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Data Access
FAO Soil Data
[rule]
Readme Contents
Data Set Overview
Sponsor
Original Archive
Future Updates
The Data
Characteristics
Source
The Files
Format
Name and Directory Information
Companion Software
The Science
Theoretical Basis of Data
Processing Sequence and Algorithms
Scientific Potential of Data
Validation of Data
Data Access and Contacts
FTP Site
Points of Contact
References
[rule]
Data Set Overview
Climate modelers need information on the water holding capacity of
global soils. Currently the best source of this information is the
Soil Map of the World, which was produced by the Food and
Agriculture Organization (FAO) of the United Nations Educational,
Scientific, and Cultural Organization (UNESCO) in 10 volumes
between 1970 and 1978. It provides the most detailed, globally
consistent soil data.
Because water holding capacity is not an explicit attribute of the
FAO soil map, the data on soil type, soil texture, soil depth, and
average slope that the soil map does provide may be used as
surrogates. The four data sets described herein were derived, by
various researchers, from the FAO soil data. For climate modelers,
a 1 degree by 1 degree grid of latitude and longitude has been
deemed adequate.
Sponsor
The production and distribution of this data set are being funded
by NASA's Earth Science enterprise program. The data are not
copyrighted; however, we request that when you publish data or
results using these data please acknowledge as follows:
The authors wish to thank the Distributed Active Archive
Center (Code 902 .2) at Goddard Space Flight Center,
Greenbelt, MD, 20771, for producing the data in its
present format and distributing them. The original data
sets where produced by:
* Soil type and soil texture were constructed by
Zobler (1986).
* Soil profile depth data set was constructed by Webb
et al. (1993).
* Slope data were originally derived from the FAO
Soil Map of the World in a 1 degree grid (GLOBTEX),
version 1.0, by the Science and Applications
Branch, EROS Data Center, Sioux Falls, South
Dakota.
* Dr. R.D. Koster performed the analyses necessary to
assign parameter values to the soil map texture
classes.
Goddard's share in these activities was sponsored by
NASA's Earth Science enterprise.
Original Archive
The original source maps are the FAO Soil Map of the World. The
Earth Sciences and Resource Institute (ESRI) digitized the data
under contract to the United Nations Environment Program (UNEP)
and the FAO in 1984. The EROS Data Center constructed the data
sets that were later used to derive the global array of average
slope.
The soil type file was acquired from Goddard Institute of Space
Science. The soil texture, slope and depth files were acquired
from the ISLSCP Initiative I CD set.
Future Updates
An international effort to develop a replacement, the Soil and
Terrain (SOTER) digital data base of the world, is under
development by the International Society of Soil Science, the
International Soil Reference and Information Center, the FAO, and
the UNEP.
The Data
Characteristics
* Summary of Parameters: Soil type contains 26 soil units, and
values for water and ice. Soil texture is characterized here
as either coarse, medium/coarse, medium, fine/medium, fine,
ice or organic. Soil profile depth is an estimate of the
depth from the soil surface to bedrock or other impermeable
layer. Slope is the surface slope, as defined by the
topography.
* Units:
o Soil type
0 = ocean/lake 11 = kastanozem 22 = vertisol
1 = acrisol 12 = luvisol 23 = planosol
2 = cambisol 13 = greyzem 24 = xerosol
3 = chernozem 14 = nitosol 25 = yermosol
4 = podzoluvisol 15 = histosol 26 = solonchek
5 = rendzina 16 = podzols 27 = ice
6 = ferrasol 17 = arenosol
7 = gleysol 18 = regosol
8 = phaeozem 19 = solonetz
9 = lithosol 20 = andosol
10 = fluvisol 21 = ranker
o Dominant soil texture index Unit-less
o Soil profile depth cm
o Average slope %
* Range:
o Soil type 1 to 26, (27=ice)
o Dominant soil texture index 1 to 7
o Soil profile depth 4 to 800 cm
o Average slope 10 to 40, (1=ice)
* Spatial Characteristics: The data are given in an equal-angle
lat/long grid that has a spatial resolution of 1 x 1 degree
lat/long. The original source map had a scale of 1:5,000,000
(1 millimeter on the map = 5 kilometers).
