Isabella,
Voici un document qui présente la classif ‘photointerprétée’, le 1.1, et les classifs SPOT sur le
supersite du Gourma, le 1.2, que j’ai surligné en jaune.
Il s’y trouve la carte des sols (fig 6), les taux de ligneux (fig 8, séparés pour l’instant en
plusieurs cartes suivant le type de sol) et les cultures (fig 7), séparées également par type de
sol. On fera le regroupement pour ces 2 cartes, mais ça te donnera quand même une petite
idée. Il y a aussi une comparaison entre la classif SPOT et le photo-interprétée.
Progress in mapping soil and vegetation in the Gourma (Mali).
AMMA WP 3.2
Pierre Hiernaux, Fanny Barbe, Sarah Guibert,
Laurent Kergoat, Bernard Mougenot, Eric Mougin
The strategy for up-scaling field site data on vegetation, soils, water, energy and gas fluxes at the
AMMA site in Mali includes three imbedded windows and scales. From bottom up these windows
are:

The Hombori super-site, a 50 x 50 km area that includes 9 of the 26 field sites (15°8’15°35’ lat N; 1°20’-1°45’longW), area = 2500 km².

The meso-scale site, 1 x 3 degree square transect area in the centre of the Gourma region (
14° 30 - 17° 30 lat N; 1° - 2° long W), area = 36 326 km²

The regional-scale, extending to the Gourma (south of the Niger river loop to the borders
with Niger and Burkina Faso, in the reach between Timbuctu and Labbezanga, at the
Niger border) and neighbour region of the Seno Mango to the South and southern
Azaouad to the north, area = 106370km²

The subcontinental-scale, extending to whole West-Africa south of the Sahara (5-20lat N;
18 long W- 25long E), area = 6 500 000 km².
This report only deals with progresses of mapping at the first three scales in chapter 1, 2 and 3
respectively and give some perspectives of future work in chapter 4.
1. Mapping soil and vegetation of the AMMA super-site. Beside its particular interest to locally
up-scale findings from the AMMA field sites concentrated around Hombori, mapping of supersite is used to test mapping methods prior to expand their use to wider areas. The diversity of
soils linked to geology, topography, geomorphology, hydrological systems, and the diversity of
land uses encountered in the super-site warrant the relevance of this test.
1.1. Since 2005, a visual interpretation of false colour composites of recent landsat scenes
(2000-2002) and systematic field data recording have been used to map land use,
vegetation, soil and hydrology over the AMMA super-site at the scale of 1/50000. To
account for the high level of micro-heterogeneity and common repetitive patterns (dunes/
inter-dunes on sandy soils, bare soil impluvium/thickets in ‘tiger bush’…) and allow for upscaling of field site observations, each landscape unit is described as a mosaic of 1 to 3
facies described separately. The relative area covered by each facies within the mosaic is
estimated, and the average size (decametric, hectometric, kilometric) and mode of
distribution of each facies (unique, repetitive regular, repetitive clumped) is described. Each
facies is defined by the vegetation, landuse, soil and hydrology. Over the 2687,8 km² of the
AMMA super-site window 579 land units have been mapped of 4,64 km² (sd 5,05) average
size. A majority of these land units are mosaics composed of three (59,2%) or two (32,3%)
facies (table 1).
1
1.1.1. There are 1452 facies described with an average area covered within land unit of
1,55 (sd 2.51) km². In a large majority of cases the distribution of these facies within
the land unit is repetitive (89,1%) under regular (54,9) or contagious (34,2%) patterns,
and facies patches within landunits are most often of decametre (49,5%) or hectometre
(47,5%) magnitude size. The facies vegetation is characterised by one or two dominant
species among the woody plants or perennial herbaceous (table 2). When two woody
species are associated in a facies, the first one is attributed 2/3 of the area, leaving 1/3
to the second one. The vegetation is also characterised by visual estimates of the
canopy cover of woody plants: either trees (maximum height superior to 4 meters), or
shrub (maximum height between 2 and 4 meters) and bushes (maximum height less
than 2m) together. These estimates are coded along the same scale (table 3) also used
to code the estimate cover and densities of the perennial herbaceous (table 4).
