African Floodplain Research Articles

Hydrodynamic Modeling of Inundation Patterns of a Large African Floodplain Indicates Sensitivity to Waterway Restoration

AbstractLarge‐scale floodplains are important features of the African continent. Regular inundation provides the means to support large populations but can also present problems such as access to health facilities and water body formation that sustain malaria vectors. Modeling of these floodplains is therefore important, but complex. In this research, we develop, calibrate, and validate a hydrodynamic model of the Barotse Floodplain, of the Upper Zambezi, Zambia. The floodplain has seen recent infrastructure developments including the restoration of the canal network and construction of a cross floodplain causeway. In order to create a robust model, a multiobjective calibration uses a time slice approach based on available Landsat satellite image overpasses. An emulator‐based sensitivity analysis indicates the significance of hydrological processes in the model. Model evaluation is undertaken for two events in the gauge record (2009 and 2018), of similar magnitude that occur before and after modifications to the floodplain. Results indicate a complex impact of infrastructure development on the hydrodynamics of the floodplain, with a higher peak flow, but with a redistribution of water throughout the floodplain. Deeper flooding is observed in some areas while others experience lower water levels. The sensitivity results also reflect a change in processes, where floodplain flows dominate the 2009 event, whilst channel process dominate the 2018 event. Overall, we show that relatively modest modifications to the floodplain have impacted flood water levels, which in turn will influence access route availability and alter malaria transmission rates.

Characterising the late Quaternary facies stratigraphy of floodplains in South Africa

AbstractSouth African river floodplains and their alluvial deposits reflect a diversity of geological and geographical drivers. We use a genetic geomorphic classification system originally developed for dryland wetlands to characterise geomorphic processes and potential successions of sedimentary fill for South African floodplains. Using case studies from the literature, we consider differences between alluvial rivers and mixed bedrock-alluvial rivers in the context of macro-scale geomorphic setting, and evaluate the impact of the setting on floodplain persistence and potential as a palaeo-environmental archive. Sedimentary facies associations represented in South African floodplains, including lateral and oblique accretion, channel, channel infill, levee vertical accretion, floodplain vertical accretion and debris flow deposits, are also evaluated.Floodplains of South Africa’s interior are typically mixed bedrock-alluvial as channel beds are set upon or close to bedrock and sediment thickness is limited. By contrast some floodplains in tectonic basin settings have sediment deposits exceeding 30 m in thickness. The resulting rivers are alluvial, and thus able to adjust their width, depth and slope to accommodate changes in discharge and sediment supply. Similarly, coastal floodplain rivers are alluvial due to downcutting during the last glacial maximum and subsequent sedimentary infilling as sea levels rose.When considering the potential of floodplains as palaeoarchives of environmental change, two considerations emerge. First, floodplain stratigraphy is not a response to a single variable due to complex process-form feedbacks. Rather, floodplain stratigraphy is an outcome of both autogenic and allogenic processes. Second, most South African floodplains are zones of sediment recycling, and as such, preservation potential is typically low. Thus, although floodplain settings of the interior may be a few million years old, the sediment within them may be only thousands to tens of thousands of years old. Our review indicates that research has historically focused on meandering river and mixed bedrock-alluvial anabranching river floodplains, while understanding of other floodplain sub-types remains limited.

Regional Scale Risk to the Ecological Sustainability and Ecosystem Services of an African Floodplain System.

The Phongolo floodplain is one of southern Africa's most important systems. In this study, we carried out a regional scale ecological risk assessment to evaluate the risk of multiple stressors associated with the use of the aquatic resources in the floodplain to selected social and ecological endpoints representing its sustainability. The floodplain has undergone significant changes as a result of the impacts of multiple stressors. This includes high risk of impact and threatened sustainability between the Pongolapoort Dam and the Ndumo Game Reserve (NGR). This compares to relatively low risk to the maintenance of the endpoints within the NGR. The reserve provides a protection and refuge function for regional biodiversity maintenance and ecosystem sustainability processes. In the study a range of scenarios were considered and demonstrate that the system will respond to protection measures and or increased resource use options. Should flood reductions or water quality pollution drivers continue on increasing trajectories, the condition of the Phongolo River and floodplain will probably deteriorate into an unacceptable, unsustainable state. Removal of the protection services of the NGR would result in an unsustainable ecosystem and loss of ecosystem services for regional vulnerable African communities. Additional evidence should be obtained from monitoring and research to refine, validate, and update the assessment in an adaptive management context. The risk assessment framework approach implemented in the Phongolo floodplain can contribute to the management of other floodplains ecosystems and the sustainability management of social and ecological attributes and processes of these important ecosystems.

