Renard et al. (1997) has an equation where you can estimate K based on the geometric mean particle size.  Soil texture information gives you the ability to quantify the GM particle size in a relative sense. 

I would infer a realistic soil structure from land use /management information and soil texture / OM content. Structure is significantly influenced by land use and management. I would try to find in the litterature previous work on structure- management relationships in conditions similar to yours.

Good luck.

Without seeing physically the soil material is difficult to estimate the soil structure and its permeability. Perhaps you can have a go an estimate b, while it goes from 1 to 4, 1:granular very fine and 4: prismatic gravel. On the other hand, if you could get the edaphology map of the area, you could estimate K based on the FAO methodology.

Regards,

Robbi Bishop-Taylor,

would be the correct perform the analyzes, but if you can not try using literature values that approximate the condition of your soil.

Regards,

@ Robbi,

Does it necessary to use the above-mentioned equation? Take a look at the attached file as a possible option. Hope it give you a direction.

Renard et al. (1997) has an equation where you can estimate K based on the geometric mean particle size.  Soil texture information gives you the ability to quantify the GM particle size in a relative sense. 

Hi Drew, I came to the same conclusion a day after posting this question: I think the geometric mean particle size method for approximating K is definitely the best and most responsible option given the data I have access to. I've run it for my dataset and the results come out very close to what has previously been modelled for my study area at a coarser scale, so it looks good. Thanks all for your help!

May I approach your question from another angle? Is your RUSLE model output sensitive to a realistic range of erodibility factors? In my experience this is a rather rare case. Mostly, the (seasonal) cover factor during the season of erosive rains dominates the outcome. I have come across two exemptions only. 

You could use the KUERY software and dataset, which determines K values (in IS system, you need to convert units after) based on OM%, rock stoniness (% at threshold in 10%), soil texture, and climate. it is based on a worldwide database, and gives a distribution of observed values, including median and percentile values.

reference is: Borselli L., Torri D., Poesen, J., Iaquinta, P. 2012. A robust algorithm for estimating soil erodibility in different climates. Catena 97, 85-94.

you find the software for free download at the main author webpage:

http://www.lorenzo-borselli.eu/kuery/

it's a very robust work and approach. best regards

For some lessons from determining K values from field measurements in Tanzania see attached paper

We have published much work on this:

http://eusoils.jrc.ec.europa.eu/library/themes/erosion/Erodibility/

 @Robbi Bishop-Taylor, interestingly i am also looking for textural data (silt, clay and sand) on some soil classes i am working on (Leptosols, Acrisols, Luvisols, plinthosols) will be great if you can provide me a link

Check out the Harmonized World Soil Database. http://www.fao.org/soils-portal/soil-survey/soil-maps-and-databases/harmonized-world-soil-database-v12/en/. The SWAT Input Data (Soil) Chapter 22 (http://swat.tamu.edu/media/69365/ch22_input_sol.pdf) prescribed an alternative equation for the computation of USLE K, based on Williams 1995.

Does anyone know if there is a comprehensive database that will indicate the % of very fine sand? If not, is there anyway to calculate it for every location worldwide ?

Great Obinna, i still need this information. But i already have the harmonized world soil database, unfortunately its only at the regional level, that is major soil level and the challenge is finding it for the soil series level. Thanks anyway 

The % of very fine sand (compared to total fraction of Sand) is found to be around 20-33.3% (literature provided

Granulometric soil texture curve is a good way to estimate very fine sand, if you have the clay, silt and total sand fractions.

Also, if you do not have soil extructure data you can not consider the part of the K-equation related with "b". The K factor can be calculated with the first part of the equation, which depends on soil texture fractions and OM. This is the value estimated in first aproximation. The "b" and "c" values are useful to estimate the K factor in second aproximation, as Wischmeier & Smith explained in their USLE Agriculture Handbook.

@Panos Panagos

Hi Mr.Panagos

Can you introduce some reference for this post??

" The % of very fine sand (compared to total fraction of Sand) is found to be around 20-33.3% (literature provided )"

this may be help

stone, R.P. et D. Hillborn, 2000, Universal Soil Loss Equation, Ontario, Canada, Ontario Ministry of Agriculture and Food (OMAFRA), [En ligne] URL : http://www.giser.be/wp-content/uploads/2012/05/USLE-infosCanada.pdf.

Spesific formula for Soil https://mepas.pnnl.gov/mepas/formulations/source_term/5_0/5_32/5_32.html

5.3.2 Soil Erodibility Factor - mepas

http://www.omafra.gov.on.ca/english/engineer/facts/12-051.htm

Universal Soil Loss Equation (USLE) - Ministry of Agriculture, Food ...

http://redac.eng.usm.my/html/publish/2011_43.pdf

Modified soil erodibility factor, K for Peninsular ... - redac, usm

http://milford.nserl.purdue.edu/weppdocs/overview/usle.html

Universal Soil Loss Equation (USLE)

http://abe-research.illinois.edu/courses/tsm352/lectures/TSM%20352%20-%20Soil%20Erosion%20II.pptx

Soil Erodibility Factor

http://www.fao.org/docrep/t1765e/t1765e0f.htm

Soil erodibility - FAO

https://www.researchgate.net/publication/323588758_Estimation_of_soil_erodibility_factor_in_rusle_equation_for_euphrates_river_watershed_using_GIS

(PDF) Estimation of soil erodibility factor in rusle equation for ...

https://www.researchgate.net/publication/253340741_Estimating_the_USLE_Soil_Erodibility_Factor_in_Sicily_South_Italy

(PDF) Estimating the USLE Soil Erodibility Factor in Sicily, South Italy

http://www.mdpi.com/2077-0472/3/3/362/pdf

Evaluating Alternative Methods of Soil Erodibility Mapping in ... - MDPI

http://www.theijes.com/papers/v3-i12/Version-2/F31202030038.pdf

Determination of Erodibility Index (K) Of Soil in Michael ... - THE IJES

Maybe you can use the standard texture :

http://www.omafra.gov.on.ca/english/engineer/facts/12-051.htm

Hey I also meet the same problem for running the RUSLE k-factor. I find an empirical equation to calculate geometric mean diameter (GMD), based on soil texture, OM, and CaCO3:

GMD=exp(1.343−2.235⁎sand−1.226⁎silt−0.0238⁎(silt/clay)+33.6⁎OM+6.85⁎CaCO3)⁎(1.0+0.006⁎layerdepth)

I have noticed there was an equation from Renard et al.(1997) estimate K based on the geometric mean particle diameter too, but diameter only related with particle size fraction.

Does any opinion which model will be better for structure classes? The OM related to structure formation makes sense to me.

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2. Article Tillage affects soil aggregation parameters linked with wind erosion

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