The Extent of Desert Dunes in Northern Nigeria as Shown by Image Enhancement
Author(s): Janet E. Nichol
Reviewed work(s):
Source: The Geographical Journal, Vol. 157, No. 1 (Mar., 1991), pp. 13-24
Published by: Blackwell Publishing on behalf of The Royal Geographical Society (with the Institute of
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The GeographicalJournal,Vol. 157, No. 1, March1991, pp. 13-24
Theextentof desertdunesin northern
Nigeriaas shownby
imageenhancement
JANET E. NICHOL
Department of Geography, National University of Singapore
This paper was accepted for publication in October 1990
On satellite imagery dune lineations can be identified in areas where surface relief and air
photos give little or no indication of the former existence of dunes. Enhancement
techniques applied to LANDSAT imagery of part of the Sudan zone of northern Nigeria
were able to extend the known area of fixed linear dunes as far south as the northern
Guinea zone, and the 1000-millimetre isohyet. Existing and new soil and land use data
were interpreted in the context of observed dune lineations on the imagery. Findings
provide a different perspective on the results of earlier soil surveys in these areas (which
were structured upon the assumption of soil catenary relationships between sampling
sites, as in traditional land systems survey), since fundamental differences in soil
properties between dune and interdune bear little relationship to topographic position
within a land system.
KEYWORDS:
Nigeria,desertdunes,imageenhancement,lineations,vegetationindex,colourspace
transform.
THE ABSOLUTE previous
ESTABLISHING
extent of desert conditions has direct relevance
to assessments of the magnitude of climatic
change in desert marginal areas, as well as for soil
surveys for agricultural and land use planning.
Numerous workers have observed on air photos the
presence of systems of fixed longitudinal sand dunes
bordering the world's deserts interpreting them as
indicators of past more extensive desert conditions,
when rainfall totals were much lower than at present
(Grove, 1958; Grove and Warren, 1968; Lancaster,
1987; Maiguet, 1983). There have, however, been
few attempts to utilize satellite imaging technology to
confirm and supplement previous observations.
After Grove, subsequent investigations of the
ancient erg of Hausaland (Falconer, 1911) have been
mainly of two types:
(a) a study of the spatial aspects of dune morphology
used as an indicator of Quaternary environmental
change (Grove and Warren, 1968; White, 1971;
McTainsh, 1984);
(b) soil surveys of the dunefields for agricultural
purposes (Sombroek and Zonneveld, 1971;
NEDECO, 1974; Bennett et al., 1981).
Areas with persisting dune forms in terms of relief
and/or land use differences between dune and interdune have been recognized by both groups. In
addition to the 'ancient erg' whose south-western
limit is shown on Figure 1, these areas include a zone
of bare, uncultivated lateritic ironstone strips farther
north, extending approximately 100 kilometres westwards from a north-south line of quartzite hills near
Kazaure (Grove and Pullan, 1963). These strips are
0016-7398/91/0001--0013/$00.20/0
oriented in the present wind direction, ENE-WSW,
similar to the ancient erg, and are believed to owe
their formation to wind funnelling between the gaps
in the hills, depositing sand in the lee of the gaps and
leaving bare those areas in the lee of the hills. The
origin of these bare strips and their relation to the
'ancient erg' remains to be fully explained. Local
relief is approximately 30 metres and the strips postdate the existing drainage system (ibid.).
Soils to the south and west of Falconer's ancient
erg, south of a line running roughly through Kano
have been regarded as largely the product of harmattan dust deposition (Bennett, 1980; McTainsh,
op.cit.) This is based on the soil textural bands
identified by the Land Resources Development Centre
from extensive soil surveys in the Kano Plains
(Bennett et al. 1981: 29). The bands, based on particle
size analysis, divide the Kano Plains into 13 regions
whose boundaries are aligned east-west and in which
particle sizes decrease from north to south. McTainsh
has confirmed these observations on the probable
aeolian origin of the soils to the south-west of Kano
with further studies of present-day dust deposition,
and recognizes a coarser size fraction in the soils than
in the present day dust, which he attributes to higher
wind speeds during Quarternary arid phases
(McTainsh, 1984).