* Spatial Coverage: The coverage is global. Data in each file
are ordered from North to South and from West to East
beginning at 180 degrees West and 90 degrees North.
* Temporal Coverage:The data was primarily collected in the
1960's and 1970's
* Temporal Resolution: The soil map typically portrays
time-invariant features.
Source
Digital data for the FAO Soil Map of the World are available from
the Land and Water Development Division, FAO, in Rome, Italy.
A) SOIL TYPE. The soil type data file was derived from the highest
level of the FAO soil units and is based on the work of Zobler
(1986).
B) SOIL TEXTURE. The soil texture data file is based on the work
of Zobler (1986) and uses the indices listed in the table below to
identify the texture of the dominant soil type within each 1
degree x 1 degree grid square. The original FAO data provided, for
the dominant soil type in a soil unit, the designation "coarse",
"medium", "fine", or a combination of these based on the relative
amounts of clay, silt, and sand present in the top 30 cm of soil.
Also listed in the table are some suggested, arbitrarily chosen
values (see caveat) for associated soil moisture transport
properties.
index soil texture n psi_s K_s b comments
Loamy
1 coarse 0.421 .0363 1.41E-5 4.26 sand
values*
Sandy
2 medium/coarse 0.434 .1413 5.23E-6 4.74 loam
values*
3 medium 0.439 .3548 3.38E-6 5.25 Loam
values*
Sandy
4 fine/medium 0.404 .1349 4.45E-6 6.77 clay
loam
values*
Clay
5 fine 0.465 .2630 2.45E-6 8.17 loam
values*
6 ice -- -- -- -- --
7 organic 0.439 .3548 3.38E-6 5.25 Loam
values*
0 (ocean) -- -- -- -- --
where
n is the porosity (dimensionless),
psi_s is the matric potential at saturation (in m)
K_s is the saturated hydraulic conductivity (in m/s), and
b (dimensionless) is the slope of the retention curve on a
logarithmic graph, used to compute transport properties of
subsaturated soils.
* CAUTION: The assignment of loamy sand transport parameter values
to coarse soils does NOT imply that the "coarse" designation
implies a loamy sand in the USDA soil texture triangle. Similarly,
a "medium/coarse" designation does not imply a sandy loam, a
"medium" designation does not imply a loam, and so on. The mapping
of transport parameter values to soil texture in the table is
highly arbitrary and technically incorrect. It is provided solely
as a suggestion for the typical large scale (GCM) modeler, who
could easily run into trouble if the "technically correct" numbers
were used.
The suggested reclassification in the table reflects the
inappropriateness of assigning hydraulic properties of soils as
measured in the laboratory to GCM soil columns that represent
extensive areas -- they tend to produce unrealistic resistance to
soil moisture diffusion. This is almost certainly due to the
inadequacy of current land surface models, which have very limited
treatments of subgrid soil moisture variability, and to the fact
that properties measured in the laboratory often do not describe
soil behavior in the field, which is strongly influenced by
spatial variability in texture, the presence of decayed root
systems, wormholes, etc. As a makeshift response to this problem,
a given soil type in the table above is arbitrarily assigned
transport parameter values for a coarser textured soil.
Determining the optimal parameter values for each type, which are
probably very different from those listed above, would require
much further research.
The values for the four transport parameters were obtained from
the study of Cosby et al. (1984), who analyzed an extensive and
diverse collection of soil samples.
C) SOIL PROFILE DEPTH. The soil profile thickness file was derived
by Webb et al. (1991, 1993) from information contained in Volumes
2-10 of the FAO/UNESCO Soil Map of the World. First, the Earth was
divided into nine continental regions: North America,
Mexico/Central America, South America, Europe, Africa,
South-Central Asia, North Central Asia, Southeast Asia, and
Australia/South Asia. For each of these regions, the FAO records
were examined to determine the profile thickness for a
representative sample of every component soil type. When a
thickness was undefined for a soil type, an arbitrary thickness of
3.6 meters was assigned; presumably the bedrock is at a greater
depth than this. All soil elements of a given type within a given
continental region were then assumed to have the same profile
thickness. The thicknesses stored in the file's 1 degree x 1
degree array are the thicknesses for the dominant soil types
within the grid squares, as determined by Zobler (1986).