Institution Shopping and Resilience Grabbing: Changing Scapes and Grabbing Pastoral Commons in African Floodplain Wetlands

This article argues on the basis of a comparative study in floodplain areas in Cameroon, Tanzania, and Zambia that land tenure issues have to be related to historical institutional changes not only to property rights in land, but also to land related common pool resources. It further outlines that in addition to privatisation of resources, pastoralists face challenges from the fragmentation of cultural landscape ecosystems, which serves to undermine local common property institutions and their reciprocal resource management arrangements that provided resilience in floodplain ecosystems. Colonisation and postcolonial constellations led to a major institutional change from common to state property in African rangelands, and created legal and institutional pluralism which enabled more powerful state and wealthy actors to shop for institutions that suit them best, especially privatisation and open access constellations (Haller 2010). The article argues that it is this institutional change that transformed the previous cultural landscape ecosystems, that conservationists and governments now want to protect by excluding pastoralists. It will analyse how governments and environmentalists increase their bargaining power in order to appropriate these landscapes in the form of land as commons and green grabbing. It will do so by combining a New Institutionalism and Political Ecology approach (NIPE) with Appadurai's notion of scapes (1996) in a way that unpacks power dynamics for legitimacy-based governmentalities that link global and local scales. The article unpacks the idea of ideoscapes and adds a four-step historical phases model to the concept. This illustrates the scapes pastoralists are in (state, neoliberal, neoliberal applied including investment and conservation, and terrorism and control scapes). These scapes provide legitimacy capital for powerful actors, shape platforms for strategic selection of institutions (institution shopping) and act as Anti-Politics Machines (APM).

Impact of Climate Change on Water Resources in the Kilombero Catchment in Tanzania

This article illustrates the impact of potential future climate scenarios on water quantity in time and space for an East African floodplain catchment surrounded by mountainous areas. In East Africa, agricultural intensification is shifting from upland cultivation into the wetlands due to year-round water availability and fertile soils. These advantageous agricultural conditions might be hampered through climate change impacts. Additionally, water-related risks, like droughts and flooding events, are likely to increase. Hence, this study investigates future climate patterns and their impact on water resources in one production cluster in Tanzania. To account for these changes, a regional climate model ensemble of the Coordinated Regional Downscaling Experiment (CORDEX) Africa project was analyzed to investigate changes in climatic patterns until 2060, according to the RCP4.5 (representative concentration pathways) and RCP8.5 scenarios. The semi-distributed Soil and Water Assessment Tool (SWAT) was utilized to analyze the impacts on water resources according to all scenarios. Modeling results indicate increasing temperatures, especially in the hot dry season, intensifying the distinctive features of the dry and rainy season. This consequently aggravates hydrological extremes, such as more-pronounced flooding and decreasing low flows. Overall, annual averages of water yield and surface runoff increase up to 61.6% and 67.8%, respectively, within the bias-corrected scenario simulations, compared to the historical simulations. However, changes in precipitation among the analyzed scenarios vary between −8.3% and +22.5% of the annual averages. Hydrological modeling results also show heterogeneous spatial patterns inside the catchment. These spatio-temporal patterns indicate the possibility of an aggravation for severe floods in wet seasons, as well as an increasing drought risk in dry seasons across the scenario simulations. Apart from that, the discharge peak, which is crucial for the flood recession agriculture in the floodplain, is likely to shift from April to May from the 2020s onwards.