However, some uncertainty regarding the origin of
the soils to the south-west of Kano is illustrated by the
following observation by Bennett et al (ibid):
Therehas neverbeen any doubt as to the aeolianoriginof
the mainlysandy materialto the north of Kano,or of the
BirninKuduarea in the east. Indeedtheir persistingdune
?
1991 The Royal Geographical Society
14
DESERTDUNESIN NORTHERNNIGERIA
Fig. 1. The mappedextentof linearfeatures
formsaresufficientevidenceof wind action.Whatwas less
certainwas the aeoliannatureof the siltyor veryfinesandy
componentof themorepedologicallydevelopedsoilsfarther
south. Rathermoreevidencethan an increasedsilt or very
finesandcontentwas neededto convincethe scepticalwhen
no otherchangein profileformwas evident.
Satellite image interpretation
The lineations comprising the 'ancient erg' were first
noticed upon visual inspection of a LANDSAT MSS
false colour print, as alternating light and dark toned
strips aligned in the prevailing wind direction. In
areas to the east of Kano where the sand dune features
exhibit considerable topographic expression, their
image appearance is pronounced, both on black and
white air photos and on satellite imagery. However,
around the edges of the dunefield where the features
are less evident, they are more easily seen on satellite
imagery due to its superior spectral characteristics.
Moreover, the assessment of spatial characteristics
such as continuity and precise orientation of individual dunes, as well as the ultimate extent of the
dunefield require the synoptic qualities of such
medium-scale satellite imagery.
Landsat MSS imagery of November 1978 was
available in digital format for the area shown on
Figure 1 and image processing was carried out of a
512 x 512 pixel extract (Fig. 1), in an attempt to
enhance the contrast between the alternating light
and dark-toned lineations representing the dunefield.
(These will subsequently be referred to as Type A, for
the light toned areas occupying the highest topographic position, and Type B for darker toned areas at
a slightly lower elevation).
Image enhancement comprised the monitor display
of individual MSS wavebands as well as image
transforms commonly used for differentiating vegetation from its background and between various types
of parent material.
Within the area of the image extract, the dune
lineations are not visible on the ground, on air photos,
(Plate I) or on individual wavebands of MSS imagery.
Also, a standard false colour composite, even when
contrast stretched, is unable to provide a clear
differentiation of dune and interdune areas (Plate II).
The coincident spectral plots (Fig. 2a) show a
considerable overlap in pixel brightness values,
although mean values for Type A are consistently
higher than those for Type B. Interestingly, Type B
areas possess brightness values whose means and
DESERTDUNESIN NORTHERNNIGERIA
.15
would in most cases have a high ratio value.
Exceptionsto this generalrule might occur in arid
regions where dry soils exhibit high reflectancein
both visible and near infra red wavelengthregions,
andvegetationmightbe expectedto havea higherred
(visible)reflectance(Tucker,1979) owingto moisture
stressat mosttimesof year.Thusvaluesfor the above
ratiowould be lower.
The large contrast observed on the vegetation
index imagebetweenType A areas(low ratio value)
and TypeB areas,(Fig.2b) is thus surprisingin view
of both the time of year (lateNovemberwhen Sudan
zone vegetation would be suffering considerable
moisturestress)and the low vegetationcanopycover
(0-20 per cent,mainlyof farmlandtrees).Townsend
and Justice (1986) found vegetationindices in the
Sudan zone of West Africa to have substantially
decreasedby mid-November.Thus significantimage
Significanceof imagetransforms
differencesowing to vegetationwould be unlikely,
Vegetationindex The most commonly used veg- anddifferencesin soil backgroundreflectancemaybe
etationindex, the NDVI, is essentiallya ratio of the assumedto be
affectingthe ratio (Elvidgeand Lyon,
nearinfra-redand visiblepartsof the spectrumsuch
1985).
that,
standarddeviationsdo not differsubstantiallyfrom
those of the whole scene, the smallerrange being
merely the result of the characteristicsof sample
populations. (Approximately 200 pixels were
sampledfor each of types A and B from a scene of
262 000 pixels).
