Earth's Systems / Biological processes / Terrestrial ecosystem functioning / Living soil system / Wind erosion: saltation, dust emission and deposition
Wind erosion: saltation, dust emission and deposition
Wind detaches and moves soil particles when friction velocity exceeds a threshold set by particle size, moisture, aggregates and cover, removing fine nutrient-rich material and delivering dust to distant ecosystems.
Boundary: where this process starts
Starts with: Surface state sets erodibility: aggregates and clods larger than 0.84 mm are non-erodible, so aggregate stability and tillage state set the erodible fraction
Boundary: where it ends
Ends after: Global dust emission is of order 1,000-3,000 Tg per year, mostly from deserts, with a smaller share from agricultural land; Windbreaks reduce wind speed over a distance of about 10-30 times their height
Line thickness = size of impact (1 to 5)% on a line = chance a fork takes that branch📄 = pieces of evidence on a stepNegative effectLoops backHypothesisResource used up (arrow into the step)Resource used, not used upResource made or released (arrow out of the step)
What am I looking at? The concepts on every map
- Participant
- Someone or something that takes part in a process or influences it: a plant, a microbe, a river, a machine, a force. Participants are the rows of a map.
- Resource
- Something a step uses or uses up: water, oxygen, sunlight, a tool. Each step has an arrow down to the resources it uses.
- Step
- One thing that happens in a process. Every step has at least one participant and carries evidence.
- Decision fork
- A step where the process can go several ways. Every outcome is labeled with a percentage, and the percentages add up to 100%. It is not limited to yes or no.
- Loop
- A line that carries the process back to an earlier step. We record whether it reinforces or balances, how strong it is (weak, moderate, strong), and its gain (0 to 1): the share of the effect that returns on each pass.
- Size of impact
- Every line says how much the earlier step affects the next one (1 to 5), in which direction (+, -, 0), and why. Thicker lines mean bigger impact.
- Evidence
- The support for a claim: a source and what it shows. Every step, line, change and vote needs it. Click any step or line to read its evidence.
- Boundary
- Where a process starts and where it ends. Every process, and every subprocess, has a start and an end.
- Subprocess
- A step that is itself a whole process, with its own boundary, steps and lines. Click a step marked ↘ to open it. Subprocesses nest as deep as the evidence goes.
- Description
- A plain-language summary of what a process is and what it covers.
- Hypothesis
- A proposed new connection between two parts of the map that nobody has linked. It carries a prediction, a way to test it, and a count of papers on each topic and on both. It is an unverified idea until someone tests it.
Every step, with its evidence
The same information as the map, as a list you can search and share. Click a participant or resource to see everywhere it appears.
Surface state sets erodibility: aggregates and clods larger than 0.84 mm are non-erodible, so aggregate stability and tillage state set the erodible fraction
Evidence (1) · added by Fern-C
- Chepil 1953, Soil Science 75:473-483; Woodruff & Siddoway 1965, Soil Science Society of America Proceedings 29:602-608: Soil fractions over 0.84 mm diameter resist wind; the erodible fraction below that size drives the wind erosion equation. (confidence 0.8)
Soil moisture raises the threshold: u*t increases by [1 + 1.21 (w - w')^0.68]^0.5 with w' = 0.0014 (%clay)^2 + 0.17 (%clay), so a 20% clay soil at 8% gravimetric water has about twice the dry threshold
Evidence (1) · added by Fern-C
- Fecan, Marticorena & Bergametti 1999, Annales Geophysicae 17:149-157: Parameterisation of threshold increase with gravimetric water content above a clay-dependent residual w'; substituting 20% clay and 8% water gives a factor of about 2.0 (derived). (confidence 0.75)
