Land use dynamics from the 1950 s to present in the district of Hombori, Gourma region in North-Eastern Mali.

Transcription

1 Land use dynamics from the 1950 s to present in the district of Hombori, Gourma region in North-Eastern Mali. Pierre Hiernaux, Cam Chi Nguyen, Manuela Grippa & Eric Mougin. Géosciences Environnement Toulouse, 14 avenue Edouard Belin 31400, Toulouse, France Juin 2013 Le terroir de Kelmi vu du sommet du Mont Hombori, Septembre 2008 Livrable D1.3 1

2 Objective This documents aims at gathering the studies and maps on the land use on the district of Hombori in the Gourma region either done for the ESCAPE project or for prior projects. Implications for the dynamics of the ecosystem and the agricultural production systems in relation with demographic and market changes are discussed in conclusion Land cover and land use maps established by photo-interpretation prior to the ESCAPE project. The IGN map (1955) The topographic map established by the French Institut Géographique National from the first aerial photographs series (in 1955) by photo-interpretation associated with field topographic survey, toponyms documentation and thematic validations, provides some indication on crop extension in addition to villages, camps, water points, roads and path. Indeed, following the legend the absence of vegetation cover pattern indicates a cropped zone (there are special patterns for rice crops and orchards). Yet the absence of vegetation cover pattern also matches with bare soils on rocks or loamy flats. Figure 1. Extract from the Hombori sheet of the IGN topographic map. The area initially with no pattern that correspond to the fields of the Sabangou and Bissékoukey camps are drawn in red with a lined pattern, while the area initially with no pattern corresponding to eroded bare soils at Tin Dambayata is mapped and covered with a brown dotted pattern. However in the case of Hombori, the association of patternless areas with villages, camps and roads or paths and the nature of soil allows distinction between cropped areas and bare soil patches (Fig. 1). The cropland areas indicated on the IGN topographic map are however quite coarse including crop fields and fallow. The total area of cropland in the district reaches 84,4 km², thus 3,63% of the 2321km². The district is on the crop front with only 0,76% of the land covered with crops over the km² of the Gourma region (excluding the Niger river valley). 2

3 Mapping rangeland units in Mopti region, including Hombori (Boudet et al. 1971) The rangeland resources have been characterised and mapped by Boudet for IEMVT in 1970 over the whole 5 th administrative region of Mali, of which the Hombori district is at the extreme East. The characterisation is based on field surveys: systematic qualitative observations on species composition sorted by the classical phyto-sociology method establish the 25 vegetation types. 10 types are characterised on sand dunes, 5 on shallow sand deposits over rocky areas or hard pans, 4 on rock or hard pan outcrops, 6 on colluvium or alluvial soils. Mapping croplands in Hombori (Marie J. and Marie J., 1975) A re-interpretation of the same 1955 IGN aerial photographs by Marie J. and J. Marie (1975) maps more precisely the crop fields distinguishing three types of fields by their edaphic location: a large majority of crop fields (63,5km²; 2,74 %; 89,1% of land cropped) are established on sandy soils and devoted to millet crop (Fig. 2a). In addition there are a few crop fields established on the pediments of the Hombori mounts (5,5km²; 0,24 %; 7,7% of land cropped), they are established on loose micro-terraces among the rock boulders (Fig. 2b). Millet is the main crop, associated to some sorghum, cowpea, roselle, voandzou. Then, there are a few fields (2,2 km²; 0,10 %; 3,1% of land cropped) cropped in one of the clay soil depression seasonally flooded located in the plain north of Hombori (Fig 2c). In total the area cropped is 71,3km²; 3,07 % of the land mapped that does not cover entirely the district of Hombori but very largely the area potentially cropped (Fig 3). Figure 3 Map of the areas cropped in the district of Hombori from Marie J. et Marie J.,

4 Figure 2: types of croplands in Hombori: a)_ top left: sandy soils, millet-cowpea (Hombori Hondo), b)_ right: micro-terraces on rocky and slopy pediment: millet-cowpea-sorrel (Kelmi) c)_ bottom: loamy-clay soils in depressions Sorghum-Ochra-Cowpea (Bilantao) Maps established by visual interpretation of satellite images PIRT, PIRL (1975) A consortium of US academics and Malian research and technical services establish a comprehensive agro-ecological zoning by visual interpretation of a mosaic of LANDSAT images (dated between 1974 and 1978, sept 1975 at Hombori) supported by field observations in sampled sites (Projet d Inventaire des Ressources Terrestes, (PIRT, 1983). Each zone is described as mosaics of soil types which relative extend is estimated. Soils are also attributed production potential: for crops, pastoral and forestry productions. Ten of these land units overlay, at least partially, the disctrict of Hombori. From the composition of the soil mosaic of these units, and grouping the soils types in four classes of soil texture (Fig 4a ) : the district appears about equally dominated by sandy soils (40%) and shallow soils on hard pan (29%) and rock outcrops (9%). Soils of valley with loamy texture on flats (10%) and clays soils in bottom lands (12%) complement the landscape. Only 12% of the land is considered arable with a marginal potential (Fig. 4b), while 19% of non arable land has a poor potential to produce fodder and wood, and 66% a very poor potential, 3% being without any production. 4