Severaltransformedimages gave better contrast
betweendune and interdune;notablythe Saturation
componentof the Colour Space Transform,and a
densitysliceof the NormalizedDifferenceVegetation
Index (NDVI) (Plates III and IV). The coincident
spectralplots of these two images(Fig.2b) show an
almostcompleteseparationbetweenbrightnesslevels
in the Saturationcomponentand a clear separation
betweenstandarddeviationsfor the Vegetationindex
(NDVI).
NDVI = NIR-VIS/NIR+ VIS
Colour space transform This transform is recogandexploitsthe generallyhigherinfraredreflectance nized as a means of enhancingsubtle differencesin
of healthy vegetationrelative to soils with higher soil and rock coloursby quantifyingand transformvisible reflectance.Thus, highly vegetated areas ing colour into its componentsHue, Saturationand
PLATEI An air photo of part of the studyarea
DESERTDUNESIN NORTHERNNIGERIA
16
PLATEII Falsecolourcomposite
(b) 250
200
a)
n
E
(a)
80
<G
^-
,
70D
Max
-
' 150
MSS7
MSS6
MSS5
MSS4
-_
601
'0
.Q 40Q,
tI
-SD
+
-
C
.O
=r
Max
-
~~
-_,
.
-i-
-Mean
100
-SD
-Mean
-SD
I
-SD
c)
50
Min
Min
3020
I'
--- Whole Scene (By subsampling)
Type A
Type B
HUE SAT INT NDVI
(Tlraining area data)
J
Fig.'2. Coincidentspectralplots
Type A
-----
Type B
DESERTDUNESIN NORTHERNNIGERIA
Intensity (Drury, 1987). The magnitude of each of
these components can then be adjusted within the
whole of the digital range before reconverting to RGB
coordinates. The effect is one of decorrelating the
wavebands and improving deficiencies in colour
contrast as a result of any of the components, Hue
(the dominant wavelength), Saturation (purity of
colour) or Intensity (brightness) (Gillespie et al.,
1986). Low Saturation values, typical of arid environments (Terhalle and Bodechtel, 1986), may inhibit
interpretability, even if Intensity and Hue values are
distinct, since the colours may not be pure. This
situation could be caused by relatively broad wavebands of imagery, such as those of LANDSAT MSS,
or by the broad spectrum reflectance characteristics
of most rocks and soils (Kaufmann, 1988). To
illustrate, in terms of the study, the differences in
colour saturation between Type A and B areas would
not be adequately enhanced by a contrast stretch in
RGB space. This was therefore achieved by first
converting to HSI coordinates, and stretching along
the Saturation axis. Reconversion to RGB coordinates would not have resulted in further contrast
improvements and therefore was not carried out.
The colour space transform gave the best separation between Type A and Type B areas (Plate III). In
particular the Saturation component exhibited little
17
overlap in digital values, (Fig. 2b) and Intensity also
gave a good contrast. Although the colour space
transform is scene specific, its application to other
image extracts provided a consistently good separation between Type A and B areas.
This transformation was applied to the whole scene
and the extent of linear features was mapped (Fig. 1).
They reveal what appears to be a relict dunefield
extending approximately 150 kilometres downwind
of the ancient erg (as previously delineated by Grove),
continuing across the Basement Complex plains
around Kano. The results of both vegetation index
and colour space transforms also suggested spectral
differences in soil colour between Type A and Type B
areas. Field investigations were subsequently carried
out to establish the nature of the contrast in terms of
soil and land use characteristics.
Spatial filters Brightness values for a 7.8-kilometre
transect across four dunes on the Saturation component (Fig. 3) also show a clear difference in mean
values between Type A and Type B, but with high
within-class variation especially in Type B areas. For
this reason, the application of spatial filters to a
density sliced image in an attempt to reduce the
within-class variation was unable to enhance the
separation since the method depends on a greater
PLATEIII Colourspacetransformedimage
18
DESERTDUNESIN NORTHERNNIGERIA
spectralhomogeneityof at least one class, than is
foundhere.
Fielddata
Collectionof new, and the use of existing,fielddata
dependedon the identificationandmanualtransferof
the dune lineationsonto 1/25 000 scale air photos.
