Re: Soilgrids water capacity data

[Tomislav Hengl]

On 13-07-17 17:20, Sonderegger Thomas wrote:
> Dear Tomislav,
>
>
>
> We are working on soil compaction in our team, which is closely related
> to soil moisture and clay content. So we were very happy to see that
> some data about soil water capacity is available in your project.
> However, we were not able to make sense of all descriptions and I really
> would appreciate some help. These are my questions:
>
>
>
> AWC: How do you define AWC? Field capacity minus wilting point?

Hi Thomas,

Yes, exactly as in https://en.wikipedia.org/wiki/Available_water_capacity
Description of the columns AWC and WWP is also available here:

http://gsif.r-forge.r-project.org/AWCPTF.html

The best technical source for this formula is probably:

http://dx.doi.org/10.1016/j.geoderma.2012.11.021

The formulas have been prepared for R by my colleagues Johan and Maria
(see contacts in http://gsif.r-forge.r-project.org/AWCPTF.html), and the
exact algorithm is available at:

https://github.com/cran/GSIF/blob/master/R/AWCPTF.R

>
> AWCtS: Is saturation equal to porosity of the soil? If not, how is it
> defined?

AWCtS is the tetaS: saturated water content from Eq.1 in
http://dx.doi.org/10.1016/j.geoderma.2012.11.021

>
> WWP: What does until wilting point mean? Is this volumetric water
> content at the wilting point?
>

WWP = moisture potential at wilting point in kPa e.g. -1585 (pF 4.2).
Here is an example of AWC derivation in R:

?AWCPTF
SNDPPT = 15
SLTPPT = 37
CLYPPT = 48
ORCDRC = 15
BLD = 1350
CEC = 45
PHIHOX = 6.4
x <- AWCPTF(SNDPPT, SLTPPT, CLYPPT, ORCDRC, BLD, CEC, PHIHOX)
x
AWCh1 AWCh2 AWCh3 WWP tetaS
1 0.0989 0.0763 0.0622 0.332 0.475

AWCh1 + WWP = 43.1% = FC (field capacity)

Example #2:

SNDPPT = 30
SLTPPT = 48
CLYPPT = 25
ORCDRC = 123
BLD = 1000
CEC = 12
PHIHOX = 6.4
x <- AWCPTF(SNDPPT, SLTPPT, CLYPPT, ORCDRC, BLD, CEC, PHIHOX)
x
AWCh1 AWCh2 AWCh3 WWP tetaS
1 0.214 0.173 0.146 0.257 0.555

AWCh1 + WWP = 47.1% = FC (field capacity)

Note that a limitation of the AWC layers is that the PTFs are based on
the limited training data, mainly from tropics / sub-tropics (Africa).
As soon as somebody makes maybe a more representative PTF (globally) we
can re-derive AWC for the whole world.

>
>
> I did not really understand the relationship between the different
> numbers and how they add up (or why they shouldn’t).
>

I do need to make more documentation for SoilGrids (as you can imagine,
our priority was really to make the best / most accurate possible
predictions, but this year we also plan to make a lot of tutorials etc).

If you need soil-hydro data for EU, maybe better use:

http://onlinelibrary.wiley.com/doi/10.1002/hyp.11203/abstract

available for download from:

http://mta-taki.hu/en/eu_soilhydrogrids_3d

All the best with your work,

--
T. (Tom) Hengl
Researcher @ ISRIC - World Soil Information
Mail: ISRIC - World Soil Information P.O. Box 353, 6700 AJ Wageningen,
The Netherlands
Location: Droevendaalsesteeg 3 (Building 101 / GAIA) room: C.002
Tel: +31(0)317-484199
Url: http://www.isric.org/tomhengl
Network: http://profiles.google.com/tom.hengl
Publications: http://scholar.google.com/citations?user=2oYU7S8AAAAJ
ORCID ID: http://orcid.org/0000-0002-9921-5129
>
>
> Thanks a lot for your time and your effort!
>
>
>
> Best, Thomas
>
>
>
>
>
> ************************************************
> Thomas Sonderegger
>
> PhD candidate
> ETH Zurich
> Institute of Environmental Engineering (IfU)
> Ecological Systems Design
>
> John-von-Neumann-Weg 9
> 8093 Zurich, Switzerland
>
> Phone: +41 44 633 60 14
> Mobile: +41 79 627 11 25
> Email: sonde...@ifu.baug.ethz.ch <mailto:sonde...@ifu.baug.ethz.ch>
>
> http://www.ifu.ethz.ch/ESD/
>
>
>
>
>