5 soil types (district of Hombori) from PIRT, % 9% 40% Sand Loam Clay hardpan Rock Soil production potential (district of Hombori) from PIRT, % 12% 19% crop marginal fodder/wood poor fodder/wood very poor none 12% 10% 66% Fig. 4, a (left) areas of soil types by top-soil texture classes, b (right) areas of soil production potential for crop (marginal), rangeland (fodder and wood either poor or very poor) and waste lands (no production). Assessed from the PIRT (1983) agro-ecological zooning over the district of Hombori (3200 km²) Visual interpretation of a mosaic of SPOT images and systematic field observations were also associated in the mapping of forestry resources over the country (PIRL, 1988). In this mapping exercise, cropped areas were identified (Nasi & Sabatier, 1988). Unfortunately the district of Hombori is located just at the edge of the area covered by the map that does not include the most arid part of Mali. Agro-ecological zoning by Pierre Hiernaux for the ILRI Project ( ) Visual interpretation of false colour composites of ancient landsat scenes ( ) and systematic field observations were used to map ecological units over the Gourma, northern Seno and the Niger river valley downstream Timbuctu in Mali at the scale of 1/ (Hiernaux, 1984). A total of 225 ecological units and km² are defined by the relative cover, expressed in deciles, of 9 generic soil types characterised by the texture of the top soil and site geomorphology (table 1). The average area size of these ecological units is 473 km² with a median at 189 km2. The area covered by each generic soil types within ecological unit is calculated by multiplying the relative cover by the unit area. Table 1. The definition of 9 generic soil types Code Texture Topo-geomorphology R Rock outcrops Eroded rock outcrop (sandstone shists) G Gravely soils and hard pans Hard pan outcrops and gravel deposits E Shallow sands (< 2m) (Sand>70%) Shallow sand deposits (wind/run-on) S Thick and flat sands (> 2m) (Sand>70%) Alluvial plains, sandy bottom slopes D Sand dune (Sand>70%) Dunes and thick wind deposits T Sandy-loam (50%<Sand<70%, Clay<30%) Colluvium, interdunes L Loamy (Clay<30% ; Sand<50%) Alluvial plain, colluvium and rivers banks A Clayed (Clay>30%) River bed and alluvial plain, depression H Permanently or extensively flooded soils either clayed, loamy-clayed or sandy-clayed soils River bed and alluvial plain of Niger river, chanels, lakes, and large ponds These area covers are then summed over the district of Hombori to characterise the edaphic environment (table 2, Fig. 5). In order to highlight the main features, the nine generic soil 5

6 types are aggregated into 3 main edaphic environments: sandy soils (aggregated S, D, E and half of T); the clay or fine textured soils (aggregated A, L, H and half of T) and the rock or shallow soils (aggregated R and G). Table 2. Area covered by the generic soil types (9) and edaphic environment (3) derived from these soils types over the District of Hombori en environment. 9 Generic soil types Area Km² Area % 3 edaphic environment area km² Area % R, rocks 216,774 16,03 Rock G, hard pan 327,401 24,2107 (R+G) 544,175 24,3413 E, sand sheet 339,132 25,0781 D, sand dunes 521,321 38,5507 Sand S, sandy plain 101,46 7,50276 (E+S+D+0,5T) T sandy loam 214,713 15, ,27 47, ,808 28,8255 L, loam 124,768 9,22632 Clay A, clay 0, ,01706 (L+A+H+0,5T) H,water 216,774 16,03 622,163 27,8297 Total 2235,61 165,319 Total 2235, Figure 5 Distribution of 9 soil surface types considered in the ecological zoning of the Gourma region (Hiernaux and Cissé, 1983) over the district of Hombori (2236 km²). The agro-ecological zoning has been used to map soils over the Gourma (Diallo and Gjessing, 1999) and supported research on the impact of the soil background in satellite based vegetation cover remote sensing (Kammerud, 1996). I has served as a base for sampling in many studies among which Franklin 1991, de Leeuw et al. 1992, Mougin et al. 1995, Tounsi 1995, Jarlan et al. 2005, Tracol et al. 2006; Frison et al. 2000, Mougin et al and Dardel et al Vegetation cover and land use estimate by low aerial survey (RIM, ) Systematic observations during three low aerial surveys have been used to assess the distribution of livestock in the Gourma region in 1983 and 1984 (Milligan 1983; Bourn & Wint 1985). In addition to livestock counts, rural dwellings were counted and the estimates were done of vegetation cover. This includes the occurrence of X types of vegetation, extends of vegetated patches of herbaceous vegetation, and proportion of land cropped. The counts are 6

