Hydrology and Earth System Sciences www.hydrol-earth-syst-sci.net 1027-5606 1607-7938 14 2 2010 10.5194/hess-14-309-2010 http://www.hydrol-earth-syst-sci.net/14/309/2010/ http://www.hydrol-earth-syst-sci.net/14/309/2010/hess-14-309-2010.html http://www.hydrol-earth-syst-sci.net/14/309/2010/hess-14-309-2010.pdf 309 324 2010-02-16 Less rain, more water in ponds: a remote sensing study of the dynamics of surface waters from 1950 to present in pastoral Sahel (Gourma region, Mali) J. Gardelle P. Hiernaux pierre.hiernaux@cesbio.cnes.fr L. Kergoat M. Grippa Centre d'Etudes Spatiales de la BIOsphère (CESBIO), UMR 5126, UPS-CNRS-CNES-IRD, 18 avenue, Edouard Belin, b.p.i. 2801, 31401 Toulouse Cedex 9, France Changes in the flooded area of ponds in the Gourma region from 1950 to present are studied by remote sensing, in the general context of the current multi-decennial Sahel drought. The seasonal and interannual variations of the areas covered by surface water are assessed using multi-date and multi-sensor satellite images (SPOT, FORMOSAT, LANDSAT-MSS, –TM, and -ETM, CORONA, and MODIS) and aerial photographs (IGN). Water body classification is adapted to each type of spectral resolution, with or without a middle-infrared band, and each spatial resolution, using linear unmixing for mixed pixels of MODIS data. The high-frequency MODIS data document the seasonal cycle of flooded areas, with an abrupt rise early in wet season and a progressive decrease in the dry season. They also provide a base to study the inter-annual variability of the flooded areas, with sharp contrasts between dry years such as 2004 (low and early maximal area) and wetter years such as 2001 and 2002 (respectively high and late maximal area).The highest flooded area reached annually greatly depends on the volume, intensity and timing of rain events. However, the overall reduction by 20% of annual rains during the last 40 years is concomitant with an apparently paradoxical large increase in the area of surface water, starting from the 1970's and accelerating in the mid 1980's. Spectacular for the two study cases of Agoufou and Ebang Mallam, for which time series covering the 1954 to present period exist, this increase is also diagnosed at the regional scale from LANDSAT data spanning 1972–2007. It reaches 108% between September 1975 and 2002 for 91 ponds identified in central Gourma. Ponds with turbid waters and no aquatic vegetation are mostly responsible for this increase, more pronounced in the centre and north of the study zone. Possible causes of the differential changes in flooded areas are discussed in relation with the specifics in topography, soil texture and vegetation cover over the watersheds that feed each of the ponds. Changes in rain pattern and in ponds sedimentation are ruled out, and the impact of changes in land use, limited in the area, is found secondary, as opposed to what has often been advocated for in southern Sahel. Instead, major responsibility is attributed to increased runoff triggered by the lasting impact of the 1970–1980's droughts on the vegetation and on the runoff system over the shallow soils prevailing over a third of the landscape. Ag Mahmoud, M.: Le haut Gourma Central, 2nd edn., edited by: Le Floc'h, R., CEFE/CNRS, Montpellier, 133~pp., 1992. Benjaminsen, T. A.: Bois-énergie, déboisement et sécheresse au Sahel: le cas du Gourma malien, Sécheresse, 7, 179–185, 1996. Boudet, G.: Désertification ou remontée biologique au Sahel, Cahiers ORSTOM, Série Biologie, 12, 293–300, 1977. Boudet, G., Cortin, A., and Macher, H.: Esquisse pastorale et esquisse de la transhumance de la région du Gourma (Mali), DIWI, Essen, Germany, 120~pp., 1971. Boudouresque, E.: La végétation aquatique du Liptako (République du Niger), thesis, Univ. Paris XI, Orsay, France, 391~pp., 1995. Bourn, D. and