Table 2: Code of the dominant species in main vegetation types in the Gourma and surrounding
areas.
code
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
Č
Woody plants
Anogeissus leiocarpus
Balanites aegyptiaca
Combretum glutinosum
Commifora africana
Euphorbia balsamifera
Prosopis africana
Grewia bicolor
Hyphaene thebaica
Isoberlinia doka
Boscia senegalensis
Butyrospermum parkii
Acacia laeta
Mitragyna inermis
Acacia nilotica
Bauhinia rufescens
Pterocarpus lucens
Combretum micranthum
Acacia raddiana
Sclerocarya birrea
Terminalia macroptera
Acacia ehrenbergiana
Salvadora persica
Adansonia digitata
Bombax costatum
Acacia seyal
Acacia albida
Combretum nigricans
code
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
y
z
Z
Woody plants
Azadirachta indica
Borassus aethiopum
Cordia rothii
Diospyros mespiliformis
Eucaliptus ssp
Burkea africana
Guiera senegalensis
Khaya senegalensis
Manguifera indica
Guibourtia copallifera
Parkia biglobosa
Leptadenia pyrotechnia
Maerua crassifolia
Tamarindus indica
Stereospermum indicum
Piliostigma reticulata
Lannea microcarpa
Ziziphus mauritiana
Acacia senegal
Terminalia avicennoides
Cadaba glandulosa
Grewia villosa
Calotropis procera
Phaenix dactylifera
Cordyla pinnata
Combretum ghazalense
Ziziphus lotus
code
Ac
Ag
Aj
As
Bm
Bs
Cj
Cm
Co
Cs
Cy
Cy
Da
Ea
Eb
Ed
Ep
Es
Ia
Ls
Ny
Ol
Pa
Pd
Pt
Sh
Sp
Ss
Sv
Ty
Vn
Perennial herbaceous
Andropogon canaliculatus
Andropogon gayanus
Aerva javanica
Aristida sieberiana
Brachiaria mutica
Bergia suffruticosa
C. jeminicus/conglomeratus
Cyperus maritimus
Cornulaca monacantha
Chrozophora senegalensis
Cymbopogon schoenanthus
Cynodon dactylon
Dichantium annulatum
Eragrostis atrovirens
Eragrostis barteri
Eleocharis duclis & ssp
Echinochloa pyramidalis
Echinochloa stagina
Ipomoea asarifolia
Lasiurus scindicus
Nymphea ssp
Oriza longistaminata
Panicum anabaptistum
Pergularia tomentosa
Panicum turgidum
Sporobolus helvolus
Sporobolus pyramidalis
Sporobolus spicatus
Stipagrostis ssp
Typha australis
Vetiveria nigritiana
2
Table 3. Canopy cover classes used to estimate trees, shrubs and bushes canopy cover. For each
category of woody plant the correspondence between crown cover and density (plants per hectare)
is established for average crown mean size of 36, 9 and 2,25 m² for trees, shrubs and bushes
respectively.
Code
0
1
2
3
4
5
6
7
8
9
Crown
cover
%
0
0–1
1–3
3–6
6 – 12
12 – 24
24 – 48
48 – 72
72 – 84
84 – 100
Trees
(> 4m, Sm = 36m²)
0
3
9
18
36
72
124
196
233
278
Plant density (plant/ha)
Shrubs
(4-2m,Sm = 9 m²)
0
11
33
67
133
267
533
800
1000
1111
Bushes
(<2m, Sm = 2.25)
0
44
133
267
533
1037
2133
3200
4000
4444
Table 4. Cover classes used to estimate the cover of perennial herbaceous. Depending on the size of
individual plants characterised by the diameter (D) or the surface area of the crown (S) plant cover
correspond to a range of plant densities calculated for three typical sizes of large, medium and small
perennial herbaceous.