7 done on two bands of 400m width on either side of the plane along regularly spaced East- West flight lines under a 0.9% rate of sampling. The results are presented as means per 5 x5 (roughly 82 km²). The district of Hombori and surroundings extends over 36 cells (2952 km²) which average cropland extent is estimated at 9.3% (interval %) of the district area. Agro-ecological zoning by Pierre Hiernaux for the AMMA project ( ) A more detailed agro-ecological zoning was achieved over the super site of the AMMA project (2687,8 km²) that extends on most of the Hombori district not on the southern fringe though (Fig. 7). This zoning is established by visual interpretation of false colour composites of LANDSAT scenes ( ) associated to systematic field data recording. 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 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 3). 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. Table 3. Statistics on the land units mapped over the AMMA Hombori super-site: number and proportions of land units with 1, 2 or 3 facies, mean area of the first, second and third facies when they exist. Frequency of the distribution unique (single facies or facies in a single patch), repetitive contagious and repetitive regular. Nb faci Land unit frequency land unit area (km²) Type of distribution Land unit frequency Size of facies Facies frequency es Nb % Mean Sd nb % elements Nb % Unique kilometre Contagious hectometre Regular decameter total All all The facies vegetation is characterised by one or two dominant species among woody plants or perennial herbaceous. 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 4) also used to code the estimate cover and densities of the perennial herbaceous. 7

8 Table 4. Coding scale used to assess canopy cover, as well as relative areas of bare soils, cropped and grazing intensity. Code Canopy cover % 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 defines 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 1). The 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 5). 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 5: 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 Water infiltration estimated after a rainfall event of: P= 10mm P=20mm P=30mm -4 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 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 4). The distribution of the 9 types of soil surfaces (Figure 7) compares to the distribution estimated from the PIRT agroecological zooning with about same extend of sandy soils (44,6 % against 40%), more fine textured soils (31,6% instead of 22%) and less shallow soils on hard pan or rock outcrops (23.8% instead of 38%). 8

9 Figure 6. Statistics on the contribution of 9 types of soils (based on top soil texture and geomorphology) over the Hombori AMMA supersite Soil texture over AMMA Supersite area (%) sand dune sand plain sand sheet loamy sand loam clay flooded hard pan rock 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%) (Table 5). 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. Table 5. 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 Area in Supersite Canopy cover % Land Land mosaic type km² % Trees bushes All woody plants Perennial herbaceous cropped % heavily grazed % RR 350,8 13,1 0,6 3,1 3,7 0,0 0,2 2,8 SR 343,7 12,8 1,8 6,4 8,2 0,0 6,8 13,0 SS 834,7 31,1 1,6 5,4 7,0 0,0 3,5 12,6 SA 74,9 2,8 8,0 17,2 25,2 0,0 6,0 24,4 AA 368,6 13,7 19,5 13,6 33,1 0,1 1,8 27,2 AR 674,0 25,1 1,7 4,6 6,3 0,0 0,7 6,4 XX 41,1 1,5 1,5 6,9 8,5 0,0 0,8 17,1 Sum/mean 2687,8 100,0 4,1 6,5 10,6 0,0 2,6 12,2 Only 2,6 % of the lands in the supersite are cropped (Fig. 7), 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 (table 5). 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 (Fig. 7). 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. 9

10 Figure 7. Relative area (%) of cropped fields, intensively grazed lands and permanently bare soils over the AMMA super-site in the Gourma region of Mali. These agro-ecological zoning is used to enforce edaphic variables in the global model tested in the intercalibration project ALMIP (Boones et al. 2009). 10

11 Land cover and land use maps established by supervised classification prior the ESCAPE project. ( ) Two attempts were made to map land use, and specifically the extent of land under cultivation, through supervised classification of either Landsat or SPOT images: the works of Bakary Sanou for the AMMA project (Sanou, 2008) and of Antoine Cheula for the ECliS project (Cheula, 2009). Methods used to classify The supervised classification is performed with the maximum likelihood option of the ENVI software with series of 20 or so Regions Of Interest (ROI) established for each surface category, based on field experience. Main idea to minimise the confusions between soil type, vegetation cover and land use type is to separate the identification of soil types performed on image dated as late as possible during the dry season, i.e. with minimal vegetation cover and no green vegetation except for the evergreen woody plants, and the crops performed during the late wet or early dry season when herbaceous in rangelands are welting while crops are reaching maximum green cover and mass. In other words, crop fields are separated from rangeland and fallow lands on the soil types that are potentially cropped (masking all other soil types) based on the shift in phenology between crops and rangeland herbaceous. Classically, the last germinate with the first rains, take a few days or weeks to establish, tiller and branch, then grow very fast through heading and flowering, and wilt soon after setting seeds by mid-september of so. On the opposite, crops are sown at much lower density, and plant density in crop fields is controlled by weeding once at least a couple of weeks after sowing. Because of much lower density it take longer for crops to cover the soil than in rangelands, however because of longer life cycle and lessened competition for water, crops remain green and grow longer than rangeland plants, seed maturity and wilting starting at the earliest in late September to mid-october. There are thus 2 to 4 weeks shift in greening optimum between crops and rangeland that are taken advantage off to map crop fields providing satellite images are available at the right time. Antoine Cheula applied a systematic approach to remote sensing on a series of LANDSAT image summarized in the chart (Fig 8), the available SPOT scenes as well as very high resolution QuickBird and Ikonos images are used to calibrate the work with LANDSAT but were not used to map the land surfaces just because they cover only a small part of the Hombori district. 11

