Journal of Hydrology 375 (2009) 1–2

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Surface processes and water cycle in West Africa, studied from the AMMA-CATCH observing system West Africa – an area covering roughly 6 millions km2 – has been under great demographic pressure over the past 50 years. This has resulted in drastic vegetation and environmental changes with primary savannah being replaced by crop lands and trees being cut for cooking wood. At the same time, the region suffered from a generalised drought that lasted unabated from the end of the 1960s to the end of the 1990s. This climatic anomaly was the largest of his kind observed worldwide over the 20th century. It had dramatic consequences for the populations and raised pressing questions to scientists, the central of these questions being:? Is this the first regional scale display of global climate change or is it predominantly linked to the specific regional land use changes observed in West Africa, as was hypothesized by Charney in the mid-1970s. Climate models are still unable to provide a trustable answer to this question and the recent IPCC report underlines that West Africa is a region where the uncertainty of the climate scenarios is the largest: some GCMs anticipate larger rainfall over the Sahel at the end of the 21st century, while other predict a lasting and even increased drought. The pressing and challenging need for the scientific community to contribute to the improvement of the performance of these models, by enlarging our knowledge of the processes governing the interactions between the atmosphere, the continental surfaces and the ocean, laid the basis for launching the international AMMA program. AMMA is made of several components. The core of the land surface program is the AMMA-CATCH long term observing system, aiming at documenting and investigating the land processes that control interactions and equilibria in the Atmosphere-Hydrosphere-Biosphere complex. AMMA-CATCH is a unique experimental setup of unparallel ambition, both in terms of spatial coverage and duration. I am thus particularly glad that Journal of Hydrology is hosting the first AMMA Special issue presenting a collection of results covering both the physical and biological processes governing the interactions between climate and hydrology in the West African monsoon system. Beyond the first results presented here, the new field data of high quality collected through AMMA-CATCH will certainly contribute in the future to further improve our understanding of these interactions through a combination of data-driven analyses and modelling studies.

0022-1694/$ - see front matter Ó 2009 Published by Elsevier B.V. doi:10.1016/j.jhydrol.2009.08.017

Table of content of the AMMA-CATCH special issue of JH (2009) Foreword. The AMMA-CATCH observing system in the perspective of the AMMA international project. J.L. Redelsperger and T. Lebel 1. AMMA-CATCH studies in the Sahelian region of West-Africa: an overview. Lebel, T. et al., doi:10.1016/j.jhydrol.2009. 03.020. 2. The AMMA-CATCH Gourma observatory site in Mali: Relating climatic variations to changes in vegetation, surface hydrology, fluxes and natural resources. Mougin, E., et al. 3. The AMMA Catch observing system in the cultivated Sahel of South West Niger- Strategy, Implementation and Site conditions. Cappelaere, B., et al., doi:10.1016/j.jhydrol.2009. 06.021. 4. Recent trends in the Western and Central Sahel rainfall regime (1990–2007). Thierry Lebel, Abdou. Ali. doi:10. 1016/j.jhydrol.2008.11.030. 5. Resilience and productivity trends of crops, fallows and rangelands in Southwest Niger: impact of land use, management and climate changes. Pierre Hiernaux, Augustine Ayantunde, Adamou Kalilou, Eric Mougina, Bruno Gérard, Frédéric Baup, Manuela Grippa and Bakary Djaby. doi:10. 1016/j.jhydrol.2008.11.032. 6. Phenological responses of the woody cover to variations in climatic and hydric factors along the latitudinal gradient in West Africa. Seghieri J., Padel K., Soubie R., Arjounin M., Boulain N., Derosnay P., Galle S., Gosset M., Mouctar A. H., Peugeot C., Seyigona Z., Soumaguel N., Timouk F. 7. Spatio-Temporal Variability of Hydrological Regimes Around the Boundaries between Sahelian and Sudanian Areas of West Africa: A Synthesis. 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. Sighomnou, D. doi:10.1016/j.jhydrol. 2008.12.012. 8. Woody plant population dynamics in response to climate changes from 1984 to 2006 in Sahel (Gourma, Mali). Pierre Hiernaux, Lassine Diarra, Valérie Trichon, Eric Mougin, Nogmana Soumaguel and Frédéric Baup. doi:10.1016/j.hydrl. 2009.01.043.

