size distribution Example 2 Draw in a sandy clay loam? Hillel (1998) page 65 I.B Mohtar

  9. Pedon Soil Structure and Functionality Clay particles Primary peds Inter-ped pore space Mineral grains Primary soil mapping unit Clay pore space Primary soil mapping unit Soil type REV Horizon = Pedostructure = Primary ped = Geomorphological unit Clay plasma porosity (micro-porosity) Vertical porosity (cracks, fissures) Interpedal porosity (macro-porosity) Pedostructure, primary peds, primary particles, are functionally defined and quantitatively determined using the shrinkage and potential curve measurement + Pedostructure + Primay particles and pedological features + Primary peds and free mineral grains I.B Mohtar Mohtar (2008)

  10. 0 Soil Water Content • Mt = Ms + Mw + Ma • Vt = Vs + Vw + Va • t = total, s = solids, w = water, a = air • ρb = bulk density = Ms/Vt≈ 1.1-1.4 g/cc (why dry basis?) • ρp = particle density = Ms/Vs ≈ 2.65 g/cc • Porosity = (Vw + Va) / Vt ≈ 25-60% • ρw = water density = Mw/Vw = 1.0 g/cc I.B Mohtar

  11. Soil Water Content – 2 • Water content wet basis: Ww = Mw / (Ms + Mw) • Water content dry basis: W = mass wetness = Mw / Ms • Volumetric water content: θ = Vw/Vt = Vw / (Vs + Vw + Va) I.B Mohtar

  12. Calc.: soil water content Soil Water Example 3. Given: • Soil with 30% water content dry basis Find? • Best guess at equivalent inches of water in the top foot of soil? I.B Mohtar

  13. Calc.: soil water content – 2 • Mw / Ms = 0.30 • Mw = Vw * ρw • ρb = Ms / Vt; Ms = Vt * ρb I.B Mohtar

  14. Calc.: soil water content – 3 • Mw / Ms = (Vw * ρw)/(Vt * ρb) = (Vw / Vt)(ρw / ρb) • θ = Vw/Vt • θ *(ρw / ρb) = 0.3; θ = (ρb / ρw) * 0.3 • θ = 0.3 *(1.3/1.0) = 0.39 • 0.39 * 1 ft * 12”/ft = 4.7” I.B Mohtar

  15. Soil Water Potentialsoil characteristic curve Hillel (1998) page 157 I.B Mohtar

  16. Soil Water Potential – 2 Cuenca (1989) page 58 I.B Mohtar

  17. Soil Water Management Ward, Elliot 1995 (Environmental Hydrology, Lewis Publishers) I.B Mohtar

  18. Soil Water Potential – 3 Fangmeier et al (2006) page 337 I.B Mohtar

  19. Soil water potential – 4 Hillel (1998) page 162 I.B Mohtar

  20. Calc.: soil water potential Soil Water Potential Example 4. • Given: • Mercury tensiometer • SG = 13.6 • Situation as shown • Find: • Total potential at point C • Is point C above or below the current water table? Cuenca, (1989) page 64 I.B Mohtar

  21. Calc.: soil water potential - 2 • Pick datum • Add pressures • Suction • Water depth • Gravity • T = z + p + pos • z = + 80 cm • p = ? • T = -86cm • Point C is above water table. Why? I.B Mohtar

  22. Soil Water Flow • q = A*K*H/L • K = (q*L)/(A*H) • K values A H L q Fangmeier et al (2006) page 261; Schwab et al (1993) page 359; Haan et al (1994) page 430 I.B Mohtar

  23. Calc.: soil water flow Darcy Law Application Example 5. • Given: • Need 50000 gpd through a 1-ft thick sand filter with K = 8 ft/d, and a total driving head of 3 ft • Find? • Required diameter for circular tank? I.B Mohtar

  24. Calc.: soil water flow – 2 q = A*K*H/L; A = (q*L)/(K*H) I.B Mohtar

  25. Soil erosion and sediment yield • Hillslope erosion • Channel system erosion • Sediment delivery to streams • Sediment transport in streams • Slope stability I.B Mohtar

  26. Hillslope soil erosion • Background • Detachment • Raindrop impact • By turbulent overland flow • Runoff • Transport downslope • By runoff Schwab et al (1993) pp:91-111; Fangmeier et al (2006) pp:134-156; Haan et al (1994) pp:238-285 I.B Mohtar

  27. Hillslope Soil Erosion Background At the top of the slope • Detachment by raindrop impact • Transport by shallow sheet flow • Sheet erosion USDA-NRCS I.B Mohtar

  28. Hillslope soil erosion background - 2 • Lower on slope • Small flow concentrations • Start to cut small channels • Rills • Roughly parallel • Head straight downslope • Random formation • Flow from sheet areas between rills • Sheet and rill erosion USDA-NRCS I.B Mohtar

  29. Hillslope Soil Erosion Background - 3 • Bottom of hillslope • Ends at concentrated flow channel • Low area in macrotopography • “ephemeral gullies” USDA-NRCS I.B Mohtar

  30. Hillslope erosion factors • Rainfall erosivity • Intensity • Total storm energy • Soil erodibility • Topography • Slope length • Steepness • Management • Reduce local erosion • Change runoff path • Slow and spread runoff => deposition I.B Mohtar