12 Landsat window 5 dates : 1-18/11/ /12/ /03/ /05/ Scene 2 mask of woody vegetation by a threshold on the NDVI + classification Scenes 2, 3, 4 (dry season) Vegetation masked Classification of Soil types Map of soil types Scenes 1 et 5 Sandy soils only Classification of landuse types Map of area cropped Figure 8. Chart of the three step method used to map land use in the district of Hombori (Cheula, 2009). In a first step the woody cover is mask by the NDVI threshold applied early in the dry season, the second step is a supervised classification of soil types by supervised classification on a dry season image, finaly area cropped are mapped on the sandy soils only in 1986 and 2997 by a statistical analyzing the NDVI seasonal contrasts: principal component analysis and SVM ( Support Vector Machine) algorithm with 8 texture indices (Cheula, 2009). Results Antoine Cheula s soil surface maps. Map of soil types (Fig. 10) illustrates the diversity of environments encountered on the district with a slight dominance of sandy soils (6 subtypes distinguished by their reflectances) which correspond to landscapes 'Bodra' of the Tamasheq terminology (Ag Mahmoud, 1992), but also a diversity of rocky surfaces with sandstones, schists and hard pas, combined with sandy sheets and silty flats which together constitute the 'asalwa' of the Tamasheq culture, and finally a network of valleys with silty clay soils by place a dense woodland, the 'idufan' in Tamasheq. These valleys occupy the peripheral depression that surrounds the Hombori Mountains and eastward extension in the Wami up to Amniganda: this is the main axis along which concentrate ponds and settlements. And this axis is completed to the North by a series of closed depressions fed by the hard sandstone crests Gonta-Belia-Koykoyra, to the South by closed depressions Ouari, Daribangui, 12

13 Dossou. Finally the district is bordered to North and South by two sets of ponds downslope of the large rock outcrop and erosion surface that extends to Gossi Inadiétafane on the northern side, and the great hard pan slope that extends between Hombori and the Seno on the Southernside. In the north, the series of ponds west to east includes Dimamou, Sabangou, Taylalelt and Agoufou. To the south are the pools Pété Ndoti, Boumboum and Kirnya. Figure 10: map of surface types: soil texture, vegetation in the district of Hombori. (Cheula, 2009) 13

14 Maps of cultivated surfaces in 1955, 1986 and 2007 are then superimposed (Fig.10) to assess the dynamics of agricultural land use. Knowing that only the map of 1955 includes terraced fields on the step rocky slopes of Mont Hombori and the few clay plains crop fields. Figure 10 : Map of the historical dynamics of land cropped in the district of Hombori, Mali. (Source : Cheula, 2009) It appears that the fields on sandy soils (more than 90% of agricultural land) extended over 1.4% of the territory of the commune in 1986, just after the droughts of and spread to 3.0 % in 2007 which represents a strong momentum even if the relative area occupied by fields remains modest (Table 1). 14