Influence of Floods and Growth Duration on the Productivity of Wet Grasslands of Echinochloa stagnina (Retz) P. Beauv. in an East African Floodplain

Echinochloa stagnina (Retz) P. Beauv., a widespread semi-aquatic perennial grass, is a key species for the sustainability of many African wetlands. In an effort to assess interactions between hydrological regimes and management strategies targeting conservation, this study focuses on characterizing the growth of a typical East African wet grassland and describing the relative influence of floods and management practices on its growth rates. Naturally flooded grassland, subjected to a factorial design protocol with different levels of irrigation and cutting frequencies, was sampled during 15 months in the Tana River Delta, Kenya. Statistical analyses of the data, using linear mixed models accounting for temporal auto-correlation, were undertaken. Reduced cutting frequencies slightly increased growth rates, except under water-limited conditions. Increased water supply had a positive effect on growth rates, the latter noting a five- and ten-fold increase from water-limited to water non-limited to flooded conditions. Floods impacted grasslands by increasing growth rates, which attained very high values of 225 kgDM ha−1 day−1, extending growth period and limiting the negative effect of severe phytomass removal through clipping. These results support the view that rangeland and water management for the Tana River Delta, and more widely for tropical floodplain grasslands, should be jointly undertaken.

The unique functioning of a pre-Columbian Amazonian floodplain fishery

Archaeology provides few examples of large-scale fisheries at the frontier between catching and farming of fish. We analysed the spatial organization of earthen embankments to infer the functioning of a landscape-level pre-Columbian Amazonian fishery that was based on capture of out-migrating fish after reproduction in seasonal floodplains. Long earthen weirs cross floodplains. We showed that weirs bear successive V-shaped features (termed ‘Vs’ for the sake of brevity) pointing downstream for outflowing water and that ponds are associated with Vs, the V often forming the pond’s downstream wall. How Vs channelled fish into ponds cannot be explained simply by hydraulics, because Vs surprisingly lack fishways, where, in other weirs, traps capture fish borne by current flowing through these gaps. We suggest that when water was still high enough to flow over the weir, out-migrating bottom-hugging fish followed current downstream into Vs. Finding deeper, slower-moving water, they remained. Receding water further concentrated fish in ponds. The pond served as the trap, and this function shaped pond design. Weir-fishing and pond-fishing are both practiced in African floodplains today. In combining the two, this pre-Columbian system appears unique in the world.

The interplay of top-down planning and adaptive self-organization in an African floodplain

Natural floodplains are complex social-ecological systems in which human livelihoods are tightly coupled with flooding dynamics. In this paper, we argue that hydraulic planning in such systems, including to mitigate extreme floods, must consider three key features of adaptive self-organizing systems in floodplains - connectivity, learning feedbacks, and rhythms - to support the resilience of floodplain communities to extreme floods. We illustrate this argument with a case study of the Logone Floodplain, Cameroon. Based on hydrological data, ethnographic research and a series of focus group interviews with floodplain villagers, we analyze the interplay of top-down planning and adaptive self-organization in response to two extreme floods in 2012 and 2015. We show that recent top-down strategies of hydraulic control in the floodplain have led to more hydraulic uncertainty for local populations, and curtailed the conditions for sustainable self-organization in the floodplain.

Studying the Logone floodplain, Cameroon, as a coupled human and natural system

African floodplains are an excellent example of coupled human–natural systems because they exhibit strong interactions among multiple social, ecological, and hydrological systems. The intra-annual and interannual variations in seasonal flooding have direct and indirect impacts on ecosystems and human lives and livelihoods. Coupled human and natural system (CHANS) is a broad conceptual framework that is used to study systems in which human and natural components interact. While there are other conceptual frameworks to study social-ecological systems, the CHANS framework offers a clear way of studying the interactions, called couplings, between human and natural systems. Core features of the framework are the following: human and natural systems are analytically separated; focus is on processes within and couplings between systems; and the goal is to build an integrative, quantitative model of the coupled system. This paper explains the conceptual framework of coupled systems, using the case study of the Logone floodplain in Cameroon. We compare the CHANS framework with other frameworks that have been used to study the same floodplain, and argue for its usefulness in the study of African floodplains.