D) AVERAGE SLOPE. The average topographical slope for each 1
degree x 1 degree square was derived from data sets constructed by
the Science and Applications Branch of the EROS Data Center in
Sioux Falls, South Dakota. Unlike the soil texture and soil
profile thickness data, the average slope data reflects all of the
soil regimes in a square, not just the dominant one. The slope
estimates are crude, however, given the qualitative nature of the
original data.
The Files
Format
Data Files
* File Size: 259200 bytes, 64800 data values
* Data Format: IEEE floating point notation
* Headers, trailers, and delimiters: none
* Land/water mask: ocean/lake mask, value 0
* Fill value: -999.0, (slope)
* Image orientation: North to South
Start position: (179.5W, 89.5N)
End position: (179.5E, 89.5S)
Name and Directory Information Naming Convention
The file naming convention for the FAO soil data set is
fao_soil.pppppp.1nnegl.ddd
where:
fao_soil./b> = data product designator
pppppp = parameter name
types = soil type
textur = soil texture
depth = soil depth
slope = average slope
1 = number of levels
n = vertical coordinate, n= not applicable
n = temporal period, n = not applicable
e = horizontal grid resolution, e = 1 x 1 degree
gl = spatial coverage, gl = global (land)
ddd = file type designation, (bin=binary, ctl=GrADS control
file)
Directory Path
/data/inter_disc/hydrology/soil/
Companion Software
Several software packages have been made available on the CIDC
CD-ROM set. The Grid Analysis and Display System (GrADS) is an
interactive desktop tool that is currently in use worldwide for
the analysis and display of earth science data. GrADS meta-data
files (.ctl) have been supplied for each of the data sets. A GrADS
gui interface has been created for use with the CIDC data. See the
GrADS document for information on how to use the gui interface.
Decompression software for PC and Macintosh platforms have been
supplied for datasets which are compressed on the CIDC CD-ROM set.
For additional information on the decompression software see the
aareadme file in the directory:
software/decompression/
Sample programs in FORTRAN, C and IDL languages have also been
made available to read these data. You may also acquire this
software by accessing the software/read_cidc_sftwr directory on
each of the CIDC CD-ROMs
The Science
Theoretical Basis of Data
The FAO Soil Map of the World is comprised of map units, which
bound areas containing an association of soil units plus texture
class and slope class. Each association of soil units can contain
a dominant soil unit, associated soil unit and included soil unit.
The area coverage, in a given map unit, for each of these
categories are as follows:
* the dominant soil unit occupies the largest area of the map
unit
* the associated soil unit occupies more than 20 percent of the
enclosed map unit area but less than the area of the dominant
soil unit
* the included soil unit occupies less than 20 percent of the
enclosed area.
Total area for each map unit is given, but area for each soil unit
is not. The soil units are values representative of designated
locations. The FAO has developed an algorithm for estimating the
area of each soil unit within a soil association based on the
number of soil units in each map unit.
The FAO system of soil classification has two levels, which are
based on a inherent profile properties system. The highest level
has 26 (used for soil type map) members and the lowest level has
106 members. Detailed profile descriptions, which are
representative of the soil units, are given in the volume
accompanying each map. The descriptions contain information on:
* texture
* structure
* color
* nutural soil layers
* total depth
* layer thickness
* underlying geologic material
Textural classes reflect the relative proportions of clay
(fraction less than 2 micrometers), silt (2-50 micrometers), and
sand (50-2,000 micrometers) in the soil. The texture of a soil
horizon is one of its most permanent characteristics. It is also a
very important one because, in combination with other properties,
it influences soil structure, consistence, porosity, cation
exchange capacity, permeability and water holding capacity.
Three textural classes are recognized by the FAO Soil Map of the
World:
1. Coarse textured: sands, loamy sands, and sandy loams with
less than 18 percent clay and more than 65 percent sand.
2. Medium textured: sandy loams, loams, sandy clay loams, silt
loams, silt, silty clay loams, and clay loams with less than
35 percent clay and less than 65 percent sand; the sand
fraction may be as high as 82 percent if a minimum of 18
percent clay is present.
3. Fine textured: clays, silty clays, sandy clays, clay loams,
and silty clay loams with more than 35 percent clay.