15 Year Surface type km² % km² % uncroppable 905,43 42, ,33 48,15 crop field 38,70 1,81 82,70 3,87 sand sheet 198,28 9,28 108,99 5,10 sand dune 441,67 20,68 331,31 15,51 loose sands 42,35 1,98 7,96 0,37 sandy plain 87,75 4,11 61,32 2,87 sands litter 309,47 14,49 239,77 11,23 sandy loam 105,44 4,94 111,20 5,21 woody on sands 6,74 0,32 164,25 7,69 All 2135,83 100, ,83 100,00 Table 1. Area of sandy soil sites retrieved from supervised classifications by Antoine Cheula (2009) of landsat images from 1986 and The class named uncroppable groups the rocky surfaces (sandstone, schist, dolomy), and the fine textured soils (loamy clay, loamy soils, clay soils, surface water) La comparaison avec la situation en 1955 peut se faire sur la fenêtre cartographiée par Jérôme et Jocelyne Marie qui et montre bien la régression des activités agricoles suite à la sécheresse : de 3,9% en 1955 la superficie cultivée s est réduite à 2,3% en 1986, puis la remontée actuelles de activités agricoles à 4,3% de la superficie (Table 2) Superficie (km²) Superficies cultivées en 1955 (%) Superficies cultivées en 1986 (%) Superficies cultivées en 2007 (%) Landsat scene 6440, Hombori district Window by J. Marie 1707, Tableau 2: Cartographie de l occupation des sols en 1955, 1986 et 2007 : estimation des superficies cultivées sur la commune de Hombori ou des fenêtre qui couvrent une partie de la commune.(cheula, 2009). Mapping the land use in 1955, 1986 and 2007:. Estimates of area cropped in the district of Hombori or in a window covering a large fraction of the district used by Marie (Cheula, 2009). L examen de la superposition du parcellaire agricole au trois date indique bien que sur sable il y a une certaine constance des terroirs qui privilégient les plaines ensablées (Wami, Bilantao, et la plaine de Hombori), et les ensablements ou les versants dunaires au contact avec la plaine (bas de versant sud et nord de la dune de Hombori, bas de versant nord et sud de la dune à Darawal, Dossou, Ouari, Daribangui, Sabangou-Taylalelt- Agoufou). C est dans ces derniers que l expansion actuelle des superficies cultivées se développe, tout particulièrement autour des points d eau qui se sont agrandis ou pérennisés (Agoufou, Dimamou, Dari Bangui) créant un risque de conflit avec l utilisation pastorale des points de ces points d eau. 15

16 The analysis of the overlay of the areas cropped at three dates indicates that consistency in selecting sandy plains (Wami, Bilantao, and Hombori plain) and bottom dune slopes at the contact with the plain (lower slope south and north of the Hombori Hondo dune down, north and south slopes of the dune at Darawal, Dossou Ouari, Daribangui, Sabangou- Taylalelt-Agoufou. It is in these that the current expansion of the cultivated area is growing, especially around water points that are enlarged or perpetuated (Agoufou, Dimamou, Dari Bangui) creating a potential conflict with the pastoral use points of these water points. Travaux de Cam Chi Nguyen The preliminary works by Bakary Sanou have been resumed in 2012 by Cam Chi Nguyen for the ESCAPE project Soil types At the spatial resolution of the landsat images the soil type identified were mostly characterised by the texture at soil surface. Among the rocky areas it is attempted to distinguish three main rock surfaces: sandstones and quartzites, schists and ferralitic hard pans. Globally, the Gourma region is a flat peneplain, however there are locally some relief, including the sharp sandstone plateau of Hombori, Boni and Gandamia resulting in some shadows on the landsat image that affecting the spectral properties of the soils. Two classes of rock shadows (deep shadow and light shadow) have been added to take care of this, these two categories are later merged with sand stone surfaces. Fine texture classes include sandy soils, sandy loams, loams, clay-loams and clays soils. However some of these classes have been subdivided to take in account the influence of dead herbaceous vegetation or litter, shadow or moisture. Sandy soils for example are classically subdivided into sandy soils covered with litter, sand dunes, shadowed sandy soils (narrow shadow of the highest dunes), burning scars on sandy soils (from fires that happened early in the dry season or late, i.e. soon before the date of the image), sandy loams plains and bare, more or less mobile sands. There are no litter, nor burning scars on loamy, clay-loam and clay soils just because herbaceous vegetation cover is too sparse and generally heavily grazed, but clay soils could be still moist in bottom lands. There is thus, when needed a additional category for moist clay soils, together with one for green aquatic vegetation and one for water surfaces in ponds in the Niger river. The subcategories related to litter cover, burning scars, soil moisture or surface water excluded, the main soil background categories should be quite stable over years, at least in the short term. This largely applies to the maps established in the dry season in 1986, 2007 and 2011 (table ).. However, soil erosion in parts of the landscapes, especially on shallow soils may explain change trends in the long run ( ) with a reduction in fine textured soils cover, an increase in rock surfaces and a about balanced situation of dominant sandy soils. 16

17 Figure Carte de l occupation des sols de la commune de Hombori en Etablie par classification supervisée d image LANDSAT de mars 2011 (types de sol, marques de feux et couvert des ligneux) et de septembre 2011 (sols cultivés sur sols sableux. Figure 1 17