Age and growth of sharptooth catfish, Clarias gariepinus (Burchell, 1822) (Clariidae), in the Lower Okavango Delta, Botswana

Managing any inland fishery requires reliable age data and accurate estimates of growth rate. Clarias gariepinus, the largest catfish species in most tropical African floodplain river systems, is an important constituent of subsistence fishery catches. We used otolith-derived age estimates to describe patterns of age and growth for C. gariepinus from the Lower Okavango Delta in northern Botswana. Edge analysis of sectioned sagittal otoliths showed that growth zone deposition occurred during the annual flood peak between July and September. The maximum recorded age was 8 years. Growth was best described by the von Bertalanffy Growth Model of the form E[L|age] = 528.70(1–e–0.72(age+1.35)). A comparison of growth parameters from this study with those from previous studies suggests two distinct populations of C. gariepinus between the Upper and Lower Okavango Delta, emphasising the importance of regional stock assessment for key fishery species. The fairly short life span and high variability in growth of C. gariepinus are important indicators that it is imperative to maintain natural habitat and flow regime for sustainable management of fishery resources in the Lower Okavango Delta.

Raptor habitat use in the Lake Chad Basin: insights into the effect of flood-plain transformation on Afrotropical and Palearctic raptors

SummaryWest African flood-plains have undergone major land-use transformations in the second half of the 20th century. To obtain insight in the effect of flood-plain development for irrigated rice cultivation on the abundance, richness, and diversity of Palearctic and Afrotropical raptors, we conducted monthly transect surveys covering dry and wet seasons in four major habitats on the Waza-Logone flood-plain of Cameroon: dry grasslands, cultivated grasslands, rice fields, and seasonally flooded grasslands resembling natural flood-plain vegetation. We recorded 36 raptor species among 2,533 individuals, dominated by Black Kite Milvus migrans, which comprised 42% of counts. Although richness and diversity were not related to land-use for Palearctic raptors, Afrotropical raptor diversity was higher on the flooded grasslands compared to the newly created cultivated habitats and dry grasslands. The abundance of Afrotropical raptors did not significantly differ across habitats but was lower in rice-fields when Black Kite and Hooded Vulture Necrosyrtes monachus were excluded. Conversely, Palearctic raptor abundance was highest in post-harvest rice fields, demonstrating the importance of the rice fields as foraging habitat for Palearctic raptors. Further transformation of West Africa’s flood-plains is expected, reducing their capacity for Afrotropical raptors, while Palearctic raptors may benefit from expansion of rice-fields, but more research is needed on their vulnerability to pesticide use.

How fit turns into misfit and back: Institutional Transformations of Pastoral Commons in African Floodplains

Haller, T., G. Fokou, G. Mbeyale, and P. Meroka 2013. How fit turns into misfit and back: institutional transformations of pastoral commons in African floodplains. Ecology and Society 18(1): 34. https://doi.org/10.5751/ES-05510-180134

Patterns of seed dispersal and establishment of the invader Prosopis juliflora in the upper floodplain of Tana River, Kenya

Invasive species in Africa have important impacts on food security and biodiversity conservation. African floodplains in arid areas are critical wildlife habitats in addition to crop production and dry season livestock grazing. The study aimed to understand the patterns of spread of the invader Prosopis juliflora in a typical African floodplain characterised by both multiple dispersers and habitats. Data was collected on faecal seed density, establishment and vegetation composition, and subjected to non-parametric tests and regression analysis. The results showed that both livestock and wildlife species played a critical role in dispersal of Prosopis juliflora, especially yellow baboons (Papio cynocephalus), bush pigs (Potamochoerus larvatus), donkeys and goats. Habitat preference of wildlife dispersers and livestock herding patterns influenced the spatial pattern of invader seed influx. Establishment of Prosopis juliflora was enhanced by ecological disturbance such as in rested crop fields and habitats outside conservation areas as compared to those inside. Establishment was also higher inside the floodplain than outside probably due to higher seasonal soil resources. Lastly, indigenous woody species diversity declined significantly as the density of Prosopis juliflora increased. We conclude that patterns of spread of Prosopis juliflora were related to ecological disturbances, type of disperser and flooplain effect.