The textural class refers to the texture of the upper 30
centimeters of the soil, which is important for tillage and water
retention. The maps often state that a dominant soil type is
composed of combinations of these textural classes (e.g., coarse
AND medium for a given soil).
100/\
/ \
90/ \10
/ \
80/ \20
/ \
/ \ 70/ \30
| / \
| 60/ \40
| / FINE \
Percent clay 50/ \50 Percent silt
/ \ |
40/ \60 |
/--------------------------\ |
30/ \70 \ /
/ \
20/ \80
/-------- MEDIUM \
10/ \ \10
/ COARSE \ \
----------------------------------------\
100 90 80 70 60 50 40 30 20 10
<--------------
Percent sand
To obtain the soil moisture transport parameters listed in the
soil texture table, points corresponding to these textures or
texture combinations were located on the U.S. Dept. of Agriculture
(1951, p. 209) textural triangle, a rough reproduction of which is
shown below:
100/\
/ \
90/ \10
/ \
80/ \20
/ \
/ \ 70/ \30
| / \
| 60/ C \40
| / /\
Percent clay 50/\ / \50 Percent silt
/ \ / SiC\ |
40/ SC \____________/______\60 |
/______\ CL \ SiCL \ |
30/ SCL \___________\_______\70 \ /
/_________/ / \
20/_ \ L / SiL \80
/ \_ SL \ / \
10/\_ \_ \_____/ ______\90
/ S \ LS \_ / / Si \
/_____\_____\________/_________/_________\
100 90 80 70 60 50 40 30 20 10
<--------------
Percent sand
The soil textures identified in the figure are:
C: Clay
SC: Sandy clay
SiC: Silty clay
SCL: Sandy clay loam
CL: Clay loam
SiCL: Silty clay loam
S: Sand
LS: Loamy sand
SL: Sandy loam
L: Loam
SiL: Silt loam
Si: Silt
The points were then arbitrarily shifted toward coarser soils, and
transport parameters for the coarser soils were taken from Cosby
et al. (1984), who used the same triangle to differentiate soil
types.
Processing Sequence and Algorithms
Soil Type & Texture:
Zobler (1986) used a transparent overlay to subdivide the original
source map into one-degree cells. The original source map
(digitized from the 1:5,000,000 scale FAO Soil Map of the World)
had a two degree grid size for latitudes below 60 degrees, a four
degree grid size for latitudes between 60 and 80 degrees latitude,
and a eight degree grid size above 80 degrees latitude. The soil
type map was created by selecting the dominant soil unit, from the
largest map unit covering the one-degree grid cell. Soil texture
was derived from the soil profile information associated with each
soil unit, using the methodology described in the SOURCE and
THEORETICAL BASIS sections of this readme.
Soil Profile Depth:
Webb et al. (1991) developed the following set of decision rules
for the soil depth data to standardize the data set, to check the
data for errors and to correct them, and to fill in missing data:
* A default basal depth of 360 cm was used when no bottom depth
was specified for a soil profile. The default depth of 360 cm
was selected to allow realistic simulation of dynamic
hydrology.
* An average depth was calculated in cases when a depth range
was given or the top and bottom depths of contiguous horizons
were not the same. Depths reported in inches were converted
to metric.
Webb et al. (1991) reported that a number of soil types from each
continent were completely missing depth data. To fill in these
areas they substituted data using the same soil type from a
different continental division. When data for a soil type were
absent from all the continents, data were substituted from an
adjacent soil type with similar descriptive characteristics.
Average Slope:
Arc/Info software was used for most processing steps in the
construction of the slope data files generated by the EROS Data
Center (which were then used to construct the average slopes),
including projection from the bipolar oblique conformal projection
to geographic (latitude-longitude) coordinates for the Americas.
The remainder of the world was projected from the Miller oblated
stereographic projection using software provided by Sprinsky
(1992).
The average slopes were generated by some simple processing of the
data sets produced by the EROS Data Center. These data sets
provide, for each 1 degree x 1 degree square, the fractions f1,
f2, and f3 of area covered by "level to gently undulating" (0-8%),
"rolling to hilly" (8-30%) and "steeply dissected to mountainous"
(>30%) slopes, respectively. For the calculation of the average
slope, the 0-8% slope category was assigned a typical slope of 4%,
the 8-30% slope category was assigned an average slope of 19%, and
the >30% slope category was assigned the arbitrary slope of 40%.