18 Woody plant cover Two categories of green woody vegetation are identified during the dry season, one on dark fine textured soils (dry or moist clay, clay-loam and loamy soils, some of the rocky areas), the other on coarse textured light soils (dry sands and loamy sands, some loamy flats). The reason why the two category separates holds in the fact that canopy are very seldom joined and that at the resolution of the LANDSAT images the signal is most of the time influenced by the soil background in addition to woody plant canopies. The same reason explains for the lack of robustness of the woody plant classification sensitive to the reflectance threshold from which a pixel that contains woody green material is classified as woody. Yet, long term changes observed locally on monitored field sites (Hiernaux et al., 2009) should be visible. Globally there is an increase in areas covered with woody plants form 1986, to 2007 and then to However this last value is perhaps over estimated compared with the two previous one as the image was dated from January instead of March, earlier in the dry season thus, when deciduous woody plants would have lost less green leaves. Trends of woody cover on shallow soils tend to decrease from 1986 to 2007 and also from 2007 to 2011 as observed on the field sites. Figure a_ Superficies (%) occupées par les categories de type de surface sol et d occupation des sols dans la commune de Hombori en 2011 b_ Superficies (%) occupées par les trois grandes unités paysagères (topographie, texture de surface et fonctionnement hydrique) ainsi que le couvert des ligneux et des terres cultivées dans la commune de Hombori en

19 Area cropped. The Gourma is on the margin of the cropping lands in Mali. Actually, when the Niger River valley that benefit of seasonal flood is excluded, there are almost no rainfed crops to the north of the Hombori district. It was already the case in 1955 as indicated by the extends of cropped areas on the IGN map (table, figure, ) and on the map established by photointerpretation on the district of Hombori (Marie and Marie, 1975). The area cropped seems to have shrunk following the major drought of and again , as they only cover. on the district of Hombori (Cheula, 2009) and % on the common area defined on the Landsat image series by Cam Chi Nguyen (table ). Area cropped appear to have increase again following the 1990 s wetter years with..% cropped over the Hombori distict and % over the landsat image (table). The dynamics of area cropped appear thus as a response to crop success of failure in relation with rainfall distribution. A closer examination of the location of these field dynamics indicates that cropland dynamics proceed both by marginal area expansion (or shrinking) of core agricultural lands and by clearing (or abandon) of satellite agricultural lands Km² % Km² % Km² % crop_field 32,256 1,516 47,038 2,200 57,927 2,710 sand loam 326,243 15, ,587 12, ,440 13,683 sand dune 317,811 14, ,191 7, ,512 8,165 sand dune-green - 237,822 11,125 7,651 0,358 sand litter 217,593 10, ,033 24, ,010 28,307 sand bare 19,205 0,903 6,281 0,294 5,962 0,279 sand_sheet 351,806 16, ,018 6,550 98,805 4,623 old burning ,038 0,470 10,118 0,473 recent burning - 0,030 0,001 0,063 0,003 tree_sand 85,721 4,028 70,596 3, ,250 5,813 tree_clay 28,076 1,319 33,127 1,550 17,456 0,817 hard pan 125,101 5, ,372 4, ,519 5,171 sandstone 182,325 8, ,636 9, ,233 5,438 sandstone_shaddow 9,362 0,440 26,362 1,233 10,586 0,495 schist_diatomite 151,298 7, ,541 4, ,328 7,408 diatomites ,085 2,484 loam 2,147 0,101 0,916 0,043 0,365 0,017 loam_clay 140,920 6,622 56,034 2,621 78,677 3,681 clay_dry 129,944 6,107 87,638 4, ,897 9,072 clay_wet 5,067 0,238 31,568 1,477 21,367 1,000 aquatic 3,081 0,145 8,849 0,414 0,003 0,000 water_body 0,001 0,000 0,042 0,002 0,047 0, , , , , , ,000 Tableau Land surface dynamics over the Hombori district : soil surface (topography and top soil texture), végétation cover (woody plants, litter) and land use (crop field) These different dynamic patterns may correspond to different process in family farming systems. Expansion (of shrinking) of core agricultural lands explain by decision taken within sedentary (villages) families to devote more of less family labour to cropping depending on expected rainfall but also available labour and competition with alternative economic 19

20 activities. In a limited number of cases the field dynamics in core croplands reflects some change in agricultural goals from pure staple to more diverse and market oriented crops. On another hand, opening or closing satellite cropped areas reveals settling (or out migration) of pastoral families. Fig Dynamique historique de l occupation des sols de la commune de Hombori, établie par classification d images Landsat Among the changes observed in core croplands the fields established on the steep slopes of the mounts pediments are increasingly abandoned, following a trend already observed in the 70 s (Marie and Marie, 1975). Reversely, there are more fields in temporary flooded bottom lands, often in the goal of diversifying crops: sorghum, okra, tobacco and vegetables. Sabangou km to the North East of Hombori is an example of satellite croplands: most of the pastoral families settled prior at least for 6 month of the year (lack of water during the late dry season) around the ancient village site of Sabangou before 1973 quitted following the drought of the because of repetitive crop failure and large losses in livestock. A partial recovery occurred in the late 70 s followed by a second abandon after the droughts of Since then, cropping recovered and settlement expanded, especially from mid 1990 s following better rains in 1990 and 1994 and also the improvement of access to water year round with the change in pond regime of Agoufou, but also local ponds in Sabangou and Taylalelt (Gardelle et al., 2009). Discussion The diversity of edaphic environments is a constant of the maps established by different remote sensing methods. Three main soil landscapes are distinguished: 1) the sandy soils which include fixed dune systems (with some moving dune crests) of sandy plains (alluvial sand) and shallow sand sheets (on rocky substrates, ferralitic hard pan or loamy-clay plains), 2) the rocky erosion surfaces are mosaics (sandstone, shale, hard pan) and silty flats enclosed in a network of gullies and ravines, more or less hierarchical, and 3) the plains and depressions that gather loamy-clayed soils of floodplains, river-beds and the ponds with a few water bodies. These three main landscape correspond to different hydric functioning: local endoreic on sandy soils, with a redistribution of rainwater at the scale of a few meters to a few 20