Code
0
1
2
3
4
5
6
7
8
9
Crown
cover
%
0
0–1
1–3
3–6
6 – 12
12 – 24
24 – 48
48 – 72
72 – 84
84 – 100
Large size plants
(D=80 cm, S = 0.5m²)
0
0,02
0,06
0,12
0,24
0,48
0,96
1,44
1,68
2,00
Plant density (plant/m²)
Medium size plants
(D=30cm, S = 0.07m²)
0
0,14
0,43
0,86
1,71
3,43
6 ,86
10,29
12,00
14,29
Small size plants
(D=10 cm, S = 0.01m²)
0
1
3
6
12
24
48
72
84
100
Up to three types of soil are described in each facies. When there are three, the first
applies to 50% of the facies area, 30 % and 20 % the second and third respectively.
When only two soil types are listed, they apply to 67 % and 33 % of the facies area
respectively. These soil types are defined by the texture of the topsoil and by a few
topographic and geomorphology traits and coded as in the map of the ecological zones
of the Gourma and surroundings (table 5).
Table 5. The definition of 9 generic soil types
Code
R
G
E
S
D
T
L
A
H
Texture
Rock outcrops
Gravely soils and hard pans
Shallow sands (< 2m) (Sand>70%)
Thick and flat sands (> 2m) (Sand>70%)
Sand dune (Sand>70%)
Sandy-loam (50%<Sand<70%, Clay<30%)
Loamy (Clay<30% ; Sand<50%)
Clayed (Clay>30%)
Permanently or extensively flooded soils either
clayed, loamy-clayed or sandy-clayed soils
Topo-geomorphology
Eroded rock outcrop (sandstone shists)
Hard pan outcrops and gravel deposits
Shallow sand deposits (wind/run-on)
Alluvial plains, sandy bottom slopes
Dunes and thick wind deposits
Colluvium, interdunes
Alluvial plain, colluvium and rivers banks
River bed and alluvial plain, depression
River bed and alluvial plain of Niger river, chanels, lakes,
and large ponds
3
A type of hydrologic behaviour is systematically associated to each soil type within
facies based on the expected run-off/run-on balance of this soils type. This balance is
coded by the value of the coefficient (α) of the empirical relationship between total
infiltration (I) resulting from a precipitation (P) and a standard precipitation of 10 mm:
I = P + α (2*P – 10)/10
Among the values taken by this coefficient, nine typical values have been retained to
characterise the hydrologic behaviour each soil type within facies, but also of each
facies and of the ecological unit all together (table 6). In addition, the hydrologic
behaviour of each facies is calculated by the average of the component soil type
hydrologic behaviour weighed by the relative area covered by each soil type.
Table 6: Codes of typical run-off/run-on balances used to characterise soil, facies and ecological
units hydrologic behaviour. The expected water infiltration in the soil following rainfall events of
10, 20 and 30 mm are calculated to help visualisation of the behaviours.
Code Qualification of the run-off/run-on balance
-4
-3
-1
0
1
3
5
8
15
High losses by run-off
Substantial losses by run-off
Light losses by run-off
Balanced run-off and run-on
Light gains by run-on
Substantial gains by run-on
High grains by run-on
Very high gains by run-on
Extremely high gains due to large external inputs
Water infiltration estimated after a rainfall event of:
P= 10mm
P=20mm
P=30mm
6
8
10
7
11
15
9
17
25
10
20
30
11
23
35
13
29
45
15
35
55
18
44
70
25
65
105
The proportion of the soil remaining bare of herbaceous vegetation throughout the
year, ‘bear soil patches’, is estimated for each facies, as well as the proportion of land
under cultivation, and that under heavy grazing pressure. The last three variables are
coded using the same scale as for canopy cover (table 3).
1.1.2. Vegetation, land use, soil and hydrology descriptors defined by facies are then
aggregated per land unit through averages weighed by the relative area of each facies.