  31. USLE/RUSLE • A = R * K * LS * C * P • A = average annual soil erosion (T/A/Y) • R = rainfall erosivity (long empirical units) • K = soil erodibility (long empirical units) • R * K gives units of T/A/Y • LS = topographic factor (dimensionless, 0-1) • C = cover-management (dimensionless, 0-1) • P = conservation practice (dimensionless, 0-1) I.B Mohtar

  32. USLE/RUSLE – background • Empirical approach been in use since 1960 • >10000 plot-years of data • International use • Unit Plot basis; LS = C = P = 1 • Near worst-case management • R from good fit rainfall-erosion • K from K = A / R • C and P from studies • Sub-factors in later versions I.B Mohtar

  33. USLE/RUSLE – approach • Lookup • Maps, tables, figures • Databases • Process-based calculations • Show changes over time • Where don’t have good data I.B Mohtar

  34. R factor – rainfall erosivity Haan et al (1994) pp:251; Haan et al (1994) Appendix 8A; Schwab et al (1993) 99(SI); Fangmeier et al (2006) pp:143(SI); USDA (1978) pp:1-5 • Maps • R(customary SI) = 17.02 * R(customary US) S4 I.B Mohtar

  35. K factor – soil erodibility • Soil surveys, NASIS, Haan et al (1994) 261-262; USDA 6 • Erodibilitynomograph: Haan et al (1994) 255; Schwab et al (1993) 101; Fangmeier et al (2006) pp144; USDA (1978) pp: 7 • No short-term OM I.B Mohtar

  36. LS – topography factor • New tables & figures • Haan et al (1994) 264; USDA (1978) 8 • Know susceptibility to rilling • High for highly disturbed soils • Low for consolidated soils I.B Mohtar

  37. C – cover-management factor • Part of normal management scheme • Lookup: Schwab (1993) 102; Fangmeier et al (2006) pp: 146; Haan et al (1994) 266; Hillel (1998) Appendix 8; USDA (1978) 9 • It Changes over time I.B Mohtar

  38. C – cover-management factor - 2 • Subfactor approach (RUSLE) • C = PLU * CC * SC * SR * SM; all 0-1 • PLU = prior land use • roots, buried biomass, soil consolidation • CC = canopy cover; % cover & fall height • SC = exp(-b * % cover) • B = 0.05 if rills dominant; 0.035 typical; 0.025 interrill • SR = roughness; set by tillage, reduces over time • SM = soil moisture; used only in NWRR I.B Mohtar

  39. P – conservation practice factor • Common practices • Contouring, strip cropping, terraces • Change flow patterns or cause deposition • Lookup tables • Schwab (1993) pp:103; Fangmeier et al (2006) pp:146; Haan et al (1994) pp: 281; USDA (1978) pp:10 I.B Mohtar

  40. Calc.: USLE/RUSLE Example 9: • Given: • Materials in handout • 3-Acre construction site near Chicago • Straw mulch applied at 4 T/A • Average 20% slope, 100’ length • Loamy sand subsoil • Fill (loose soil) • Find: • Erosion rate in T/A/Y I.B Mohtar

  41. Calc: USLE/RUSLE – 2 • R = 150 (HO.1) • K = 0.24 (HO.7) • LS = 4 (HO.8-high rilling) • C = 0.02 (HO.9) • P = 1.0 • A = R * K * LS * C * P = 2.9 T/A/Y I.B Mohtar

  42. Calc: USLE/RUSLE – 2.1 Example 10: • Given: • Materials in handout • 16-A site near Dallas, TX • Silty clay loam subsoil • Average 50% slope, 75’ length • Cut soil • Find: • By what percentage will the erosion be reduced if we increase our straw mulch cover from 40% cover to 80% cover? I.B Mohtar

  43. Calc: USLE/RUSLE – 2.2 • Only thing different is C • Only subfactor different is SC • SC = exp(-b * %cover) • For consolidated soil, b = 0.025 • SC1 = exp(-0.025 * 40%) = 0.368 • SC2 = exp(-0.025 * 80%) = 0.135 • Reduction = (0.368 – 0.135)/0.368 = 63% I.B Mohtar

  44. Sediment delivery • USLE/RUSLE for hillslopes • Erosion • Delivery • Erosion critical for soil resource conservation • Delivery critical for water quality • Movement through channel system I.B Mohtar

  45. Sediment delivery – 2 I.B Mohtar

  46. Sediment delivery – 3 • SDR (Sediment Delivery Ratio) • Hillslope erosion • Empirical fit for watershed delivery • Channel erosion/deposition modeling • Erosion • Transport • Deposition I.B Mohtar

  47. Sediment Delivery Ratio • Haan et al (1994) pp:293-299 • SDR = SY / HE • SDR = sediment delivery ratio • SY = sediment yield at watershed exit • HE = hillslope erosion over watershed I.B Mohtar

  48. Sediment Delivery Ratio – 2 • Area-delivery relationship Haan et al (1994) pp:294 I.B Mohtar

  49. Sediment Delivery Ratio – 3 • Relief-length ratio • Relief = elev change along main branch • Length = length along main branch Haan et al (1994) page .294 I.B Mohtar

  50. Sediment Delivery Ratio – 4 • Forest Service Delivery Index Method Haan et al (1994) pp:295 I.B Mohtar

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