Previouslyunidentifieddifferencesin toneandtexture
on the air photos became evident once the major
imagery,and precise
trendswere known from the
fieldlocationscould then be found.
Fielddata comprisedinformationon soil, vegetation and land use fromthreesources:
a) A studyof land use and woody vegetationto the
westof Kanowas previouslycarriedout by theauthor
as partof the KanoRuralEnergyProject(Cline-Cole
et al., 1989). Projectsitesmarkedon airphotoscould
be accuratelylocated on either dune or interdune.
Figure4 shows the dunelineationssuperimposedon
x metrevegetation
landusemap.Twelve250 250
the
which fell exactly on either a dune or
quadrats
location were selectedfrom this previous
interdune
for which informationon woody vegetation
study,
a
available (Table II).The data comprised
was
each
within
shrubs
and
trees
of
list
comprehensive
includingspecies,basalareaand height.At
quadrat,
timeof thispreviousstudylargespatialvariations
the
were observed,however, no attemptwas made to
explain these becausethe presentlydescribedlineations were not then apparent.
b) New soil data were collectedfrom 15 sites to the
west of Kanowithin the area coveredby the 512 x
512 pixel extract. Transectsperpendicularto the
dune lineationswere markedon the air photos and
soil samples were obtained from locations correspondingdistinctlyto eitherTypeA or TypeB areas.
Soils were sampled at a consistent depth of
in all cases
40 to 50 cm, corresponding
approximately
the limit
approximately
is
This
to the mid-Bhorizon.
to which soils can be sampled in the dry season
without mechanicalmeans, and also corresponds
approximatelyto rootingdepth.
c) Existingsoil data from the EnvironmentalData
Bankof NorthernNigeria(locatedin the Department
by
of Geography,BayeroUniversity,Kano),collected
the
Land Resources Development Centre of used
the
of Overseas Surveys were also
Directorate
land
et al., op cit). This survey adopted a
(Bennett
corresites
sampling
thus
and
approach
systems
to toposequences.Data from 5 toposesponded
27 siteswereused(e.g.E.D.B.on
comprising
quences
west of
4). Data were used only for areasto the on the
Fig.
evident
were
lineations
distinct
where
Kano
(areashadedon Figure1).
imagery
indeximage
IV Vegetation
PLATE
DESERTDUNESIN NORTHERNNIGERIA
19
Fig.3. Brightnessvaluesfor a transectacrossfour dunes
Results
Soils Soil differences were immediately evident in
the field. Type A soils are dark grey-brown, (2.5Y 4/4
and 10YR 4/3 to 5/4), Fine Sandy Loam and Fine
Sandy Clay Loam, gleyed with mottles. Type B soils
are yellowish red (7.5YR 4/4 and 5YR 5/6), Sands,
Loamy Sands and Sandy Loams. Of the soil samples
from 42 different sites, only 3 do not fall within these
colour class limits for Type A and Type B areas
respectively. Furthermore, changes from one type to
the other occurred rather suddenly across the dune
lineations although relief varied only slightly, in the
order of 10 metres over distances of approximately
one to two kilometres. Type A occupied the highest
topographic position where any relief was evident. In
the highest areas Type A soils were blocky, dark grey
and hard.
Farmers interviewed were aware of differences in
both soil colour and agricultural properties, though
preferences varied between farmers. Those working
the grey soils (locally termed 'laka') recognized them
for their water and nutrient retention properties and
their ability to grow rice, though the soils were
heavier to work. The lighter, reddish soils ('jigawa')
were preferred for being easier to work, though they
did not retain organic fertilizer and therefore farmers
tended not to apply it. Apart from rice, crops grown
on each soil type were similar.
Particle size analysis using the Bouyoucos hydrometer method was carried out for the 15 newly
collected soil samples (Table I), but only broad
textural classes were available for the E.D.B. data.