21 kilometers from dune slopes to inter-dune depressions; regional endoreic for the other landscapes, erosion surfaces being upstream providing runoff to the loamy-clayed soil of the plains and depressions downstream, ending in ponds. Soil surface maps agree on areas of these main landscape units over the Gourma region as well as over the district of Hombori. However, methods for supervised classification of multispectral high-resolution images may differ in detail for different reasons. One of the expected deviations between soil surface maps is due to interference by vegetation cover. The canopy cover of woody plants generates some of these interferences. Only the crowns of tall trees can cover a full pixel, although joined crowns of smaller trees could also fill whole pixels as for tiger bush of riparian thickets, but in most cases the woody crowns only cover a fraction of pixel. The decision to award that pixel to tree cover only depends on the selected threshold. Moreover, woody cover only distinguishes during the dry season when the woody plants are foliated at this season, which depends on the specific date, on species phenology and on the particular year climate (Hiernaux et al. 1994). Mapping 'tree-cover on clay' and 'tree cover on sand is therefore qualitatively valid (the pixel classified as treecover are effectively located in areas of concentration of woody plants) but remain quantitatively imprecise to the point that changes in tree-cover observed between images over time are not safely interpretable. Water bodies are again easily identifiable, and together with the area of aquatic vegetation the total area of water with or without aquatic vegetation is fairly reliable in the dry season. However, changes in area of surface water (free water and aquatic vegetation) are highly dependent on the date of the image and the fill level of the ponds in the previous rainy season and therefore have limited interest. For similar reasons, the relative proportions of soils 'clay wet' and 'dry clay soils' are variable in the short-term during the dry season and thus they are better to merge in a single clayed soil category. If silty flats are easily distinguished as highly reflective surfaces almost devoided of vegetation, the difficulty comes from the transitional class of loamy-clayed soils as the transition is gradual. This justify grouping all the fine textured soils to check on the conservation of that group overtime even if the relative extend of components varies over time. Les affleurements rocheux devraient à priori être aisément identifiables, néanmoins il n est pas toujours aisé de distinguer les affleurements de grès et de schiste d une part, et ces derniers se confondent largement avec les diatomites qui occupent le fond de plaines lacustres fossiles du nord du Gourma (Oufar, Kakas, Tiouaz, Tin Orfan) séparés de la vallée du Niger par les cordons dunaires de l Edjerew). Or les affleurements de diatomites se rapprochent davantage d un sol argileux de plaine temporairement inondable que d un affleurement rocheux. En outre les réflectances des affleurements rocheux sont sensibles aux rayonnements, et particulièrement à l angle. Une classe particulière à due être consacrée aux ombres qui peuvent se confondre avec des surfaces en eau ou des surfaces incendiées (dans les deux cas peu réfléchissantes). Dans les cas extrême (ombre épaisse des monts Hombori, il a fallu forcer la classification). Rocky outcrops should a priori be easily identifiable, however it is not always easy to distinguish sandstone from schist outcrops; then schists reflectance signature largely overlap with the diatomite one. Yet these can hardly be considered as rock outcrops, behaving closer to clayed soils temporarily flooded in the bottom of fossil lacustrine plains in the north 21

