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Estimation of cultivated area, number of farming households and yield for major rice-growing environments in Africa.

(eds M.C.S. Wopereis et al.) 35 Upland environments are situated at the high end of the toposequence, where rice depends solely on rainfall as the water table is out of the reach of rice roots for much of the growing season. At the lower end of the toposequence, rice plants can reach the water table or profit directly from flood water. Along the toposequence, interactions exist between environments (e.g. water and nutrient flow from upland to lowland). These externalities influence the environmental sustainability of rice farming. Furthermore, interlinkages can blur the boundaries between rice environments, as in the hydromorphic zones, between the upland and lowland areas, which rather than being flooded for long periods have a water table close enough to the surface for rice roots to reach during the growing season. Another unclear transition exists between rainfed and irrigated lowlands, where a continuum of water management exists ranging from the strictly rainfed (no water control or only drainage), which may evolve (via investments in water control measures) to the fully irrigated lowlands. In this chapter, lowlands with partial water control (intensified lowland) are classified as rainfed lowland environments. Introduction