FABLE
I
Soil texturalclasses(%)
Mean
Coarse Sand
Fine Sand
Very Fine Sand
Silt
Clay
Type A
9.7
47.7
14.3
21.8
6.8
Type B
57.3
24.0
5.3
4.3
9.2
Figure 5 indicates distinct textural distributions for A
and B type soils. While none of the samples contained
more than 40 per cent silt and clay combined, (the
generally agreed size limit for aeolian dust suspension
over long distances, [Bennett et al., 1981: 31; Pye,
1987: 125]), all of the Type B samples contained less
than 20 per cent. The high sand content of the Type B
samples compares with sandy soils of the dune crests
in the dunefields farther east at 12?12'N, 10?6'E
(Hudu, 1986), and with the sand sheet (Couverture
Sableuse) of the ancient erg of southern Niger at
approximately 13?N (White, 1971). Particle sizes for
Type A soils also appear to be similar in their high
20
DESERTDUNESIN NORTHERNNIGERIA
'
A
B
~:
A
"B
,t
^
'- ---"'"'^
j
x5
vBr ;
^-
_W<
n
'2
^a~...
B
_ ..
_
_
B
---
-
^
To\
o
B i.' i..--^,<
B
/'^
~
JitO..t,..,.,.
<~
|
I::.'-' Uncultivoted
Cultivoted
Settlement
Fig. 4. Landuse in part of the studyarea,sixkilometresw~estof Kano
- - -CLAY--~
100
j
I
- VF SANO--F. SANO
- '
'
'
',
-
SILT
i
80-
-.
-
^
COARSE
SAND
I TYPEA/
yt-
t
-(r-
C3 60s0fl
.
-
-
!
---^^
^^ .
,
'
X
PARTICLESIZE ( Microns)
Fig. 5. Meanparticlesize distributions
v
DESERTDUNESIN NORTHERNNIGERIA
Fine Sand and Very Fine Sand contents, to those
described by McTainsh as 'Zaria aeolian mantles' for
this same area to the south-west of Kano. The high
Coarse Sand content of Type B soils, however,
renders them coarser than those of McTainsh but
similar to McTainsh's 'Dunesand' samples for areas
to the north-east of Kano e.g. at Wurma. It is
maintained, however, that results are not directly
comparable since McTainsh did not distinguish
between dune and interdune sites, and specific
sampling locations are not given. In the case of the
soil surveys by Bennett et al., (1981) sampling
locations were mapped and the data could thus be
related to either Type A or B. Specific particle size
data were not available but the broad textural
groupings for Type A and B sites were found to be
comparable to those in the present study. Twelve out
of 13 sites on Type B areas contained at least 50 per
cent of particle sizes over 100rm (Fine Sand and
coarser), the modal group being Fine Sandy Loam,
while in Type A the modal group was Fine Sandy Clay
Loam.
Type A soils were found to be more acidic than
Type B soils, with pH values of 5.6 and 6.7
respectively.
Land use Around Kano, differences in soil suitability for agriculture do not greatly affect land use,
owing to land pressure. Nevertheless, field observations and land-use mapping from air photos
indicated a higher proportion of uncultivated shrubland on the Type A areas (Table II and Fig. 4).
Villages appear to cluster on the lighter soils of Type
B although larger settlements such as a university
campus and an army barracks are sited on Type A.
TABILE
11
Land use and woody vegetation
% cultivated
%?settled
No. of villages
No. of trees/ha.
Basalarea(sq.m/ha.)
Type A
Type B
76.0
2.0
3.0
26.2
0.5
92.0
1.0
19.0
23.5
2.0
Woody vegetation consists of stunted thorny shrubs
in uncultivated areas and cultivated trees on farmland. Wood production is four times higher in Type B
areas, owing to larger tree sizes (Table II).
The secondary drainage network appears to be
controlled by the dune lineations, in that tributary
streams are aligned in the same ENE-WSW direction
and frequently follow the edge of Type A areas or the
boundary between Types A and B (Fig. 4).
Discussion
The recognition of linear dunes on multispectral
satellite imagery in areas where they are not visible
21
either on the ground or on black and white air photos
is thought to be the result largely of differences in
reflectance properties between the grey soils of Type
A and the red soils of Type B, and to a lesser extent to
differences in land use and vegetation. These produce
high and low reflectance values for Types A and B
respectively in all wavebands and waveband combinations, except for the vegetation index image in
which Type B has higher values. The very high visible
reflectance of Type A areas must partially account for
their lower vegetation index (NIR/VISIBLE)values.
