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Can global soil organic carbon maps be used in policy decisions on practical agricultural management?
Mats Söderström & Kristin PiikkiInternational Center for Tropical Agriculture (CIAT)Swedish University of Agricultural Sciences (SLU)
Digital Soil Mapswith national, continental or global coverage
SoilGrids250m / ISRIC World Soil Information(Hengl et al. 2017, PLOS ONE 12(2): e0169748)
1. How accurate is Rwandan SOC farm averages and Sector averages derived from SoilGrids 250-m maps?
2. Can we combine the SoilGrids 250-m raster with available local SOC samples to produce a downscaled, improved map?
Questions
A Sector unit (50 km2) was deemed appropriate for management advice
RwandaComparing local data and SoilGrids250m maps
800+100 soil analyses from IFDC and CropNuts Lab
topsoil SOC according to SoilGrids 250-m raster
Söderström M, Piikki K, Cordingley J. 2016. South African J. of Plant & Soil
RwandaComparing local data and SoilGrids250m maps
topsoil SOC according to SoilGrids 250-m raster
topsoil SOC according to interpolation of 800 soil samples
Söderström M, Piikki K, Cordingley J. 2016. South African J. of Plant & Soil
RwandaValidation at soil sample locations ’Truth’: the 100 soil samples
SOC
Söderström M, Piikki K, Cordingley J. 2016. South African J. of Plant & Soil
SoilGrids
RwandaValidation of Sector averages ’Truth’: Sector means of the 800 Rwandan soil samples
Söderström M, Piikki K, Cordingley J. 2016. South African J. of Plant & Soil
SoilGrids
1. How accurate is Rwandan SOC farm averages and Sector averages derived from SoilGrids 250-m maps?
-Local validation was needed to judge.2. Can we combine the SoilGrids 250-m raster with available local soil samples to produce a downscaled, improved map?
-Yes.
Questions and answers
YES(r: ***) YES
(r: **)
NO(r: ns)
NO(r: ns)
NO(r: ns)NO
(r: ns)
Is the map correlated to the samples?Test against 50 soil observations / area
105,000 km2
25,000 km2
8,600 km2
400 km2
2 km2
5 km2
NO(Bias: ***)
NO(Bias: ***)
NO(Bias: ***)NO
(Bias: ***)
Is the map at the right level?Test against 50 soil observations / area
105,000 km2
25,000 km2
8,600 km2
400 km2
2 km2
5 km2
NO(Bias: ***)
NO(Bias: ***)
YES(DMAE: *) YES
(DMAE: ***)
Is the map improved by downscaling?Test against 50 soil observations / area
105,000 km2
25,000 km2
8,600 km2
400 km2
2 km2
5 km2
YES(DMAE: ***)
YES(DMAE: ***)
YES(DMAE: **)
YES(DMAE: ***)
Summary• The DSM community must help users assess whether the
maps are appropriate for their intended use.
• As a user – do not take for granted that a global soil map can be used without local validation
• It may be possible to improve it through local adaptation using local soil observations.
• We recommend further studies on approaches for interactive validation and local improvement of digital soil maps.
Summary• The DSM community must help users assess whether the
maps are appropriate for their intended use.
• As a user – do not take for granted that a global soil map can be used without local validation
• It may be possible to improve it through local adaptation using local soil observations.
• We recommend further studies on approaches for interactive validation and local improvement of digital soil maps.
Thanks to: International Fertilization Development Center (IFDC), One Acre Fund (OAF), Swedish Board of Agriculture and CropNuts Lab for letting us use their soil data. Funding was received by Formas and the Swedish International Development Cooperation Agency, Sida (contract no. 220-2013-1975).
Mats Söderström [email protected]
Validation - maybe you get the answer you ask for?
Validation set independence
RMSEPM
AE
E
R 2RPD
This says little about the map accuracy between the clusters…
Validation support
The map may describeregional variation well..
…but it may be of little use at an individual farm
Validation extent
The error at point support may be large
But the farm averages may still be quite good
Is it the method that is validated – or is it the map?
© K. Piikki, 2015
This does!