Thematic maps are established based on these weighed mean attributes per land unit
(see maps in annex). Soil maps are based on the weighed mean proportions of sandy,
clayed and loamy, and rocky soils (figure A.1.); there are also maps of the relative
proportion of each of the nine soil types, and maps in which each land unit is classified
into one of the 7 types of soil mosaics identified. Land use maps are established with
the mean area of bare soils, cropped field, and land subject to heavy grazing (figure
A.2). From the hydrologic behaviour of each soil type within facies, weighed averages
have been calculated and mapped for each land unit (figure A.3) Thematic maps are
generated of the canopy cover of trees, shrubs, all woody plants, and perennial
herbaceous (figure A.4). There are also a set of maps established with the relative
contribution to woody plant canopy cover of the 32 main woody species (figure A.5 to
A.11).
1.1.3. The share of the main three edaphic environment in the Hombori supersite is close
from their share in the overall Gourma (see chap. 3), with more fine textured soils to
the detriment of sandy soil at Hombori (Figure 1 and 2). The relative contribution of
the 7 types of soils mosaics in term of main edaphic environments and soil types
(figure 3 ) indicate that these mosaics are sufficiently contrasted to serve as edaphic
indicator agains which the other variables can be averaged or plotted. In turn the land
unit attributes are averaged over the AMMA supersite window weighed by the relative
4
area of the land units in order to characterise the average situation per edaphic situation
and for the supersite as a whole (table 7 and 8).
Fig 2 Charts of the area covered by the 3 main edaphic environment, the 9 generic soil types
and the 6 type of soil type mosaics defined for the ecological units mapped over the Hombori
supersite.
Superficie des ensembles édaphiques sur le
supersite (% )
23,8
44,6
sand
clay
rock
31,6
Superficie des 9 types de sol sur le supersite (% )
6,9 2,0
14,2
8,4
R
G
E
15,4
D
S
5,5
T
L
A
H
16,9
15,3
15,4
2%
Classement
edaphique
des unités
cartographiées30%
du supersite
14%
AA
25%
13%
3%
13%
AR
RR
SA
SR
SS
XX
5
Figure 3. Statistics on the contribution of 3 main edaphic environments, or 9 types of soils
(based on top soil texture and geomorphology) to the 6 types of soils mosaics defined for each
land unit mapped over the Hombori AMMA supersite
Contribution of the 3 main edaphic environments to the
6 types of soil mosaics in the land units of the
supersite ecological map
100%
80%
rock
clay
sand
60%
40%
20%
0%
RR
SR
SS
SA
AA
AR
XX
Contribution of 9 soils types to the 6 types of soil
mosaics described in land units of the supersite
ecological map
100%
80%
60%
40%
20%
0%
RR
SR
SS
SA
AA
AR
XX
H
A
L
T
S
D
E
G
R
Table 7. Weighed mean soil and hydrolic attributes of the Hombori super-site land units sorted
by type of soil mosaics and all together.
Soil mosaic type
RR: rocky
SR: sandy/rocky
SS: sandy
SA:sandy/clayed
AA: clayed
AR: clayed/rocky
XX: :all mixed
Sum/mean
Area in Supersite
km²
%
350,8
13,1
343,7
12,8
834,7
31,1
74,9
2,8
368,6
13,7
674,0
25,1
41,1
1,5
2687,8
100,0
Edaphic environment (%)
sandy
Clayed
Rocky
7,4
14,6
77,9
61,2
17,3
21,5
93,9
5,6
0,5
50,9
46,7
2,4
3,1
92,1
4,9
16,9
44,9
38,2
37,7
34,8
27,4
44,6
31,6
23,8
Bare soil %
83,5
30,7
6,0
16,1
36,5
72,3
46,3
41,0
Run-on/off
index
-3,5
0,0
0,0
2,2
4,5
-1,5
-0,2
-0,1
6
Table 8. Weighed mean canopy cover of woody plants and perennial herbaceous, and landuse
attributes of the Hombori super-site land units sorted by type of soil mosaics and all together.