Another contributory factor is the higher woody
biomass in Type B areas thus increasing the near
infra-red reflectance (see Basal Area in Table II),
although this factor is probably less significant since
at the image date, over 80 per cent of the scene
comprised bare soil.
The colour space transform gave the best separation between Type A and B lineations by enhancing
the inherent colour differences, and particularly the
variation in Saturation across the image (Fig. 2b). The
transform is likely to have increased the differences
observed in the field between the grey soils of Type A
and the red soils of Type B by increasing the purity
(Saturation) of the colours.
The dune features are, in places, of uneven width
and spacing. When the light toned Type A lineations
are extended upwind from the study area they
frequently meet high points on maps representing
large rocks or inselbergs (Fig. 6). This is not always
the case, however, and in some instances the lineations appear to traverse high ground. It seems
possible that, like the Kazaure laterite ridges, the
features are controlled topographically, owing their
formation or their preservation, or both, to elevated
terrrain. White observes that the erg landscape in
southern Niger is best preserved on the higher land
surfaces, being part of a once continuous sand sheet,
now degraded and unrecognizable in the lower sites
(White, op. cit.).
Grove and Pullan surmise that in the Kazaure
region, sand moved westwards through gaps in the
hill masses, while strips in the lee of the hills remained
bare for long distances downwind. Thus areas to the
lee of the gaps became covered with sand and remain
today as cultivated areas with sandy soils, though the
majority of the dune sand has since been removed.
(Grove and Pullan, op. cit.).
The grey soils of Type A are similar in colour,
consistency, their high degree of mottling and their
acid reaction, to hydromorphic soils in the same
region (Klinkenberg and Higgins, 1968), and probably correspond to former interdune hollows, the
relief having been inverted. Smaller seasonal streams
which once followed the interdune depressions (as in
the ancient erg farther east) still do follow the former
interdunes, whereas the main watercourses have
eroded laterally across the dune landscape. This
22
DESERTDUNESIN NORTHERNNIGERIA
drainagepattern can be seen on Figure 4, and is
readilyapparentby close examinationof air photos
throughoutthe additionalareas of relict dunefields
delineatedon Figure1.
These observations accommodate Grove and
Pullans'theory for the uncultivatedironstoneridges
west of Kazaure,that the sanddepositedin the sandy
corridorshas beenremovedduringa subsequentarid
phase (Grove and Pullan, op. cit.). However, their
theorythat the Kazauredunespost-datethe drainage
system(ibid.) does not appearto applyto the Kano
dunes.In this respect,as in the fact that the Type A
(formerinterdune)soils are mainly cultivated,the
Kanoarearesemblesthe dunefieldsfarthereastwhere
thedarkgreysoils of the interdunesarevaluedforrice
cultivation,and settlementsare located on the red,
better-drainedsoils.
A sequenceof eventsrelatingthe Kanodunesto the
farthereast and to the Kazaureironstone
dunefields
can be hypothesized,takinginto consideration
ridges
atleastfourlateQuaternaryaridphases,(Servantand
1980). During one of these arid
Servant-Vildary,
extendedfromthe SaharaDesert
dunes
linear
phases,
atleastas far south-westas the areamapped(Fig.1),
althoughthe dune-forming processes near the
marginsof the dunefieldwere more influencedby
local relief than in other areas. This was the result
both of the derivationof dune sand from the areas
upwind,thus setting up a funnellingeffect between
highpoints, and also the fact that rockyoutcropsare
moretypicalof BasementComplexgeology (Fig. 1).
This would explainthe unevenwidth and spacingof
the Kazaurelineationsin particular.Those observed
aroundKanoare moreregularlyspaced,for example
than to the south-eastof Kano city where they are
approximately2-3 kilometresapart,suggestingless
topographiccontrol and more resemblanceto the
sand sheet of south-westNiger describedby White
(op. cit.). The influenceof topographyin the Kano
region may be as much in preservation as in
formation.