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Editorial / Journal of Hydrology 375 (2009) 1–2

9. Sahelian rangeland response to changes in rainfall over two decades in the Gourma region, Mali. Pierre Hiernaux, Eric Mougin, Lassine Diarra, Nogmana Soumaguel, François Lavenu, Yann Tracol and Mamadou Diawara. doi:10.1016/ j.jhydrol.2008.11.005. 10. Rainfall regime over the Sahelian climate gradient in the Gourma, Mali. Frédéric Frappart, Pierre Hiernaux, Françoise Guichard, Eric Mougin, Laurent Kergoat, Marc Arjounin, François Lavenu, Mohamed Koité, Jean-Emmanuel Paturel, Thierry Lebel. doi:10.1016/j.jhydrol.2009.03.007. 11. The organization and kinematics of tropical rainfall systems ground tracked at mesoscale with gages: First results from the campaigns 1999–2006 on the Upper Ouémé Valley (Benin). Christian Depraetere, Marielle Gosset, Stéphane Ploix, Henri Laurent. doi:10.1016/j.jhydrol.2009. 01.011. 12. Surface thermodynamics and radiative budget in the Sahelian Gourma: seasonal and diurnal cycles. Françoise Guichard, Laurent Kergoat, Eric Mougin, Frank Timouk, Frederic Baup and François Lavenu. 13. Response of sensible heat flux to water regime and vegetation development in a central Sahelian landscape. Timouk F, Kergoat L, Mougin E, Lloyd C.R., Ceschia E, de Rosnay P, Hiernaux P, Demarez V, Taylor C.M. (HYDROL7143). 14. Towards an understanding of coupled physical and biological processes in the cultivated Sahel - 1. energy and water. David Ramier, Nicolas Boulain, Bernard Cappelaere, Franck Timouk, Manon Rabanit, Colin R. Lloyd, Stéphane Boubkraoui, Frédéric Métayer, Luc Descroix, Vincent Wawrzyniak. doi:10.1016/j.jhydrol.2008.12.002. 15. Towards an understanding of coupled physical and biological processes in the cultivated Sahel – 2. vegetation and carbon dynamics. Boulain N., Cappelaere B., Ramier D., Issoufou H.B.A., Halilou O., Seghieri J., Guillemin F., Oï M., Gignoux J., Timouk F. doi:10.1016/j.jhydrol.2008.11.045. 16. Combining scintillometer and an aggregation scheme to estimate area-averaged latent heat flux during AMMA Experiment. Ezzahar Jamal, Chehbouni Abdelghani, Hoedjes

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Joost, David Ramier, Nicolas Boulain, Stephane Boubkraoui, Bernard Cappelaere, Luc Descroix, Bernard Mougenot and Timouk Frank; doi:10.1016/j.jhydrol.2009.01.010. Combined analysis of energy and water budgets to consolidate latent heat flux estimation using an infrared scintillometer. Adrien Guyot, Jean-Martial Cohard, Sandrine Anquetin, Sylvie Galle. doi:10.1016/j.jhydrol.2008.12.027. Multiscale soil moisture monitoring and characterization over the Sahelian mesoscale site of Gourma. P. de Rosnay, C. Gruhier, F. Timouk, F. Baup, E. Mougin, P. Hiernaux, L. Kergoat, V. LeDantec. doi:10.1016/j.jhydrol.2009.01.015. ERS Scatterometer surface soil moisture analysis of two sites in the south and north of the Sahel region of West Africa; M. Zribi, M. Pardé, P. De Rosnay, F. Baup, L. Descroix, C. Ottlé, B. Decharme. doi:10.1016/j.jhydrol.2008.11.046. Hydrological modelling and associated microwave emission of a semi-arid region in South-western Niger T. Pellarin, J.P. Laurent, B. Decharme, L. Descroix, B. Cappelaere doi:10. 1016/j.jhydrol.2008.12.003. Conditional simulation schemes of rain fields and their application to rainfall runoff modeling studies in the Sahel. Théo Vischel, Thierry Lebel, Sylvain Massuel, Bernard Cappelaere. doi:10.1016/j.jhydrol.2009.02.028. Water and Energy budgets simulation over the Niger super site spatially constrained with remote sensing data. S. Saux-Picart, C. Ottlé, B. Decharme, C. André, M. Zribi, A. Perrier, B. Coudert, N. Boulain, B. Cappelaere. doi:10.1016/ j.jhydrol.2008.12.023. Editors Jean-Luc Redelsperger Chair of the International Scientific Steering Committee of AMMA Thierry Lebel Chair of the International Coordination and Implementation Committee of AMMA