Soil
mosaic
type
RR
SR
SS
SA
AA
AR
XX
Sum/mean
Area in Supersite
km²
350,8
343,7
834,7
74,9
368,6
674,0
41,1
2687,8
%
13,1
12,8
31,1
2,8
13,7
25,1
1,5
100,0
Trees
0,6
1,8
1,6
8,0
19,5
1,7
1,5
4,1
Canopy cover %
All woody
bushes
plants
3,1
3,7
6,4
8,2
5,4
7,0
17,2
25,2
13,6
33,1
4,6
6,3
6,9
8,5
6,5
10,6
Perennial
herbaceous
0,0
0,0
0,0
0,0
0,1
0,0
0,0
0,0
Land
cropped
%
0,2
6,8
3,5
6,0
1,8
0,7
0,8
2,6
Land
heavily
grazed %
2,8
13,0
12,6
24,4
27,2
6,4
17,1
12,2
The canopy cover of perennial herbaceous is extremely low throughout the Hombori
supersite, while that of woody plants riches 10,6% shared between bushes and shrubs
(6,5%) and a few trees (4,1%). Woody plants are inequally distributed with 33,1 and
25,2 % cover over the 13,1 % low land clayed soils and the 2,8 % bottom slope mixes
of sandy and clayed soils respectively. The eroded rocky surfaces are only covered
with 3,7% woody plant canopies, mainly from bushes, while all the other situations
range between 6,3 and 8,5% woody plant canopy.
Only 2,6 % of the lands in the supersite are cropped, mostly on sandy soils, while the
12,2% heavy grazed are more extended over clay and sandy-loam soils, often close to
water point, and spread to all other soil types. Permanently bare soils extend to 41% of
the supersite, with up to 83,5 and 72,3% on rocky soils and rock-clay mosaics, but
only 6% on sandy soils. The run-off run-on balance index almost nul over the whole
supersite landscape (-0,1), is negative, indicating dominant run-off on rocky soils (3,5) and rock-clay mosaics (-1,5), close to nul on sandy soils, and largely positive,
indicating dominant run-on on clay (+ 4,5) and sandy-loam (+ 2,2) soils in low lands.
Species contributions to cover are not only weighed by the area of the land unit but
also by the mean canopy cover within land unit in order to estimate the species
contribution to the canopy cover across the AMMA supersite window (figure 4).
Three dominant species contribute equally for 13% each to the woody plant cover in
the supersite: Acacia raddiana, predominantly but not exclusively on sandy soils, the
ubiquitous Balanites aegyptiaca and Acacia seyal who dominates in the low land forest
on flooded clay soils. Acacia ehrenbergiana and Boscia senegalensis, common on
sallow and sandy-loam soils contribute each to 9%. Another Three species account for
6 or 5% each: Combretum glutinosum on sandy soils, Anogeissus leiocarpus on
flooded loamy soils and Acacia nilotica on fooded clay soils. Together these 8 species
account for 73% of the cover, the 13 other species all account for less than 5% of the
woody cover.
7
Figure 4. Dominant woody species contribution to woody canopy cover over the AMMA Hombori
supersite. The contribution is weighed by the mean cover of woody plants within land units and the
area of these units.
Species contribution to woody cover over the
Hombori supersite
2%
ACACEHRE
1%
6% 1%
2%
9%
3%
4%
5%
13%
13%
5%
1%
3%
3%
2%
1%
9%
1%
13%
2%
ACACLAET
ACACSEYA
ANOGLEIO
CALOPROC
COMBMICR
COMMAFRI
BALAAEGY
MAERCRAS
BOSCSENE
GREWBICO
ZIZIMAUR
ACACSENE
ACACRADD
ACACNILO
PTERLUCE
MITRINER
PILIRETI
LEPTPYRO
COMBGLUT
GUIESENE
The edaphic specificity of each species is highlighted by the distribution of species
(mean species contribution woody plant cover) across the 7 different types of soil
mosaics (figure 5).