The average slope was then taken to be:
f1*4% + f2*19% + f3*40%
average slope = -------------------------
f1+f2+f3
Special Corrections/Adjustments:
A few of the 1 degree x 1 degree squares that were designated by
Zobler (1986) as ice in the soil texture and soil type files are
listed as land squares in the ISLSCP Initiative I vegetation data
sets. To correct this inconsistency the "ice" squares, in the
original soil data sets, that are designated as "tundra" in the
vegetation data set have been changed to coarse (soil texture) and
regosol (soil type).
The soil texture, depth, and slope data (that were originally
archived on the ISLSCP Initiative I CD set) had the ISLSCP
land/sea mask, applied to them. This mask had areas of land which
were classified as ocean in the original Zobler (1986) Webb et al.
(1991, 1993), and EROS data center files. The soil data on the
ISLSCP Initiative I data was modified so that these grid cells
were re-classified as land with an appropriate parameter value.
The soil data describe in this readme, and available as part of
the Interdiscipline Data Collection (IDC) has been revised. Areas
which were re-classified on the ISLSCP Initiative I soil data, as
land, have been changed back to ocean on the IDC soil data. This
process does not apply to the soil type data which never had the
ISLSCP Initiative I land/sea mask applied to it.
The Goddard DAAC converted the soil files to IEEE floating point
notation and re-oriented the soil type file to it's present form.
Scientific Potential of Data
The four soil data files are provided mainly for use in defining
land surface properties for general circulation model (GCM)
applications. Many land surface models coupled to GCMs require
estimates of soil profile depth, surface slope, and soil moisture
transport properties (as obtained from soil texture) for their
runoff, soil moisture storage, and drainage parameterizations.
Inherent in the data are large-scale spatial variations in the
soil properties, which presumably are realistic even if values at
various grid squares are inaccurate. This large-scale structure
can be important for defining GCM climate.
Given that climate modelers are the expected users of the data,
the danger of using the data for other applications must be
stressed. Extracting a soil texture, slope, soil profile depth or
soil type from the files for a specific small-scale region (even a
region composed of numerous 1 degree x 1 degree squares) is
foolhardy without further research into the reliability of the
data in the region, as determined, for example, from the original
FAO Soil Map of the World. At some squares, the data is
undoubtedly unreliable. Even if the reliability were high, soil
texture and profile depth are provided only for the dominant soil
component of the 1 degree x 1 degree square, and thus the
appropriate values in a subgrid region of interest can easily be
missed. The moisture transport parameter values listed in the soil
texture table are undoubtedly inaccurate and are provided ONLY to
give climate modelers a consistent basis for performing
intercomparison studies.
The data can be spatially aggregated by averaging the values in
adjacent grid cells to create, for example, a 2x3 degree grid or a
3x5 degree grid. Although the grid cells are not equal area, and
large errors would be introduced if a cell at the equator were
averaged with a cell at the north pole, the errors from averaging
adjacent cells will be within the accuracy limits for the data
set.
Validation of Data
The original FAO data represent a generalization of more detailed
data, which may be available in various countries, and which are
in turn a generalized representation of reality. As stated by
Zobler (1986), "about 11,000 maps were reviewed [to construct the
FAO Soil Map of the World]; they varied widely in reliability,
detail, precision, scales, methodologies, etc." As with any soil
map, some of the variability in the actual soils is not shown on
the map. Errors may have been introduced in the digitizing and map
projection process.
The soil type, soil texture, and profile depth files contain data
for the dominant soil type in each 1 degree x 1 degree square and
thus ignore contributions from potentially significant secondary
components. Soil texture, profile depth and average slope values
were not available for all of the soil types mapped by Zobler
(1986). The profile depths are generally based on depths measured
for an equivalent soil elsewhere on the continent; depths are not
actually measured at each square. As stated by Webb et al.(1991),
the soil profile thickness in many cases represent minimum
possible values because profile descriptions do not always extend
to subsurface bedrock. The soil thicknesses range from 10 cm for
Lithosol to 800 cm for Distric Nitosol in Africa. The spatial
distribution of soil profile thickness can be summarized as
thickest in the well-developed soils of tropical low latitudes and
thinnest in the poorly developed soils of high latitudes. The soil
profiles are thin in mountainous regions such as the Himalayas and
Andes and are thick in mid-latitude peatlands such as those found
in northern Europe and North America. For further discussion of
the limitations of these data sets, see Zobler (1986) and Webb et
al. (1991, 1993).