22 Gourma (Oufar, Kakas, Tiouaz Tin Orfan), separated from the Niger valley by the Edjerew sand dunes. In addition, the reflectances of the rock outcrops are sensitive to the solar angle and particular categories had to be devoted to 'rock shadows' that can be confused with surface water or burnt areas (in both cases reflecting little in the visible). In extreme cases ('thick' shadows of the Hombori Mounts), the classification had to be locally enforced (no water, nor burning on outcrops). L interférence de l état du couvert végétal est plus important sur les surfaces sableuses qui occupent plus de la moitié du Gourma. Elle est due à l état de la strate herbacée qui en saison sèche est constituée de pailles et litières. Lorsqu elles sont abondantes, elles atténuent les réflectances dans le visible suffisamment pour être cartographiées séparément comme sablelitière, à l opposé le sable-vif des dunes vives et des sables érodés (ravines, cône de déjection) est aussi distingué des sables dunaires ( sable-dune ) médians. En saison sèche une partie des champs de mil peut être classée en sable vif une autre en sable-dune et une troisième partie se rattache aux sables limoneux justifiant le recours à une cartographie séparée sur une image de fin de saison des pluies. Très tranchées apparaissent les zones incendiées, d abord très sombre (confusion possible avec ombre) puis s éclaircissant progressivement au cours de la saison sèche avec de multiples confusion possibles avec les ensablements de surface, les sables limoneux, voir les affleurements de schiste et ou de grès. Deux autres catégories de surface sableuses assurent les interfaces : ensablement de surface avec les affleurements rocheux, et sables limoneux avec les sols limono-argileux. The interference by vegetation cover is more important on sandy soil surfaces that occupy more than half of Gourma. These interferences are due to the state of the herbaceous layer made of straws and litter during the dry season. When abundant, straws and litter reduce the reflectance in the visible enough to be mapped separately as 'sand-litter', opposite the bare 'moving sands' at the crest of some sand dunes, and in eroded patches (ravines, alluvial fan) and also distinguished from 'sand-dune' that stand in between. In dry season a millet crop field could be classified as 'bare sand or 'sand-dune ' or else as silty sand. This confusion justify the use of a separate image from the late-season to map croplands. Burned areas first appear very contrasted just after the fire event, very dark (with possible confusion with shadows), then they gradually lighten with multiple possible confusion with silt surface, silty sands, schist outcrops or even sandstone. Enforce corrections had to be done in some of these cases. Two other categories of sandy soil areas provide interfaces: shallow sand sheet intergrade with rock outcrops, and' silty sands ' intergrade with loamy-clayed soils. En conclusion, hormis des surfaces occupées par l eau et les ligneux, l interprétation des changements d état de surface sol requiert de procéder à des regroupements entre catégories distinguées pour éviter les confusions grands types de surface mais pas forcément robustes. Certaines d entre elles, comme les aires incendiées, mais aussi les sols sableux couverts de litières sont circonstancielles. A l échelle de quelques décennies, les surfaces des principaux états de surface devraient se conserver. Néanmoins, des études de cas dans le Gourma (bassin versant de Tin Adjar, de Fintrou, d Edjerit et de Timbadiawal) ont montré (Ramarohetra, 2009) qu au moins localement, le jeu des érosions éoliennes et hydriques pouvaient modifier les superficies des états de surface dans des proportions notables: extension des affleurements rocheux au détriment des surfaces limoneuse et limono-argileuse. Il est peu probable cependant que les classifications supervisées faites à des dates différentes à l échelle d une région toute entière permettent de mesurer ces dynamiques locales. 22

23 In conclusion, apart from areas classified as water or woody cover, the interpretation of changes in the soil surface requires the aggregation of soil surface classes initially distinguished from one another to avoid confusion between major surface types because their separation is not always robust. Some of them, such as burned areas, or sandy soil with litter are circumstantial. At the scale of a few decades, the area of the main surface states should remain about constant. However, case studies in Gourma (watershed Tin Adjar of Fintrou, of Edjerit and Timbadiawal) showed that wind and water erosion could change the areas of main surface states in significant proportions (Ramarohetra, 2009). The expansion of outcrop areas is going on at the detriment of silty and loamy-clay surfaces. It is unlikely, however, that the supervised classifications made at different times across an entire region to measure the local dynamics. On the other hand, mapping soil conditions in the dry season is essential to the mapping crop fields. Indeed, it is unrealistic to attempt a direct supervised classification of crop fields without first limiting the scope to particular soil types as the risks of confusion are many. Map soil surfaces was used to stratify the landscape and only retain sandy and silty-sandy soils which harbor a large majority of the croplands. The few fields set on the rocky slopes of the Hombori mounts are not identifiable because millet is very sparse, in a mosaic with roc boulders, weeds and shrubs, and also because of the steep slopes that increases shadowing. A few other crop fields have been set in fine textured soils temporarily flooded during the wet season. There identification is hindered by the interferences with the woody plants in open stand, and also the variable woody plant phenology. Crop fields on sandy and silty-sands have the greatest momentum. The area cropped slowly increased from the 1950s to early 70s, then declined following the droughts of and They have increased again since the mid-1990s Already observed in the mid-19th century by Heinrich Barth (1857), and in the early 20th century by de Gironcourt (1912) and colonial administrators stationed in Hombori (Mourgues 1932), rain-fed millet has been practiced for a very long time at Hombori which has long been on the northern front of rainfed crops. But this front appears static, affected only by variations in areas cropped in adaptation to changes in rainfall volume and distribution pattern, but also affected by the social environment and political events (Clauzel 1962 Gallais 1975 Marie 1983 Gáname 2002). As such, the impact of the civil unrest since 2010 has not yet been observed. Acknowledgements This work has been funded by the French National Research Agency (ANR) through the ESCAPE project (ANR-10-CEPL-005) 23

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