8
Figure 5. Distibution of dominant woody species to canopy cover in the 7 classes of soil mosaics: :
RR dominantly shallow soils, SS dominantly sandy soils, AA dominantly fine textured soils, RS
mix of sandy and shallow soils, RA mix of shallow and fine-textured soils, SA mix of sandy and
fine textured soils, XX mix of all three types of substrate.
Relative
contribution
of Acacia
ehrenbergiana
to the canopy
cover of woody
plants in
Relative
contribution
of Anogeissus
leiocarpus
to the canopy
cover of woody
plants in
Weighed
mean over
the supersite:
16,0%
45,0
38,5
40,0
35,0
29,228,0
30,0
25,0
20,0
14,0
15,0
7,9
7,8
10,0
5,0
0,5
0,0
RR SR SS SA AA AR XX
8,0
7,0
SR
SS
SA
5,0
0,3
0,0 0,0 0,1
0,5 0,1 0,0
Relative
contribution
of Balanites
aegyptiaca
to the canopy
cover of woody
plants in
0,0
25,0
20,0
5,0
XX
SR
SS
10,0
SA
3,7
5,0
1,2
4,1
0,0 0,8
0,0
Relative
contribution
of Boscia
senegalensis
to the canopy
cover of woody
plants in
12,0
10,0
7,9
8,0
SS
Weighed
mean over
the supersite:
5,4%
6,0
4,6
1,8
0,0 0,6
0,0
RR SR SS SA AA AR XX
AA
AR
1,9
XX
25,0 22,4
Weighed
mean over
26,6
the supersite:
13,4%
0,0
SR
SS
15,0
XX
RR
18,9
20,0
5,0
10,8
8,6
SA
7,4
AA
AR
2,3
XX
RR SR SS SA AA AR XX
RR
SR
SA
AA
4,0
SA
6,9
30,0
AR
SS
2,0 0,8
SR
8,7
10,0
AA
13,0
14,0
AR
RR
0,0
RR SR SS SA AA AR XX
Relative
contribution
of Acacia
senegal
to the canopy
cover of woody
plants in
AA
RR SR SS SA AA AR XX
RR
13,6
15,0
SA
22,5
15,4
14,2
15,0
RR SR SS SA AA AR XX
Weighed
mean over
the supersite:
6,5%
SS
XX
Weighed
mean over
the supersite:
10,4%
19,5
10,0
AR
0,0
20,0 18,0
2,5
AA
2,0
Relative
contribution
of Maerua
crassifolia
to the canopy
cover of woody
plants in
4,3
SR
RR SR SS SA AA AR XX
SA
2,2
5,2
0,0
SS
3,0
RR
Weighed
mean over
the supersite:
3,9
15,0
XX
SR
4,0
21,3
20,0
10,0
AR
RR
5,0
25,0
AA
Weighed
mean over
the supersite:7,1
1,1%
6,0
1,0
RR
Relative
contribution
of Acacia
seyal
to the canopy
cover of woody
plants in
Relative
contribution
of Acacia
raddiana
to the canopy
cover of woody
plants in
28,4
25,5
Weighed
24,1
30,0
25,0
mean over
the supersite:
13,5%
20,0
SA
7,7
10,0
5,0
XX
0,0
SR
SS
15,0
AR
RR
0,4
1,1 1,9
AA
AR
XX
RR SR SS SA AA AR XX
9
Relative
contribution
of Leptadenia
pyrotechnica
to the canopy
cover of woody
plants in
8,5
9,0
8,0
Weighed
7,0
mean over
6,0
the supersite:
5,0
3,1%
3,4
4,0
3,0
2,0
0,4
1,0 0,0
0,0 0,1 0,0
0,0
RR SR SS SA AA AR XX
RR
SR
SS
SA
AA
Relative
contribution
of Combretum
glutinosum
to the canopy
cover of woody
plants in
30,0
24,9
25,0
20,0
Weighed
mean over
the supersite:
9,9%
SR
SS
12,9
15,0
RR
SA
10,0
AA
AR
5,0 2,1
XX
0,0
0,3 0,0 0,8
2,4
AR
XX
RR SR SS SA AA AR XX
The thematic maps derived from the integrated ecological map and the attached
databases are integrated to the geographic information system that also includes base
maps for geology, topography, hydrology and the position of field sites and deployed
instruments to facilitate up-scaling from field sites to super-site. They provide a
training tool to calibrate the remote sensing approaches under development to expand
the mapping exercise at smaller scale, especially over the meso-scale Gourma
windows of the AMMA project.