Duringa subsequentwetterperiodthe dune sands
may have been stabilizedfor long enough to have
aquiredtheirredstaining,mainlyas a resultof in situ
of silicatemineralsin a savannaclimate
weathering
and
(Cooke Warren,1973). Iron-richwaterfromthe
would contributeto iron segregationmottling
dunes
inthe interdunes,this giving rise to groundwater
lateritesunder a regime of seasonally fluctuating
watertables.The Kazaurelateriteridgesrepresenta
advancedstagein thetransitionof the indurated
more
layerto ironpanby surfaceexposureand a
plinthite
Fig. 6. The relationshipbetweentopographyand linearfeatures
DESERTDUNESIN NORTHERNNIGERIA
declining water table, especially in this area with
higher relief and more arid climatic conditions than in
Kano. The process of relief inversion is also more
advanced in the Kazaure area with uncultivated
laterite ridges being elevated to a height of 30 metres
above the cultivated sandy strips (Grove and Pullan,
1963). This later stage of transition to ironpan has
only limited distribution in the Kano study area and is
seen typically along stream banks as, for example,
adjacent to the Watari River where there are ironstone outcrops which have very high reflectance and
appear as light toned areas on imagery (uncultivated
areas in Fig. 4). The fixed dunes farther east, which
retain their dune topography, may have been formed
during a subsequent, less extensive arid phase.
Conclusion
Observations using enhanced satellite imagery have
extended the known area of relict dune features in
northern Nigeria as far south and west as the north
Guinea zone, and from the 750-millimetre (Grove,
1958), to the 1000-millimetre isohyet. This finding
challenges statements (based largely on observations
of particle sizes of soil samples) about the predominantly loessic nature of the soils to the south-west of
Kano.
Particle sizes measured for areas to the south-west
of Kano, both in the present study as well as that by
McTainsh, contain a considerable Very Fine Sand
and are coarser than
component (50-100lm)
present-day dust (McTainsh and Walker, 1982). Soils
sampled from Type B areas are much coarser than
present-day dust, containing over 80 per cent of
particles over 100btm. The present modal size of
harmattan dust particles at Kano is 31.2btm (ibid.).
While Very Fine Sand particles may be transported
for short distances in suspension, the same may not be
true of particles larger than 100am, even under high
wind conditions. (In fact, McTainsh and Walker cite
12btm modal size for dust carried as far as 900
kilometres, (McTainsh and Walker, op. cit.).
These observations can only be upheld either by
invoking higher wind speeds or by re-evaluation of
the theory of the loessic origin of the soils. McTainsh
uses the former explanation (McTainsh, 1987). It is
recognized in addition, that mixing may occur on the
23
margins of dunefields between loess and dune sands
and that this could be used to explain the presence of
some coarser textured material among loess deposits.
This would in fact give a bi-modal distribution, but
would be unlikely to account for proportions of over
80 per cent of medium and coarse sand in an allegedly
loess mantle.
The clear differences in soil colour and soil texture
between Type A and B soils observed in the present
study, from field sampling and from the samples
contained in the Environmental Data Bank, suggest
that distinct and individual processes have operated
in adjacent sites corresponding to former dunes and
interdunes.
These findings from soil data and satellite image
interpretation have relevance to estimations of the
magnitude of climatic change in desert marginal
areas. Assuming an upper annual rainfall limit of 150
millimetres for dune building to take place, though
some workers suggest less (Talbot, 1980), an increase
of the magnitude of 850 millimetres annual rainfall is
suggested, as opposed to the previous implication of
600 millimetres (Grove, op. cit.).
The findings may also advocate a more cautious
approach to soil survey in desert marginal areas
known to have been subject to climatic change in the
recent past. However, they are supported by only
limited field data and could usefully be supplemented
by additional information as well as a more extensive
examination of the data contained in the
Environmental Data Bank for northern Nigeria.
Although the 80-metre resolution of LANDSAT MSS
imagery was found to be appropriate for the study,,
the superior spectral sensitivity of LANDSAT TM
imagery could usefully be investigated.
Acknowledgements
The author is grateful to Professor M. J. Mortimore
for his interest in this and previous research projects
in West Africa, and for his comments on the draft of
this paper. A. T. Grove's advice and assistance in
discussion have also been invaluable, as have Grant
McTainsh's comments on the characteristics of
harmattan dust in the Kano region.
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