An obvious source of error in the average slope file is the
arbitrary choice of 40% to represent all steep slopes, when all
that is known is that they exceed 30%. Also, for the files used to
compute the average slopes, assumptions were made on the
percentage composition of the components. The vector data sets
were gridded as separate data sets, and the data sets were merged
in grid form. Some overlaps between data sets were removed
manually.
Confidence Level/Accuracy Judgment:
Some measure of reliability was provided for the original FAO
source maps, but these measures were not considered when
constructing the soil texture, depth, and slope files, and
corresponding arrays of reliability estimates are not available.
The accuracy of the data is, of course, severely limited by the
errors.
Measurement Error for Parameters and Variables:
The published FAO Soil Map of the World contains inset maps
showing three categories of reliability for the source data used
to make the map. Those interested in the reliability at a specific
site should consult this source; again, digitized global
reliability estimates are not available. Detailed soil surveys
were performed only over selected areas of each continent.
Known Problems with the Data:
The FAO Soil Map of the World is becoming out-of-date because of
recent soil surveys and new techniques for measurement and data
handling. An international effort to develop a replacement, the
Soil and Terrain (SOTER) digital data base of the world, is under
development by the International Society of Soil Science, the
International Soil Reference and Information Center, the FAO, and
the UNEP.
Data Access and Contacts
FTP Site
Points of Contact
For information about or assistance in using any DAAC data,
contact
EOS Distributed Active Archive Center (DAAC)
Code 902.2
NASA Goddard Space Flight Center
Greenbelt, Maryland 20771
Internet: daacuso@daac.gsfc.nasa.gov
301-614-5224 (voice)
301-614-5268 (fax)
References
Cosby, B.J., G.M. Hornberger, R.B. Clapp, and T.R. Ginn, 1984. A
statistical exploration of the relationships of soil moisture
characteristics to the physical properties of soils, Water
Resources Research, 20:682-690.
Food and Agriculture Organization (FAO) of the United Nations,
1970-78, Soil map of the world, scale 1:5,000,000, volumes I- X:
United Nations Educational, Scientific, and Cultural Organization,
Paris.
Sprinsky, William H., 1992. The inverse solution for the Miller
oblated stereographic projection: Presented at the 27th
International Geographical Congress, Washington, D.C.
U.S. Dept. of Agriculture, 1951. Soil Survey Manual. U.S. Dept. of
Agriculture Agricultural Handbook, 18, 503pp.
Webb, R.S., C.E. Rosenzweig, and E.R. Levine, 1991. A global data
set of soil particle size properties, NASA Tech. Memo. 4286, NASA,
34pp.
Webb, R.S., C.E. Rosenzweig, and E.R. Levine, 1993. Specifying
land surface characteristics in general circulation models: soil
profile data set and derived water-holding capacities, Global
Biogeochemical Cycles, 7:97-108.
Zobler, L., 1986. A world soil file for global climate modeling.
NASA Tech. Memo. 87802, NASA, 33pp.
Zobler, Leonard, 1987. A world soil hydrology file for global
climate modeling: International Geographic Information Systems
Symposium: The Research Agenda, November 15-18, 1987, Arlington,
Virginia, Proceedings. 1:229-244.
------------------------------------------------------------------------
[NASA] [GSFC] [Goddard DAAC] [cidc site]
NASA Goddard GDAAC CIDC
Last update:Thu Jun 19 18:42:11 EDT 1997
Page Author: James McManus -- mcmanus@daac.gsfc.nasa.gov
Web Curator: Daniel Ziskin -- ziskin@daac.gsfc.nasa.gov
NASA official: Paul Chan, DAAC Manager -- chan@daac.gsfc.nasa.gov