Wint, F.: Wet season distribution and abundance of livestock populations and human habitants in the Gourma region of Mali, Resource Inventory and Management, St Hellier, Jersey, UK, 53~pp., 1985. Cheula, A.: Dynamique de l'occupation des sols en milieu sahélien. Espaces cultivés et couverture ligneuse dans la commune de Hombori, Mali. Mem.Master2 Télédétection et Géomatique Appliquées à l'Environnement, Univ. Paris Diderot, 44~pp., 2009. d'Herbès J.M., Valentin C., Thièry J.: La brousse tigrée au Niger: synthèse des connaissances acquises. Hypothèses sur la genèse et les facteurs déterminants les différentes structures contractées, in: Fonctionnement et gestion des écosystèmes forestiers contractés sahéliens, d'Herbès, edited by: Ambouta, J. M. K. and Peltier, R., John Libbey Eurotext, Paris, 131–152, 1997. Défossez, M.: Contributions à l'étude géologique et hydrogéologique de la boucle du Niger, Mémoire BRGM 13, Paris, 174~pp., 1962. Desconnets, J.-C.: Typologie et caracterisation hydrologique des systèmes endoréiques en milieu Sahélien (Niger-degré carré de Niamey), thesis, Univ Montpellier II, France, 326~pp., 1994. Descroix, L., Mahé, G., Lebel, T., Favreau, G., Galle, S., Gautier, E., Olivry, J. C., Albergel, J., Amogu, O., Cappelaere, B., Dessouassi, R., Diedhiou, A., Le Breton, E., Mamadou, I., and Sighomnou, D.: Spatio-temporal variability of hydrological regimes around the boundaries between Sahelian and Sudanian areas of West Africa: a synthesis, J. Hydrol., 375(1–2), 90–102, doi:10.1016/j.jhydrol.2008.12.012, 2009. Dregne, H. E. and Chou, N. T.: Global desertification: dimensions and costs, in: Degradation and restoration of arid lands, edited by: Dregne, H. E., Texas Tech Univ., Lubbock, Texas, 249–282, 1992. Favreau, G., Cappelaere, B., Massuel, S., Leblanc, M., Boucher, M., Boulain, N., and Leduc C.,: Land clearing, climate variability, and water resources increase in semiarid south-west Niger, Africa: a review, Water Resour. Res., 45, W00A16, doi:10.1029/2007WR006785, 2009. Frappart, F., Hiernaux, P., Guichard, F., Mougin, E., Kergoat, L., Arjounin, M., Lavenu, F., Koité, M., Paturel, J.-E., and Lebel, T.: Rainfall regime across the Sahel band in the Gourma region, Mali, J. Hydrol., 375(1–2), 128–142, doi:10.1016/j.jhydrol.2009.03.007, 2009. Gond, V., Bartholome, E., Ouattara, F., Nonguierma, A., and Bado, L.: Surveillance et cartographie des plans d'eau et des zones humides et inondables en régions arides avec l'instrument VEGETATION embarqué sur SPOT4, Int. J. Remote Sens., 25(5), 987–1004, 2004. Haas, E. M., Bartholomé, E., and Combal, B.: Time series analysis of optical remote sensing data for the mapping of temporary surface water bodies in sub-Saharan western Africa, J. Hydrol., 370(1–4), 52–63, doi:10.1016/j.jhydrol.2009.02.052, 2009. Hiernaux, P.: The crisis of sahelian pastoralism: ecological or economic? Overseas Development Institute, London, Pastoral Development Network, 39a, 20~pp., 1996. Hiernaux, P., Diarra, L., Trichon, V., Mougin, E., Soumaguel, N., and Baup, F.: Woody plant population dynamics in response to climate changes from 1984 to 2006 in Sahel (Gourma, Mali), J. Hydrol., 375(1–2), 103–113, doi:10.1016/j.jhydrol.2009.01.043, 2009a. Hiernaux, P., Mougin, E., Diarra, L., Soumaguel, N., Lavenu, F., Tracol, Y., and Diawara, M.: Sahelian Rangeland response to changes in rainfall over two decades in the Gourma, Mali, J. Hydrol., 375(1–2), 114–127, doi:10.1016/j.jhydrol.2008.11.005, 2009b. Hiernaux, P. and Gérard, B.: The influence of vegetation pattern on the productivity, diversity and stability of vegetation: the case of the `brousse tigrée' in the Sahel, Acta Oecol., 20(4), 147–158, 1999. Lacaux, J. P., Tourre, Y. M., Vignolles, C., Ndione, J. A., and Lafaye, M.: Classification of ponds from high-spatial resolution remote sensing: Application to Rift Valley Fever epidemics in Senegal, Remote Sens. Environ., 106, 66–74, 2007. Leblanc, M., Favreau, G., Massuel, S., Tweed, S., Loireau, M., and Cappelaere, B.: Land clearance and hydrological change in the Sahel: SW Niger, Global Planet. Change, 61(4), 135–150, 2008. Leduc, C., Favreau, G., and Shroeter, P.,: Long term rise in a Sahelian water-table: the Continental Terminal in South-West Niger, J. Hydrol., 243, 43–54, 2001. Leprun, J. C.: Etude de quelques brousses tigrées sahéliennes~:structure, dynamique, écologie: in L'aridité une contrainte au développement, edited by: Le Floc'h, E., Grouzis, M., Cornet, A., and Bille, J. C., Orstom Editions, Paris, 221–244, 1992. Liebe, J., van de Giesen, N., and Andreini, M.: Estimation of small reservoir storage capacities in a semi-arid environment. A case study in the Upper East region of Ghana, Phys. Chem. Earth, 30, 448–454, 2005. Mahé, G., Leduc, C., Amani, A., Paturel, J. E., Girard, S., Servat, E., and Dezetter, A.: Augmentation du ruissellement de surface en région soudano-sahélienne et impact sur les resources en eau, in: Hydrology of Mediterranean and Semiarid regions, edited by: Servat, E., Najem, W., Leduc, C., and Shakeel, A., Proc. Conf., Montpellier, France, 003, IAHS Pub., 278, 215–222, 2003. Mahé, G., Olivry, J. C., and Servat, E.: Sensibilité des cours d'eau ouest africains aux changements climatiques et environnementaux~: extrêmes et paradoxes, in; Regional hydrological impacts of climatic change - hydroclimatic variability, Proceedings of symposium S6 held during the seventh IAHS scientific Assembly at Foz de Iguaçu, Brazil, April 2005, IAHS pub., 296, 169–177, 2005a. Mahé, G., Paturel, J. E., Servat, E., Conway, D., and Dezetter, A.: The impact of land use on soil water holding capacity and river flow modelling in the Nakambe River, Burkina Faso, J. Hydrol., 300, 33–43, 2005b. Marie, J. and Marie, J.: La région de Hombori, Essais de géographie régionale en zone sahélienne, Mém. Maitrise, Inst. Géographie, Univ. Rouen, France, 300~pp., 1974. Mougin, E., Hiernaux, P., Kergoat, L., Grippa, M., de Rosnay, P., Timouk, F., Le Dantec, V., Demarez, V., Lavenu, F., Arjounin, M., Lebel, T., Soumaguel, N., Ceschia, E., Mougenot, B., Baup, F., Frappart, F., Frison, P. L., Gardelle, J., Gruhier, C., Jarlan, L., Mangiarotti, S., Sanou, B., Tracol, Y., Guichard, F., Trichon, V., Diarra, L., Soumaré, A., Koité, M., Dembélé, F., Lloyd, C., Hanan, N. P., Damesin, C., Delon, C., Serça, D., Galy-Lacaux, C., Seghieri, J., Becerra, S., Dia, H., Gangneron, F., and Mazzega, P.: The AMMA-CATCH observatory site in Mali: relating climatic variations to changes in vegetation, surface hydrology, fluxes and natural resources, J. Hydrol., 375(1–2), 14–33, 2009. Nicholson, S. E.: Climate and environmental change in Africa during the last two centuries, Clim. Res., 14, 123–144, 2001. Olsson, L.: On the causes of famine: drought, desertification and market failure in the Sudan, Ambio, 22, 395–403, 1993. Puech, C.: Plans d'eau sahéliens et imagerie SPOT: inventaire et évaluation des capacités d'exploitation, in: Eau, Environnement et Développement, Intern. Coll. Nouakchott, 20–22 mars 1994. Roussel, L.: Suivi temporal de la couverture ligneuse au Sahel par télédétection à très haute résolution spatiale, Rapport de stage GGAT, Ecolab, Univ P Sabatier, Toulouse, 28~pp., 2009. Tracol, Y., Mougin, E., Jarlan, L., and Hiernaux, P.: Testing a sahelian grassland functioning model against herbage mass measurements, Ecol. Model., 193, 437–446, 2006. Rouse, J. W., Haas, R. H., Schell, J. A., and Deering, D. W.: Monitoring vegetation systems in the Great Plains with ERTS. Third ERTS Symposium, NASA SP-351 I, 309–317, 1973. Verdin, J. P.: Remote sensing of ephemeral water bodies in Western Niger, Int. J. Remote Sens., 17, 733–748, 1996.