Erosion, Erosion, Everywhere

Transcription

1 Erosion, Erosion, Everywhere

2 Main Objectives 1.Capable of describing the magnitude of accelerated soil erosion in the past and at the present. 2.Comprehend the mechanics and the factors influencing water erosion 3.Understand wind erosion in terms of particle movement on a surface, and the factors that change soil erosion by wind.

3 Key Terms and Concepts Raindrop splash Sheet erosion Rill and gully erosion Universal Soil Loss Equation Wind erosion equation

4 1. The problem of soil erosion Natural (geologic) erosion (<2 Mg/ha/yr. Forest <0.1 Mg/ha/yr) vs. accelerated erosion (up to 1000 times higher) (1) On-site: loss of topsoil (nutrient, water, etc.) (2) Off-site: Sedimentation Pollution (eutrophication, etc) From D. Pimentel et al. 1995, SCIENCE, Vol.267: : Of the 75 billion tons of soil eroded worldwide each year, about two-thirds come from agricultural land. If we assume a cost of $3 per ton of soil for nutrients, $2 per ton for water loss, and $3 per ton for off-site impacts, this massive soil loss costs the world about $400 billion per year, or about $67 per person per year. So, soil erosion is everybody's business."

5 Rates of soil erosion? Soil erosion is a natural process that averages a loss of 0.2 tons per acre annually. The average annual rate of loss is 0.5 tons per acre for managed forests. The rate is 1.5 to 20 tons per acre for pasture and cultivated land. For unprotected construction sites the rate may jumps to 150 to 200 tons per acre.

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7 Figure 14.1 The downward spiral of land degradation resulting from the feedback loop between soil and vegetation. As the natural vegetation is disturbed, soil becomes exposed to raindrops and wind leading to erosion and loss of soil, including organic matter and nutrients. The now impoverished soil can support only stunted crops or other vegetation, which leaves the soil with even less protective cover and root mass than before. Soil loss becomes severe, such that the soil depth and the capacity to hold water are greatly reduced and vegetation can barely survive, leaving extremely degraded soil. Incapable of providing nutrients and water needed to support healthy growth of natural vegetation or crops, the site continues to erode, polluting rivers with sediment and impoverishing the people who attempt to grow their food on the land. (Diagram courtesy of R. Weil)

8 2. Nature of water erosion Raindrop splash Sheet erosion Channelized flow (rill and gully) associated with runoff. 3. Factors affecting water erosion and its control The USLE: Universal Soil Loss Equation A = R K Ls C P A: erosion in tons per acre per year R: rainfall factor (storms vs. mists) K: soil erodibility (0.01 to 1.0) Ls: field length or slope factor (see Table 14.3) C: vegetative cover (types and density of plants/residues, Table 14.2)) P: practices used for erosion control (contours, terraces, strip cropping)

9 Water erosion occurs in three phases: Particle detachment (by raindrop impact or moving water, or by wind) Sediment transport Sediment deposition

10 Figure 14.5

11 GEP Technical Report No

12 Figure 14.6

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16 4. Nature of wind erosion Wind erosion mostly occurs on loose and dry soil surfaces. Suspension: Saltation: Surface Creep: 5. Factors affecting wind erosion and its control WEQ: Wind Erosion Equation: E = f (I, K, C, L, V) E: estimated annual erosion loss in tons per acre (multiplies by 2.24 mega grams per hectare) f: function of ( ) I: erodibility factor (e.g., texture and aggregation) K: surface roughness factor (e.g., ridges) C: climate factor (Gardon City, KS = 1) L: unsheltered length of field factor (how open is the field) V: vegetative cover factor (cover type, density etc)

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18 United States Geological Survey

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20 Figure Wind erosion control in an area of productive Histosols (Saprists) in central Michigan. Prior to being cleared and drained, this area was a partially forested bog. When dry, cultivated Histosols are very light and fluffy and susceptible to wind erosion. The rows of trees (mainly willows) were planted perpendicular to the prevailing winds to slow the wind velocity and protect these valuable organic soils from erosion. Wetting the soil surface is another effective means of reducing wind erosion, as seen by the darker-colored field in the background (where the water table was raised) and the darker circles in the foreground where sprinkler irrigation was used. Note that the photo was taken in early spring before most crops were planted and before the trees had fully leafed out. (Photo courtesy of USDA Natural Resources Conservation Service)

21 6. Soil erosion and tillage Agricultural practices of tillage have the most direct consequences on soil erosion. Can you discuss the relationship between the two?

22 (a) (b) (c) Conventional and conservation tillage practices. (a) In conventional tillage, a moldboard plow inverts the upper soil horizon, burying all plant residues and producing a bare soil surface. (b) A chisel plow, one type of conservation tillage implement, stirs the soil but leaves a good deal of the crop residues on the soil surface. (c) In no-till systems, one crop is planted directly into a cover crop or the residue of a previous cash crop, with only a narrow band of soil disturbed. No-till systems leave virtually all of the residue on the soil surface, providing up to 100% cover and nearly eliminating erosion losses. Here soybeans are planted into a cover crop that will be killed with a herbicide (weedkilling chemical) to form a surface mulch. (Photos courtesy of R. Weil)

23 From: David Pimentel et al. 1995, Science Vol267:

24 Total Soil Erosion on Cropland in United States USDA National Resources Inventory (2007)

25 From USDA National Resources Inventory (2007)

26 From USDA National Resources Inventory (2007) Each blue dot represents 100,000 tons of water erosion per year. Each red dot represents 100,000 tons of wind erosion per year.

27 Prevention of Erosion

28 CLAS, UC Berkeley

29 Plastic covering

30 Leaky Dam

31 Two methods of establishing vegetative cover on steep, unstable slopes. (1) A hydroseeder allows vegetative cover to be efficiently established on difficult-to-reach areas. The machine is spraying a mixture of water, chopped straw, grass seed, fertilizer, and sticky polymers that hold the mulch in place until the grass seed can take root. (2) Erosion-control mats made of plastic netting or natural materials like jute are laid down over newly seeded grass to hold the seed and soil in place until the vegetative cover is established. (Photos courtesy of R. Weil)

32 Silt fence/sediment ponds

33 Wind Breaks

34 Cover crops

35 Mulch

36 7. Soil erosion and agricultural sustainability How does soil erosion figure into agricultural sustainability? What is your opinion about the so-called T value (erosion Tolerance level)? Is 5 tons/acre/year a good value?

37 From: Helmsath et al Nature 388: Tennessee Valley, Marin county, CA

38 Ruins of Babylon USDA W.C. Laudermilk 1939

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40 Take-home message: Soil erosion still is an major issue for human societies.