Roughness elements such as plants, residue and stones absorb wind momentum, raising the soil threshold friction velocity by about 1.5-2.5 times at only 5% lateral cover
Evidence (1) · added by Fern-C
- Raupach, Gillette & Leys 1993, Journal of Geophysical Research 98:3023-3029: Threshold ratio = [(1 - m sigma lambda)(1 + m beta lambda)]^(1/2); with beta about 90 small cover lambda multiplies the threshold substantially (range here is an estimate). (confidence 0.5)
Wind friction velocity above the threshold lifts loose particles; the minimum threshold, about 0.2 m/s, occurs for grains near 0.1 mm
Evidence (1) · added by Fern-C
- Bagnold 1941, The Physics of Blown Sand and Desert Dunes; Shao & Lu 2000, Journal of Geophysical Research 105:22437-22443: Threshold friction velocity has a minimum near 100 micrometre grains at about 0.2 m/s; smaller grains are held by cohesion. (confidence 0.75)
Saltation: grains hop in ballistic paths and, far above threshold, horizontal flux scales with friction velocity cubed, so a 20% rise in u* raises flux about 1.7 times
Evidence (1) · added by Fern-C
- Bagnold 1941, The Physics of Blown Sand and Desert Dunes: Saltation flux q proportional to (rho/g) sqrt(d/D) u*^3; 1.2^3 = 1.73. (confidence 0.8)
Transport partition of horizontal mass flux: creep 7-25%, saltation 55-72%, suspension 3-38%
Evidence (1) · added by Fern-C
- Chepil 1945, Soil Science 60:305-320, Dynamics of wind erosion I: Measured shares of soil movement by surface creep, saltation and suspension. (confidence 0.65)
Impacts of saltating grains break aggregates and eject dust, so vertical dust flux is proportional to saltation flux with a ratio that rises with soil clay content (sandblasting)
Evidence (1) · added by Fern-C
- Kok et al. 2012, Reports on Progress in Physics 75:106901; Marticorena & Bergametti 1995, Journal of Geophysical Research 100:16415-16430: Dust emission is driven by saltation bombardment, with efficiency depending on soil texture. (confidence 0.7)
Eroded dust is enriched in organic carbon, nitrogen and phosphorus relative to the source soil, so fields lose fertility faster than they lose mass
Evidence (1) · added by Fern-C
- Larney, Bullock, Janzen, Ellert & Olson 1998, Journal of Soil and Water Conservation 53:133-140: Wind-eroded material was enriched in organic C, N and P compared with the source soil. (confidence 0.7)
Loss of fines and organic matter coarsens the surface and weakens aggregates, raising erodibility
Evidence (1) · added by Fern-C
- Lal 2001, Soil degradation by erosion, Land Degradation & Development 12:519-539: Erosion preferentially removes the organic-rich fine fraction and degrades structure, increasing further susceptibility. (confidence 0.6)
Coarse lag and non-erodible aggregates armour the surface, so loss slows once the erodible fraction is exhausted
Evidence (1) · added by Fern-C
- Chepil 1950, Soil Science 69:149-162, Properties of soil which influence wind erosion II: Erosion removes erodible particles until non-erodible clods and lag cover the surface, after which loss declines. (confidence 0.6)
Dust travels far: the Sahara supplies about 28 Tg of dust a year to the Amazon basin, bringing about 22,000 t of phosphorus
Evidence (1) · added by Fern-C
- Yu et al. 2015, Geophysical Research Letters 42:1984-1991: Satellite lidar estimates 28 Tg/yr dust deposition in the Amazon with about 22 Gg phosphorus, comparable to hydrologic losses. (confidence 0.7)
Global dust emission is of order 1,000-3,000 Tg per year, mostly from deserts, with a smaller share from agricultural land
Evidence (1) · added by Fern-C
- Kok et al. 2021, Atmospheric Chemistry and Physics 21:8169-8193: Global dust emission and its source regions constrained to order 1,000-3,000 Tg per year (estimate). (confidence 0.55)
Windbreaks reduce wind speed over a distance of about 10-30 times their height
Evidence (1) · added by Fern-C
- Brandle, Hodges & Zhou 2004, Agroforestry Systems 61:65-78: Shelterbelts lower wind speed leeward over distances of 10-30 barrier heights depending on porosity. (confidence 0.65)