Late Quaternary dynamics of a South African floodplain wetland and the implications for assessing recent human impacts

Knowledge of the long-term geomorphological dynamics of wetlands is limited, so currently there is an inadequate scientific basis for assessing anthropogenically induced changes and for developing conservation, remediation, and/or sustainable management guidelines for these fragile ecosystems. Along the upper Klip River, eastern South Africa, geomorphological and sedimentological investigations, geochronology, and remote sensing have been used to establish the late Quaternary dynamics of some internationally important floodplain wetlands, thus providing a reference condition against which to assess the extent of recent human impacts. Optically stimulated luminescence dating reveals that the wetlands have developed over at least the last 30 ky as a result of slow meander migration (< 0.2 m y − 1 ), irregular cutoff events, and infrequent avulsions (approximately one every 3–6 ky) that have occurred autogenically as a natural part of meander-belt development. Following European settlement in the Klip valley (late nineteenth century), however, modifications to local flora and fauna, as well as the initiation of local wetland drainage schemes, have had major impacts. In particular, proliferation of exotic willows and associated debris jams, and the artificial excavation of a 1.2-km-long channel section across the wetlands have initiated an ongoing avulsion that is characterised by failure (gradual abandonment) of the main channel and rapid incision of a headcutting channel. Compared to the pre-settlement condition, little change in lateral migration activity has occurred, but this avulsion provides a clear example of anthropogenically accelerated change, occurring only ~ 1 ky after the last natural avulsion and in a part of the wetlands where avulsions have not occurred previously. Subsequent human interventions have included installing weirs in an attempt to control the resulting erosion and promote reflooding, but ongoing maintenance has been required. In areas that were not glaciated during the Quaternary, many other floodplain wetlands may be of similar antiquity, but the Klip River illustrates their sensitivity to direct and indirect human impacts.

Market-resource Links and Fish Vendor Livelihoods in the Upper Zambezi River Floodplains

This paper examines small-scale fish vending in a southern African floodplain from two perspectives: as a link between natural resource use and consumption, and as a livelihood in itself. We used a combination of observation, surveys and semistructured interviews in a market in Katima Mulilo, Namibia, to determine sources of fish, preferences and constraints to vending, average investment and profit, as well different routes into fish vending and perceptions regarding vending. Most vendors come from fishing households, but their stock is often an accumulation of purchases from other fishers. There is little evidence of formal arrangements between fishers and vendors, yet most adapt to the highly variable natural and social environments of the region. Although all vendors ranked selling fish as their most important livelihood activity, a wide range of investment and profit exists among individuals. Our findings indicate that fisheries management proposed for the area must be developed with a careful understanding of how changes in access and use will affect vending livelihoods.

Interactions between fire and flooding in a southern African floodplain system (Okavango Delta, Botswana)

A series of 98 satellite images was analysed to reconstruct the fire and flood history of a floodplain system in southern Africa (Okavango Delta, Botswana). The data was used to investigate interactions between fire and flooding, and to determine the relevance of rainfall and flood-events for fire occurrences on floodplains and on drylands. The aims of the study are (1) to analyse and compare the fire frequency on floodplains and on adjacent drylands, (2) to investigate the influence of rainfall and flooding on the fire occurrence and (3) to determine correlations between fire frequency and flood frequency. The analyses show higher fire frequencies on floodplains than on drylands because of higher biomass production and fuel loads. The fire occurrence on drylands shows a correlation with annual rainfall events, while the fire frequency on floodplains is in principle determined by the flood frequency. Between floodplain types, clear differences in the susceptibility to fire where shown by analysing flood frequency vs. fire frequency. Here, the highest potential to burn was found for floodplains that get flooded about every second year. By calculating mean fire return intervals, the potential to burn could be specified for the different floodplain types.