1.2. In 2007, an alternative mapping approach has been initiated over the AMMA Hombori
super-site using of a set of 10 georeferenced SPOT images dated from 2005 and 2006. Six
of these images (06/07/2005, 22/07/2005, 8/08/2005, 8/10/2005, 23/10/2005, 13/04/2006)
were corrected of atmospheric interferences and further used for thematic mapping through
supervised classification using the reflectances in four channel bands (Green, G: 0,500,59µm, Red, R: 0,61-0,68 µm, Near Infra-red, NIR: 0,79-0,89 µm and Mid-Infra-red,
MIR: 1,58-1,75µm), as well as the Normalised Difference Vegetation Index calculated
from two of these reflectances: NDVI = (NIR-R)/(NIR+R). The objective was to map some
of the landscape attributes mapped empirically by visual interpretation and field
observations (previous chapter) automatically from satellite data namely: the extent of soil
types, that of land cropped (mostly in millet) and the woody plant cover.
1.2.1. From field knowledge, between 12 and 24 training sites were identified for each soil
type. After many trials, the more efficient mapping was obtained in using the three
bands of the images dated from 23/10/2005 and 13/04/2006 and the ‘optimum
likelihood option’ of the ENVI software. Because the reflectances of some of the
targeted soil types are heterogeneous, theses classes are first split into subcategories
mapped separately and later grouped. That’s for example the case of low-land clayed
soils first subdivided into tree and tree less sites mapped separately and further grouped
(table 9).This also applies to dune sandy soils depending on burning history: burnt
sandy soils and un burnt sandy soils are classified separately and later merged. On the
other way round, one type of rock surface, the schist outcrops, was not separable from
the gravels and ferrugineous hard pans and thus separated from the sandstone outcrops
and mapped together with the gravels and hard pans (Figure 6). The performance of
the classification is assessed in analysing the classification of the pixels within training
units reached an overall accuracy of 91,2 % (10660/11686 pixels ) with a Kappa
coefficient of 0.90 (table 10).
10
Table 9. Comparison of soils types mapped by empirical interpretation of Landsat images and
the soil types mapped by supervised classification a set of Spot images.
Empirical interpretation (Landsat)
codes Definition
R
Rock outcrops
G
E
S
Gravely soils and hard pans
Shallow sands (< 2m)
Thick and flat sands (> 2m)
D
Sand dune
T
L
A
Sandy-loam
Loamy
Clayed
H
Extensively flooded
Supervised classification (Spot)
Working classes
Sand-stone outcrop
Sand-stone outcrop + shadow
Schist outcrop
Ferruginous hardpans
Shallow sands
Thick sands with no recent fire
Thick sands with recent fire
Sand dune with no recent fire
Sand dune with recent fire
Mobile sand dunes
Sandy-loam plains
Loamy flats
Clayed low lands with no trees
Clayed low lands with trees
Flooded
Final soil classes
Sand-stone outcrop
Schists and ferruginous hard
pans
Shallow sands
Thick sands and sand dunes
Mobile sand dues
Sandy-loam plains
Loamy flats
Clayed low lands
Flooded
11
Figure 6. Map of the main soil types over the Hombori AMMA super-site (Gourma, Mali).
Sandstone outcrops= red; hard pan outcrop= pink; mobile sand dunes (bright yellow), fixed sand
dunes (orange-yellow), superficial sand sheat (light brown), sandy loam (light green); loamy soils
(light blue); loamy-clay soils (purple); open water (white); unclassified (black).
