Shared Waters, Shared Opportunities: Hydropolitics in East Africa

The Large-Scale Irrigation Potential of the Lower Rufiji Floodplain: Reality or Persistent Myth?

219 The Large-Scale Irrigation Potential of the Lower Rufiji Floodplain: Reality or Persistent Myth? Olivier Hamerlynck, Stéphanie Duvail, Heather Hoag, Pius Yanda & Jean-Luc Paul Introduction The Rufiji River Basin is located entirely in Tanzania and drains some 180,000 km², about a fifth of mainland Tanzania. The river has a strong seasonal flow pattern, with a flood peak around April. With an annual mean flow of 800 cubic meters per second, it is one of Africa’s largest rivers after the Congo, Zambezi, Niger, Nile, and Volta. The annual rainfall is highly variable, depending on altitude and distance from the coast and generally displays two peaks with short rains in October-November and long rains from March to May. Three main rivers: the Kilombero, the Great Ruaha and the Luwegu, join up in the Selous Game Reserve to form the Rufiji which tumbles down to its wide Lower Floodplain through the 100 m deep Stiegler’s Gorge before branching out in a wide delta covered by the largest stand of mangrove in East Africa (Figure 1). Administratively, the Lower Floodplain is located entirely in the Rufiji District which has some 200,000 inhabitants. Figure 1: The Rufiji River Basin 220 Description of Agriculture in the Rufiji District The agricultural potential of the Rufiji District cannot be confined to the floodplain but has to be framed in a wider land-use context. Traditionally livelihoods strategies in Rufiji show a strong integration of activities between the floodplain farming system, the surrounding wooded and forested terraces and the floodplain associated lakes.1 In Rufiji District the agricultural potential is mainly determined by soil quality and water supply (rainfall, floods and gr oundwater). On the basis of soil type and natural vegetation cover, which is the result of the combination of soil type and the availability of water, there are four main zones in Rufiji District outside of the Selous Game Reserve (part of which covers 25% of the District but will not be considered here): Figure 2: The Lower Rufiji Valley • 425,000 ha of coarse sands, covered with miombo woodland and coastal forest mosaics, lying between 50 m and 150 m Above Mean Sea Level (ASL), (‘the terraces’) • 270,000 ha of finer sands (predominantly red soils), covered with coastal forest, between 150 m and 700 m ASL (‘the hills’) • 170,000 ha of rich fertile alluvium interspersed with sands, covered by tall grass and occasional trees lying between 5 m and 50 m ASL (‘the floodplain’) • 130,000 ha of fine salty clay, covered by mangrove and grasslands below 5 m ASL (‘the delta’) The Lower Rufiji floodplain starts at Mloka where the river leaves the Selous Game Reserve. At Mloka, the floodplain has a width of some 12 km and is 221 situated at an altitude of about 50 m ASL. From there, it gradually slopes down to the Ocean, about 120 km to the east, its width varying from 7 km (Utete) to 30 km (edge of the delta). Floods that cover agriculturally significant parts of the floodplain occur, on average, in 4 years out of 10.2 These floods, which transport enormous quantities of fine elements, provide the natural fertiliser to the floodplain fields (shamba in Swahili). The coarser sands are deposited in areas with relatively high current velocities, in the various riverbeds and on the banks when the river overflows them. When the sediment-laden floodwaters enter the plains, the current velocities drop sharply and the fine elements are deposited, as a thick layer of fertile silt. Local farmers estimate that yields are halved if a shamba is not flooded for 3 consecutive years.3 The high dynamics of the river, which continuously erodes and deposits different materials, meanders or opens new braided channels, has resulted in a patchy distribution of soils.4 This is most pronounced in the western and central parts of the floodplain. Therefore, in any single year only a relatively small proportion of the floodplain is cultivated, at present some 15,000 ha. Fallows represent about twice as much surface area which means local farmers consider that about 45,000 ha are ‘worth cultivating’ in a highly opportunistic and flexible system. For an individual plot, the series of years it is cultivated continuously can vary between 5 and over 30 years, depending on the flooding frequency. In the absence of floods, yields will decline strongly after 3 years but...