Common Property Resource Management, Institutional Change and Conflicts in African Floodplain Wetlands

Most contemporary discussions on African development since independence forty years ago emphasize the notion that Africa is still “mal parti”. Many show discontent for what has been achieved in this time, despite that “Africa works” as is suggested by Chabal and Daloz in their widely discussed book (1999). I will focus on the issue of sustainable development in Africa. This will be illustrated by the presentation of a common property resource management research project (on fisheries, pastures, wildlife, water for irrigation, and forests). The question of why the overuse of natural resources and conflicts over resources are occurring in modern day Africa is addressed here. This research project is called “Common Property Institutions and Power Relations: Resource Management, Change and Conflicts in African Floodplain Wetlands”. It focuses on six African floodplain wetlands in semi-arid zones (Internal Niger Delta in Mali, Hadejia-Jama'ara in Northern Nigeria, Logone Floodplain in Northern Cameroon, Pangani Floodplain in Tanzania, Okavango Delta in Botswana and Kafue Flats in Zambia (Haller 2001)). This work will be conducted by students from the Department of Social Anthropology, University of Zurich and by local students in these wetland areas, and will be under my supervision. The aim is to make a comparative analysis of these wetland areas, which should lead to a better understanding of the processes of resource use responsible for environmental problems and conflicts in these areas. The project is integrated in the Swiss National Centre of Competence in Research (NCCR) with the title “NCCR North-South: Research Partnerships for Mitigation Syndromes of Global Change”. In this competence centre, research for mitigation of diverse problems concerning development in the Third World will be done over the next 10 years by a large group of Swiss research institutions who will conduct this research in partnership with local scientists in Third World Countries (SARPI 2000). The competence centre consists of different subprojects, one of them being the IP6 “Institutional Change and Livelihood Strategies”, in which the presented research project on African floodplain wetlands is integrated. To illustrate the aim of the project and to give some reflections on institutional change and conflicts, two cases from Northern Nigeria and Mali will be presented. These two cases show how common property resources (CPR) such as fisheries were institutionally managed in former times, and how the traditional institutions have changed over the last forty years. The aim of the wetlands project is to test the validity of the New Institutionalism theories on CPR Resource Management in a specific ecological and political setting. Firstly, the design principles by Elinor Ostrom on robust institutions for the sustainable use of CPR-resources (Ostrom 1990, Becker and Ostrom 1995) are compared with local CPR-institutions in the selected wetlands areas. Secondly, theories and approaches of the New Institutional Economics and Anthropology (North 1990, Ensminger 1992) will be tested. Finally, Jean Ensminger's explanations concerning institutional change and bargaining power are looked at in particular. (African Anthropologist: 2002 9(1): 25-35)

Methane sources and sinks in a periodically flooded South African savanna

Methane (CH4) fluxes were measured over a 2‐year period at subtropical savanna and floodplain sites located in South Africa. No significant differences were detected in fluxes from the nutrient‐rich and nutrient‐poor savannas. Savannas were not always sinks and showed some CH4 emissions during the wet, summer season with fluxes in the range of −1.6 to 1.68 mg m−2 d−1. Soil temperature did not have a significant effect on the flux, while CH4 consumption decreased and emissions increased with an increase in soil moisture between 5% and 50% water‐filled‐pore‐space (WFPS). Understanding the factors which control the production and emission of CH4 in aerobic environments is extremely important for predicting net emissions from a region. Dry floodplain soil fluxes were near zero and did not differ significantly from savanna fluxes. During a flood, saturated sites emitted CH4 at an average rate of 69.4 mg m−2 d−1. Flooded areas with a water depth of 0.3 ‐ 0.4 m emitted CH4 from the water surface at rates between 0.48 and 466.3 mg m−2 d−1 with the highest emission occurring during summer. Emission rates were exponentially related to sediment temperature, which had a greater influence on the emission rate than the flood regime. The length of the dry period preceding the flood and the extent of a flood did not have a significant effect on CH4 fluxes from saturated and flooded sites. Emission rates were highest when the water level was between 0.1 m below the soil surface and 0.4 m above the soil surface, with emission rates declining to near zero as the water became deeper than 0.4 m. Savannas were estimated to consume an average of 0.04 g CH4 m−2 d−1, with southern African savannas consuming 0.23 Tg CH4 yr−1. Saturated and flooded sites were estimated to produce 25.3 and 57.2 g CH4 m−2 d−1, respectively. Southern African floodplains are estimated to produce between 0.2 and 10 Tg CH4 yr−1(excluding the effects of vegetation‐mediated emissions) and therefore produce more CH4 than the savannas consume.