Table 10. Confusion matrix of the soil type classification and 20 training units selected for each
soil type (total 11686 pixels)
12
Soil classes
Sand-stone outcrop
Schists and ferruginous
hard pans
Shallow sands
Thick sands and sand dunes
Moving sand dunes
Sandy-loam plains
Loamy flats
Clayed low lands
Flooded
Commission %
Omission %
Prod. Acc.
%
User acc.
%
4.39
0.30
1.07
1.69
98.93
98.31
95.53
99.7
19.81
39.57
4.97
4.83
19.92
7.95
0.00
19.15
10.55
19.90
31.88
0.39
7.34
0.27
80.85
89.45
80.10
68.12
93.22
87.99
99.31
80.19
60.43
90.04
89.73
82.23
83.76
100.00
13
1.2.2. Attempts were made to map cropped fields based on the observed gap between the
growth curves of natural rangeland and millet fields. Indeed, because millet is sown
only after the first rains, and because millet fields are weeded one or twice at the early
stages, the growth curve of millet raises later and at a slower pace than that of
neighbouring rangelands. At the end of the season, on the opposite, millet generally
remains green later than wild annual herbaceous (except in wetlands) and reaches
higher standing mass than rangeland in comparable soils. Thus the ratio of NDVI
between late wet and early wet season should be higher on crop fields than on
rangelands. Unfortunately, soil background on one hand, and the spatial differences in
crop growth due to the pattern of water redistribution at the soil surface or differences
in soil texture or fertility on the other hand, are interfering on the ratio as well. Better
results were obtained in separating cropped lands within each soil types that harbour
most of the fields i.e. sandy-loam soils, thick sandy soils, shallow sandy soils. NDVI
values in late wet and early wet seasons were extracted over known training fields
located in each of these three soil types and compared to that of neighbouring
rangelands in order to identify specific threshold in NDVI ratios that could separated
the cropped fields from un-cropped areas (table 11). The area mapped as cropped
(Figure 7) cover %, % and % of the three soil types and thus % of the whole supersite. The classification accuracies calculated on the confusion matrices established on
the training areas for each of the three soil type are poor (table 12).
14
Figure 7. Map of the area cropped on shallow and thick sandy soils, and loamy sand soils the
Hombori AMMA super-site (Gourma, Mali).
Supervised classification of millet crop fields by soils
types for the three sandy soil types : dunes (top left),
sandy loam plains (top right), and thin sand sheets
(bottom left).
1.2.3. Mapping woody plant cover is also based on gaps in plant phenology: indeed, woody
plant are keeping leaves (evergreens) or renewing then during the late dry season prior
to the germination of annual herbaceous. Most woody plants (except short cycle
deciduous species) also remain on leave or renew their foliage (Acacia albida, Boscia
ssp, Maerua crassifolia…) early in the dry season while annuals have wilted. However,
absolute values of NDVI are largely affected by soil type, thus the separation of the
woody plant canopies is conducted by soil type. NDVI values of identified trees or
woody plant concentration generally over one or a few pixels, are compared to NDVI
of area with same soil type but with no or a few scattered trees to identify thresholds
(table 14). The extent of woody cover is then mapped by using these thresholds in a
decision tree in which the first threshold is the NDVI at the end of the dry season, and
the second the NDVI of early dry season. These thresholds very slightly between soil
15
types (table 14). The classification accuracies based on confusion matrices constructed
with the resulting classification on the training pixels over/outside woody plant
canopies are acceptable although they vary with the nature of soil background (table
15). Ounce identified and mapped for each soil type separately the areas markedly
covered by woody plants are grouped altogether (figure 8).
Figure 8. Map of the area covered by woody plant canopies in the different soil types over the
Hombori AMMA super-site (Gourma, Mali).
Pixels with large woody plant cover (green) in all sandy soils (top left), clay soils (top,right), rocky soils
(bottom, left), thin sandy soils (bottom, right).
16