Floodplains and the Development of Complex Society: Comparative Perspectives from the West African Semi‐arid Tropics

The diverse nature of iron‐using societies in the semi‐arid tropics of West Africa is illustrated by the very different sequences of growth and change in the floodplains of the Middle Senegal Valley and the Inland Niger Delta during the first millennium A.D. The Inland Delta is characterized by a pattern of rapid population growth over the course of centuries‐long occupation in very large, high‐density settlement mound clusters. The Middle Senegal Valley has low‐density occupation in fairly regularly spaced small villages. These differences are due, in part, to differences in the size, distribution of critical resources, and nature of subsistence scheduling conflicts and opportunities in the two floodplains. Faced with similar risks to agriculture posed by chaotic variation in the timing and extent of annual flooding, societies on both floodplains have responded in broadly similar ways historically by developing subsistence specializations that are integrated through exchange. Despite the large scale and considerable integration of first millennium societies in the Inland Niger Delta, the clustered settlement pattern that persisted even through the period of maximum population density at around A.D. 1000 indicates that centralizing pressures did not succeed in producing an effective administrative hierarchy at that time. Rather, various models of heterarchical organization provide a better “fit” for the archaeological data currently available. In the Middle Senegal Valley, on the other hand, the small and relatively dispersed nature of settlement around A.D. 900 should not immediately be assumed to indicate simple, autonomous, village‐level organization. Here, the possibility that complex territorial units composed of overlapping territories of subsistence specialists had already emerged in the first millennium deserves consideration. As archaeology works to counterbalance its long‐standing focus on vertical hierarchies as the primary modality of complexity, the archaeological sequences in these two African floodplains offer insights into the various ways that specialization, trade, population heterogeneity, and even urban growth may have emerged and flourished relatively unconnected to the development of social stratification and centralized power.

Classification of multi-temporal SPOT-XS satellite data for mapping rice fields on a West African floodplain

The use of satellite imagery data to accurately map agricultural land use in semi-arid Africa has proven difficult due to the small size of cropped fields, low vegetative cover within and outside cropped areas, and high correlation between the seasonal trajectories in the signatures of cropped and uncropped surfaces. This paper evaluates the potential of computer-aided classification of multi-temporal imagery data to map traditionally-managed rice fields in Sahelian West Africa. SPOT-XS imagery data covering the southwestern corner of the Inland Niger Delta of Mali were acquired on three dates in 1988. These dates encompass the pre-flood period when ploughed surfaces contrast strongly with surrounding land surface and the first half of the flood period. Two alternative classification approaches were evaluated. Supervised and unsupervised classification of a database formed by principal component reduction proved unsuccessful as a result of the high spectral heterogeneity of ploughed and unploughed surfaces. An alternative classification approach was developed which utilizes a series of steps (unsupervised classification, stratification, and supervised classification) that progressively reduce image heterogeneity in such a way to better aid interpretation and comparison with training data. Seventy-one per cent of ploughed-field reference sites were classified correctly using this approach, while 8-20 per cent of reference sites for cover types with a potential for being confused with ploughed fields were misclassified as ploughed fields. A fuller evaluation was performed using a posterior accuracy assessment of classifications produced at five intermediate stages of the procedure. The suitability of the procedure for land-use classification in spectrally-heterogenous areas is discussed.